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TEMPORARY REMOVAL: Evolution of textures, crystal size distributions and growth rates of plagioclase, clinopyroxene and spinel solidified at variable cooling rates from a mid-ocean ridge basaltic liquid
Journal Pre-proof Evolution of textures, crystal size distributions and growth rates of plagioclase, clinopyroxene and spinel solidified at variable cooling rates from a mid-ocean ridge basaltic liquid
Letizia Giuliani, Gianluca Iezzi, Francesco Vetere, Harald Behrens, Silvio Mollo, Federica Cauti, Guido Ventura, Piergiorgio Scarlato PII:
S0012-8252(19)30778-0
DOI:
https://doi.org/10.1016/j.earscirev.2019.103063
Reference:
EARTH 103063
To appear in:
Earth-Science Reviews
Received date:
25 November 2019
Accepted date:
10 December 2019
Please cite this article as: L. Giuliani, G. Iezzi, F. Vetere, et al., Evolution of textures, crystal size distributions and growth rates of plagioclase, clinopyroxene and spinel solidified at variable cooling rates from a mid-ocean ridge basaltic liquid, Earth-Science Reviews(2019), https://doi.org/10.1016/j.earscirev.2019.103063
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© 2019 Published by Elsevier.
Journal Pre-proof Evolution of textures, crystal size distributions and growth rates of plagioclase, clinopyroxene and spinel solidified at variable cooling rates from a mid-ocean ridge basaltic liquid
Letizia Giuliania (corresponding author), Gianluca Iezzia,b, Francesco Veterec-d, Harald Behrensd, Silvio Mollob-e, Federica Cautia, Guido Venturab, Piergiorgio Scarlatob
Affiliations: Dipartimento di Ingegneria & Geologia (InGeO), Università G. D’Annunzio di Chieti-Pescara,
of
a
Istituto Nazionale di Geofisica e Vulcanologia, INGV, Via di Vigna Murata 605, 00143, Roma,
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b
na
Institute for Mineralogy, Leibniz University of Hannover, Callinstrasse 3, Hannover, D-30167,
Germany
Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale A. Moro 5, 00185
Roma, Italy
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e
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Dipartimento di Fisica & Geologia, Università degli studi di Perugia, Piazza dell’Università 1,
06123, Perugia, Italy d
re
Italy c
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Via dei Vestini 31, 66100, Chieti, Italy
Keywords: MORB, cooling rate, solidification, texture, CSD, growth rate. Abstract Mid-ocean ridge basalts (MORB) are the most abundant eruptive tholeiitic products on Earth. Many experimental investigations reproduce and model the solidification of basalts, but under limited thermal ranges of cooling (ΔTc) and cooling rates (ΔT/Δt). Previous studies report a set of experiments run between 1300 °C and 800 °C under ΔT/Δt of 1, 7, 60, 180, 1800 and 9000 °C/h. These experiments allowed to determine the glass-forming ability (GFA) of sub-alkaline silicate
Journal Pre-proof liquids, but do not give information on their textures. Here, we quantify the evolution of sizes, shapes, number per area and CSDs of plg (plagioclase), cpx (clinopyroxene) and sp (spinel) using image analysis on thousands of crystals. These textural measurements are the most complete dataset ever obtained in laboratory, since reflects rapid, intermediate and sluggish cooled parts of MORB from liquidus to solidus. Faceted plg grows only for ΔT/Δt ≤ 60 °C/h, while cpx and sp became dendritic at ΔT/Δt between 60 and 180 °C/h. As ΔT/Δt increase, crystal size ranges decrease from 1000-10 μm (1
of
°C/h) to 100-1 (60 °C/h) µm, 400-8 µm (1°C/h) to 25-0.5 µm (1800 °C/h) µm and 90-6 µm (1 °C/h)
ro
to 6-0.5 (1800 °C/h) µm, respectively for plg, cpx and sp. The 2D aspect ratio and calculated 3D
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shapes of all phases do not evidence trends as a function of ΔT/Δt. The number of crystals per area
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(#/A) increases with increasing ΔT/Δt, except for cpx between 60 and 180 °C/h. As ΔT/Δt increases, CSDs of plg, cpx and sp increase their slopes (m) and population densities per size (n0), reduce the
lP
size ranges and tend to be log-linear. At low ΔT/Δt, CSDs are composed by several log-linear
na
segments, which slopes are related to different pulses of nucleation, followed by the crystal growth by coarsening. The average m of CSDs linearly scale with the corresponding n0 and both m and n0
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for plg, cpx and sp are highly correlated with ΔT/Δt. The calculated maximum (Gmax) and average (GCSD) growth rates increase as ΔT/Δt increases; GCSD for plg, cpx and sp changes from 10-9 to 10-8 cm/s, from 10-9 to 10-7 cm/s and from 10-9 to 10-8 cm/s, respectively. The Gmax of cpx is highly correlated with m and n0 and semi-quantitatively to Lmax. Therefore, the CSD parameters of cpx in MORB can be used to retrieve either ΔT/Δt and Gmax. The experimental plg and cpx crystals with dimensions between 0.1 and 1 mm are abundant at low ΔT/Δt. In volcanic rocks, these crystal sizes are referred to phenocrysts and microphenocrysts formed in intra-telluric environments. Our data demonstrate that phenocrysts and microphenocrysts may grow also after eruptions. The continuous evolution of textures in response to ΔT/Δt variations implies that the solidification of MORB can be fully captured only by considering the effects of kinetics. As a result, the widely accepted assumption that phenocrysts represent the products of evolution processes, e.g. fractionation, in conduits or reservoirs could be not valid for some basaltic lavas.
Journal Pre-proof
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1. Introduction
ro
The effusion of magmas from mid-ocean ridges (MORs) is by far the most relevant
-p
mechanism of crust formation on Earth (Crisp, 1984; Soule, 2015). MORs erupt basalts (MORB)
re
with a prevailing tholeiitic composition characterized by SiO2 contents ~ 50 wt.%, Na2O + K2O < 3 wt.% (Rogers, 2015) and, with some exceptions (Ligi et al., 2005), H2O mostly close to 0.1 - 0.2
lP
wt.% (Wallace et al., 2015). Although extremely abundant, the textural features of MORB rocks
na
have been investigated in detail only in few studies (Coish and Taylor, 1979; Jafri and Charan, 1992; Lesher et al., 1999; Zhou et al., 2000; Faure and Schiano, 2004). These studies highlight the
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general aphyric and sub-aphyric texture of these rocks in the more rapidly cooled outermost parts of lava flows with tiny minerals dispersed in a glassy matrix. The innermost and sluggishly cooled portions are crystal-rich (Soule, 2015). Submarine MORBs are frequently erupted under superheated or close to liquidus conditions (Wallace et al., 2015). The analysis of both solidified and still molten basaltic lavas has improved our knowledge on the crystallization of silicate liquids and magmas (Muir and Tilley, 1966; Bryan, 1972; Mevel and Velde, 1976; Coish and Taylor, 1979; Schiffman and Lofgren, 1982; Long and Wood 1986; Rowland and Walker, 1988; Degraff et al., 1989; Jafri and Charan, 1992; Cashman, 1993; Armienti et al., 1994; Crisp et al., 1994; Lesher et al., 1999; Zhou et al., 2000; Burkhard, 2005; Chistyakova and Latypov, 2009; Lanzafame et al., 2013, 2017; Robert et al., 2014; Soldati et al., 2018 and references therein). In the last five decades, the basaltic rocks on Earth and Moon have been
Journal Pre-proof extensively investigated with experiments focused on the nucleation and crystal growth processes of natural melts and magmas (Lofgren et al., 1974; Usselman et al., 1975; Walker et al., 1976, 1978; Bianco and Taylor, 1977; Grove and Bence, 1977; Lofgren, 1977; Walker et al., 1977; Grove, 1978; Lofgren et al., 1979; Kirkpatrick, 1981; Lesher, 1999). Most of these processes are summarized in several review studies (Dowty, 1980; Lofgren, 1980; Kirckpatrick, 1981; Cashman, 1991; Lasaga, 1998; Higgins, 2006; Jerram and Higgins, 2007; Marsh, 2007; Hammer, 2008; Iezzi et al., 2009; Vetere et al., 2015; Mollo and Hammer, 2017). More recently, some authors have investigated the
of
role of strain rate and oxygen fugacity on the solidification of basaltic liquids (Kolzenburg et al.,
ro
2018; Vona et al., 2011; 2017; Vetere et al., 2017; 2019a; 2019b). The majority of anhydrous
-p
solidification experiments were conducted at atmospheric pressure and melt redox conditions close
re
to air through the application of variable (mainly linear) cooling rates (Hammer, 2008; Vetere et al., 2015; Mollo and Hammer, 2017). Alternatively, the solidification of hydrous basaltic magmas was
lP
investigated by degassing-induced decompression experiments (Applegarth et al., 2013; Fiege et al.,
na
2015; Arzilli et al., 2016). Some cooling experiments were also performed at high-P under either anhydrous or hydrous conditions (Del Gaudio et al., 2010; Shea and Hammer, 2013). Nevertheless,
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the solidification process induced by cooling at atmospheric pressure appears by far the most appropriate condition for understanding the crystallization path of aphyric MORBs and H2O-poor basaltic magmas.
In this study, we first focus on the imposed thermal ranges of cooling (ΔTc) and cooling rates (ΔT/Δt) imposed on basaltic compositions from literature. Then, we extend the experimental results reported in Vetere et al. (2013, 2015) on a tholeiitic basalt (sample B100 from Iceland) cooled between 1300 and 800 °C (ΔTc) at ΔT/Δt of 1, 7, 60, 180, 1800 and 9000 °C/h. These data point out that the abundance (area%) of plagioclase (plg), clinopyroxene (cpx) and spinel (sp) phases decreases as ΔT/Δt increases, making possible to model the glass-forming ability of the melt. On this premise, we reconstruct the overall evolutionary path of plg, cpx and sp by quantifying the textural maturation of crystals at different ΔT/Δt through the following parameters:
Journal Pre-proof size and 2D-3D shape of crystals, number of crystals per area (#/A), crystal size distribution (CSD) and growth rate (G). These new data rely on the measurement of several thousands of textural data collected by image analysis of crystals formed under very rapid to intermediate and extremely sluggish ΔT/Δt applied from liquidus to solidus conditions (ΔTc). Our textual dataset constrains the crystallization of aphyric basaltic lava flows, pillow lavas, bombs, and decimeter-to-meter thick surface dikes. These experimental outcomes have implications for (1) the interpretation of MORB cooling paths, (2) a better understanding of crystal formation and growth before and after the
ro
of
eruption and (3) the reconstruction of magma solidification conditions.
-p
2. Previous experimental investigations
re
More than 50 basaltic compositions investigated by cooling-induced crystallization experiments are present, to our knowledge, in the literature (Fig. 1 and Table 1S). The anhydrous
lP
starting compositions of these experiments range from picro-basalt to basalt to basaltic andesite to
na
trachybasalt, and consist of glassy materials from synthetic or natural products melted and rapidly quenched at 1 atm. Cooling experiments were performed at log fO2 ranging from air (-0.68 at
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1600°C) to NNO-6.2 (where NNO refers to the nickel-nickel oxide buffer; Table 1S). The thermal range of cooling significantly changes among the different studies, with the largest applied ΔTc of 1000 °C (from 1600 to 600 °C; Fig. 1S). The most frequent ΔTc is generally initiated below 1250 °C and terminated above 900 °C (Fig. 1 and Table 1S). However, the melting (or liquidus) temperature (Tm) and the glass transition temperature (Tg) of MORB liquids are ~ 1250 and 650 °C, respectively (Vetere et al., 2015; Iezzi et al., 2017). Hence, a significant part of the crystallization path of dry basaltic liquids is not considered by most of the previous studies (Fig. 1 and Table 1S). The experimental cooling paths applied to basaltic systems are at constant ΔT/Δt rates, with only few investigations with more complex thermal paths (Table 1S). Notably, a few experiments covering a large ΔTc (above 1300 °C and down to 800 °C) explored only high ΔT/Δt rates, whereas experiments with very slow ΔT/Δt were quenched above 900 °C (Fig. 1 and Table 1S). This implies
Journal Pre-proof that the overall knowledge of the MORB solidification process requires a more systematic ΔT/Δt change with ΔTc between liquidus and solidus temperatures (Fig. 1). Consequently, the ΔT/Δt and ΔTc used in Vetere et al. (2013; 2015) are appropriate to capture the whole solidification paths of MORB units, albeit possible reheating processes by the emplacement of other successive units are
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na
lP
re
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ro
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not considered.
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f o
l a
o r p
r P
e
n r u
o J
Fig.1. Thermal paths of the most relevant experimental solidification studies from literature (Table 1S) and the related melt compositions plotted in the TAS diagram (La Maitre, 2002). The green field highlights the experimental conditions investigated in this study (Table 1S; Vetere et al., 2013, 2015).
Journal Pre-proof
authors Backer and Grove (1985) Bianco and Taylor (1977)
Bowles et al. (2009)
Brachfeld and Hammer (2006)
Conte et al. (2006)
Donaldson et al. (1975) Faure et al. (2003) Faure and Schiano (2005)
Table 1S (to be deposited as supplementary). Bulk compositions (wt.%) and experimental conditions of basaltic melts from previous solidification experiments ΔT/Δt pre-cooling SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O Cr2O3 total TAS Ti (°C) Tf (°C) (°C/h) phases basaltic 1008÷ 50, 10, liquid + 54.0 0.9 17.9 7.7 0.1 5.9 8.7 3.3 1.0 99.5 1170 andesite 1131 5, 1, 0.5 plg + ol 43.3
2.9
8.8
23.7
0.4
7.0
0.4
0.2
-
0.7
99.1
44.3
3.7
12.2
17.7
0.4
9.4
10.6
0.4
0.1
-
0.6
99.2
51.2
1.6
8.7
18.9
0.5
7.0
8.4
2.3
0.8
0.5
0.1
100.0
51.2
1.6
17.0
8.5
0.5
7.0
10.5
2.3
0.7
0.5
0.1
8.7
19.0
-
7.1
8.5
2.3
0.8
-
50.4
0.9
15.4
7.6
0.2
7.7
15.5
2.3
1.9
0.5
-
54.2
0.8
16.0
7.0
0.1
6.3
9.8
2.9
1.8
0.2
-
100.8
55.5
0.7
15.0
7.4
0.2
6.4
9.7
2.3
1.6
0.2
-
99.9
45.8
2.9
10.4
16.2
0.3
11.7
11.8
51.3
-
14.1
-
-
17.1
17.4
J
r P
99.5
1290 1200
1075 1100
2, 5, 16, 48
liquid and liquid + plg
NNO-5.3
basalt
1205
f o
400÷ 600°C
5.7, 18.7, 72.4, 231
liquid
NNO-0.8 NNO-4.8
1210
285÷ 510
3, 6, 19, 72, 231
liquid
NNO-4.8 NNO-0.8 NNO NNO+4.2
99.9
1200
1175 1150 1125 1100
1, 9000
liquid
NNO
2.7÷1430
liquid
NNO-4.8
1÷ 1890
liquid
air
liquid
air
liquid
air
liquid
NNO-3.8
liquid
NNO+1.2
liquid
-
basalt
basalt
0.3
l a -
-
0.4
99.7
basalt
1300
-
-
-
-
100.0
basalt
1466÷ 1364
1110÷ 1047 1322÷ 986 1318÷ 1166
rn
u o
-
e
1.6
basaltic andesite
16.6
17.4
-
-
-
-
100.0
basalt
1394÷ 1361
-
17.1
17.4
-
-
-
-
100.0
basalt
1394÷ 1364
1082÷ 1226
15.9
0.5
18.3
6.7
0.1
-
b.d.
0.7
100.5
basalt
1435
1115, 909
15.0
8.4
0.2
7.6
10.7
2.6
0.4
0.3
-
100.0
basalt
1198
1162.41÷ 1141
13.6
19.1
0.2
5.3
13.1
0.3
-
-
0.2
100.0
basalt
1225
1210÷ 990
-
14.3
-
-
51.3
-
14.1
-
49.8
0.6
6.0
Gibb (1974)
48.7
1.7
Grove and Bence (1977)
47.1
1.2
NNO-0.8
picro basalt
o r p
100
51.5
51.6
Faure et al. (2007) First and Hammer (2016)
11.9
log fO2
2, 1639÷1730, 1, 225, 525, 991, 1890 2, 1890, 1639÷2182 1, 5, 10, 28, 50 ÷ 1000 10, 5, 2.5, 2, 1, 0.5, 0.25, 0.1, 0.05 0.5, 1.75, 3.75, 10, 30, 60, 150
Journal Pre-proof Table 1S (continued) (to be deposited as supplementary). Bulk compositions (wt.%) and experimental conditions of basaltic melts from previous solidification experiments ΔT/Δt pre-cooling authors SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O Cr2O3 total TAS Ti (°C) Tf (°C) (°C/h) phases 0.5, 1.7, Grove 1010÷ 47.1 0.9 12.6 20.2 0.3 6.3 12.8 0.2 0.2 100.59 basalt 1200 10, 40, liquid (1978) 950 150, 600 Grove and 1204 ÷ 48.4 2.3 12.7 20.2 0.2 3.2 11.7 0.3 0.1 0.1 99.23 basalt 1045 150 liquid Raudsepp, (1978) 1039 Grove and 1207÷ 1164÷ 47.1 0.9 12.6 20.2 0.3 6.3 12.8 0.2 0.2 100.6 basalt 0.48 ÷ 689 liquid Bence (1979) 1196 953 Grove 822, 846, 50.2 1.7 16.0 9.9 7.6 11.2 2.9 0.1 0.2 99.8 basalt 1195 2, 9.5, 110 liquid (1990) 876 Hammer (2006; 2009)
Kennedy et al. (1993)
Kolzenburg et al. (2016)
Kolzenburg et al. (2018)
Lattard and Partzsch, (2001)
o r p
51.4
1.6
9.1
18.9
-
7.3
8.8
2.1
0.8
0.5
-
45.0
0.2
5.8
16.7
0.3
29.4
2.3
0.2
0.4
-
0.6
100.9
54.8
-
3.7
16.2
-
31.9
1.2
0.7
0.1
-
-
98.8
56.9
-
4.0
13.9
0.1
23.1
1.2
-
-
-
rn
55.4
-
6.6
7.1
0.1
30.4
2.6
-
54.8
-
3.7
16.2
-
21.9
1.2
0.7
56.9
-
4.0
13.9
0.1
23.1
1.2
48.9
1.9
13.3
15.0
-
6.8
49.7
1.8
13.8
12.0
0.2
7.0
48.0
47.8
1.9
2.9
17.2
13.9
10.9
12.8
0.1
-
5.0
6.5
e
1300÷ 1150
1438
1425
2
1250
1129÷ 1107
30, 60, 180, 300
liquid
1300
1122÷ 984
30, 60, 180
liquid
air NNO-2.9
1180
1146 1133 1110 1103 1098 1091
1.5, 2, 3
liquid
NNO+0.2 ÷ NNO–3.8
98.8
-
0.1
-
-
99.1
basaltic andesite
11.9
2.3
-
-
-
100.0
basalt
12.4
2.7
0.2
0.2
-
99.8
basalt
2.8
0.4
-
-
98.2
100, 933, 2191 1, 5, 100, 1000, 2000
basaltic andesite
-
10.9
NNO-5.3
1430
99.1
97.3
liquid + nuclei*
basaltic andesite
-
-
liquid
NNO-4.8 NNO-0.8 NNO NNO+4.2
1200
0.1
0.6
NNO-0.8
1620
102.3
2.1
-
basalt
-
3.5
-
1210
basalt
-
10.5
2.8, 5.7, 18.7, 72.4, 231
-
310÷ 401
-
u o
J
l a 0.1
100.4
r P
f o
log fO2
trachybasalt
basalt
air
Journal Pre-proof
authors
Lesher et al. (1999)
Lofgren et al. (2006)
Lofgren and Lainer (1991) Lofgren et al. (1974) Lofgren (1977) Lofgren et al. (1978) Lofgren et al. (1979) Lofgren and Smith (1980)
Table 1S (continued) (to be deposited as supplementary). Bulk compositions (wt.%) and experimental conditions of basaltic melts from previous solidification experiments ΔT/Δt pre-cooling SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O Cr2O3 total TAS Ti (°C) Tf (°C) (°C/h) phases 48.7
2.9
14.0
11.6
-
6.5
11.0
2.8
0.4
-
-
97.9
48.4
3.0
13.9
12.0
-
6.6
11.0
2.8
0.4
-
-
98.3
48.2
3.0
13.6
12.6
-
6.6
10.8
2.8
0.4
-
-
98.4
47.6
2.9
13.5
13.3
-
6.5
10.7
2.8
0.4
-
-
98.1
48.2
2.9
13.8
12.8
-
6.4
10.9
2.6
0.3
-
-
98.3
48.5
1.1
14.0
13.6
0.2
8.0
11.1
2.2
0.1
-
-
98.8
46.2
0.2
3.6
14.2
0.4
30.4
1.8
0.9
0.1
-
0.4
98.1
43.3
2.0
9.3
10.1
0.1
10.7
22.8
-
-
e
0.2
-
0.1
r P
99.7
picro basalt
0.1
0.1
-
0.4
99.3
basalt
0.4
-
0.1
99.5
basalt
0.4
0.2
0.2
0.3
99.9
basalt basalt
2.0
9.3
10.2
0.1
10.5
23.0
48.0
1.8
11.2
18.0
0.3
9.2
10.3
48.8
1.2
20.5
7.7
-
7.8
46.2
1.3
17.6
11.6
0.2
10.3
47.5
1.7
10.1
20.7
0.3
u o
0.6
10.2
0.2
-
-
0.4
99.8
49.1
1.4
16.9
8.1
0.2
9.3
3.4
0.5
0.2
-
100.0
J 8.7 7.2
11.6
-
l a
rn
0.2
98.7
1199÷ 1194
f o
o r p picro basalt
0.2
44.3
12.5
basalt
1265
log fO2
1173÷ 1006
10, 50, 100, 1000, 2000
liquid
1000÷ 1225
5, 250, 100, 1000, 3000
liquid
NNO-3.6
NNO-0.8
1400÷ 1250
1000
5 2÷2100
liquid liquid+nuclei
NNO-4.8÷ NNO-3.6; NNO-2.8; > NNO-0.8; NNO-4.5
1280÷ 1240 1300÷ 1280 1280÷ 1230
1085÷ 1042
1.2÷1260
liquid
NNO-5.8
1000
2, 4.5, 33.6, 5
liquid liquid+plg
935÷ 890
2.6÷1.8
liquid
NNO-5.3
1225
925÷ 980
4÷136
liquid
basalt
1215÷ 1180
1000
0.5, 2
liquid
< NNO-0.8
0.5, 2, 5, 10, 50 5, 100, 2500, 2000
liquid, liquid+ol liquid+ol+plg
NNO-0.6
liquid liquid+opx+ol
NNO-4.8
50.6
1.4
16.6
8.2
-
7.5
9.2
3.2
0.5
0.2
-
100.0
Lofgren (1983)
51.9
1.2
15.9
8.3
0.2
7.0
8.7
3.0
0.8
0.2
0.1
99.5
basalt
1300÷ 1180
1012÷ 899
Lofgren and Russell (1986)
55.2
0.1
2.8
9.6
0.3
28.9
1.5
0.7
0.1
-
0.6
99.7
basaltic andesite
1525÷ 1425
1225÷ 1125
Journal Pre-proof authors
Lofgren (1989)
Mollo et al. (2012) Mollo et al. (2013a, 2013c) Mollo et al. (2013b) Nabelek et al. (1978) Ni et al. (2014) Pupier et al. (2008) Sato (1995) Schiffman and Lofgren (1982) Sossi and Neill (2016) Toplis et al. (1994)
Toplis and Carroll (1995)
Table 1S (continued) (to be deposited as supplementary). Bulk compositions (wt.%) and experimental conditions of basaltic melts from previous solidification experiments ΔT/Δt pre-cooling SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O Cr2O3 total TAS Ti (°C) Tf(°C) (°C/h) phases 56.5
-
4.5
8.0
-
27.6
1.9
1.0
0.1
-
-
99.7
53.3
0.1
1.9
17.5
0.3
24.2
1.2
0.5
0.1
-
0.3
99.4
1600 1585 1570 1550
1300÷ 975
5, 100, 1850
liquid
NNO-4.8
1250
1100
2.5, 10, 50, 150
liquid
NNO-0.8; NNO+1.5
trachybasalt
1250
1100
2.5, 10, 50
liquid
NNO-0.8; NNO+1.5
basalt
1250
1000
30, 126,180, 564, 900
liquid
NNO+1.5
99.5
basalt
1320÷ 1265
994÷ 1001
10, 2
liquid liquid + plg
NNO-5.1
-
98.6
basalt
1240
600
50÷2000
liquid
-
-
97.2
basalt
1291÷ 1186
1179÷ 1109 1100÷ 1050 1000÷ 850 1434.5÷ 1350
1, 0.2, 3
liquid
NNO-0.8
1800÷10
liquid
NNO-0.8
2400÷0.5
liquid
NNO-0.8
30, 150, 90, 390, 360
liquid
NNO-2.4
basalt 54.8
-
3.7
16.2
-
21.9
1.2
0.7
0.1
-
-
99.0
46.9
-
5.7
12.5
0.1
28.9
2.5
0.9
0.1
-
-
97.6
42.0
2.6
9.8
10.0
0.1
11.2
22.9
n.d.
-
0.1
0.2
98.9
54.5
0.7
17.2
6.1
0.2
7.1
11.3
1.9
0.9
0.1
-
100.1
49.7
1.4
15.3
8.6
0.2
7.0
11.5
3.8
1.4
0.4
-
99.5
47.6
1.7
16.8
10.6
0.2
6.5
11.1
3.3
1.8
0.5
-
48.8
1.2
20.5
7.7
n.d.
7.9
12.5
0.6
0.4
-
0.1
49.4
0.8
15.8
7.7
0.2
8.0
12.7
2.3
48.8
-
14.9
13.1
-
6.5
10.9
2.7
52.6
1.3
14.5
12.0
0.2
4.6
10.1
53.6
1.9
13.7
9.3
0.2
4.5
45.2
0.2
3.7
10.9
0.2
49.5
4.3
11.5
14.6
-
48.8
2.9
14.9
13.1
49.5
4.3
11.5
48.7
4.3
11.5
l a
log fO2
r P
f o
o r p
e 100.0
basaltic andesite
1.9
-
0.4
-
-
97.7
8.1
rn
0.3
3.0
1.1
0.3
-
98.7
32.2
5.3
0.4
0.2
-
0.5
100.3
basalt
1500
4.8
10.0
2.9
0.5
-
-
98.1
basalt
1130
1072
3
liquid
NNO+1.2 NNO-2.8
-
6.5
10.9
2.7
0.3
-
-
100.1
14.6
-
4.8
10.0
2.9
0.5
-
-
98.1
basalt
1300
1158÷ 1050
3, 5, 10, 20
liquid
16.2
-
4.6
9.7
3.0
0.5
-
-
98.5
NNO-0.8, NNO+0.2, NNO-1.8, NNO-2.8
u o
J
2.0
basaltic andesite basaltic andesite
1200 1140
Journal Pre-proof Table 1S (continued) (to be deposited as supplementary). Bulk compositions (wt.%) and experimental conditions of basaltic melts from previous solidification experiments authors
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O
Cr2O3
total
TAS
Ti(°C)
Tf(°C)
ΔT/Δt (°C/h)
pre-cooling phases
log fO2
Usselman et al. (1975)
42.3
10.6
8.8
17.6
0.2
7.9
10.8
0.9
0.3
-
0.2
99.5
picro basalt
1250
1125÷ 920
2÷210
liquid
NNO-5.1 ÷ NNO6.2
48.0
1.0
15.6
10.2
0.2
9.4
13.2
1.8
-
0.1
-
100.7
basalt 1300
800
1, 7, 60, 180, 1800, 9000
liquid
air
1187÷ 1157
1162÷ 1101
0.16÷ 100.44
liquid
-
1162÷ 1059 1080÷ 600
0.5÷ 2000
liquid
-
NNO-5.3
NNO-5.8
This study (B100) Vetere et al. (2013, 2015) Vona and Romano (2013) Walker et al. (1976)
Walker et al. (1978)
Zieg and Lofgren (2006)
53.0
0.8
15.0
9.5
0.2
7.6
10.8
2.2
1.0
-
-
100.0
45.8
0.4
8.4
12. 2
0.2
23.0
8.7
0.9
0.2
-
0.5
100.3
f o
basaltic andesite
o r p basalt
50.4
0.9
17.2
1.7
0.2
6.3
11.6
2.3
1.9
0.5
-
99.2
48.2
1.7
17.1
2.5
0.2
5.4
10.3
3.7
1.9
0.4
-
99.4
1182÷ 1131
43.8
2.8
8.0
22.0
0.3
13.5
8.2
0.2
-
-
0.7
99.4
picro basalt
1350
43.7
2.9
8.2
21.8
0.4
12.5
8.4
0.1
-
-
0.8
98.7
43.1
2.4
8.8
22.6
-
9.9
9.3
-
-
-
-
e
trachybasalt
96.2
44.0
3.3
9.6
21.3
-
8.7
10.0
-
-
-
96.9
46.0
3.5
10.2
19.5
0.3
7.7
10.4
-
0.6
98.4
46.0
3.8
11.1
18.8
0.3
45.6
3.8
11.4
19.3
0.3
45.6
4.4
11.1
19.6
0.3
44.9
4.7
10.8
19.8
0.4
49.2
1.0
12.2
18.1
49.8
0.1
7.2
14.4
7.0
rn
l a
11.2
u o
0.2 -
-
r P -
0.5
picro basalt
98.7 basalt
6.4
11.2
0.1
-
-
0.4
98.5
6.2
11.6
0.1
-
-
0.4
99.4
6.0
11.5
0.2
-
-
0.4
98.7
picro basalt
0.5
7
10.6
0.5
0.1
0.1
-
99.8
basalt
0.1
24.5
3.2
1.1
0.1
-
-
100.5
basalt
J
1200÷ 1149
1159÷ 806
0.5÷2000
liquid liquid+px +plg+sil+sp +ilm+sulf
1545
1515÷ 1176
92
liquid+ol
Footnotes: plg = plagioclase, ol = olivine, px = pyroxene, opx = orthopyroxene, sil = silica, sp = spinel, ilm = ilmenite,sulf = sulphide; Ti is the initial temperature of cooling (regardless of the heating treatment adopted) and Tf is the quenching temperature. When the temperatures and/or the cooling rates are not clearly explained in the paper, we have used the symbol “”. All the cooling rates have been converted in °C/h, log fO2 is expressed as a function of NNO buffer and the total amount of Fe is reported as FeO. The compositions are plotted in the TAS diagram according to La Maitre (2000), with the relative thermal paths (Fig. 1). The starting material B100 has density of 2.76 g/cm3, the liquidus temperature (Tm) is 1233°C, the glass transition (at 1012 Pa/s) occurs at Tg of 651°C and Trg is 0.53 (Vetere et al. 2015).
Journal Pre-proof 3. Cooling of basaltic solidifying bodies with variable thicknesses Previous cooling rate studies on basaltic lava flows/lakes and domes quantified only the shallower thermal conditions of the cooling bodies and, thus, they principally refer to the heat released at the outermost surface (Flynn and Mouginis-Mark, 1992; Cashman, 1993; Hon et al., 1994; Cashman et al., 1999; Witter and Herris, 2007; Kolzenburg et al., 2016). On the other hand, 2D and 3D numerical models can properly simulate the heat released from a cooling basaltic body (e.g., a lava flow) once the melt composition, lava thickness/volume and the temperature of the
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surrounding medium(s) are quantified (Brandeis et al., 1984; Brandeis and Juapart, 1987; Neri,
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1998; Wholetz et al., 1999; Patrick et al., 2004, 2005; Kattenjorn and Schaefer, 2008; Whittington
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et al., 2009; Nabelek et al., 2012; Blundy and Annen, 2016). We also stress that this approach has
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been already applied to reproduce the cooling history of basaltic lava flows (Neri, 1998; Patrick et al., 2004, 2005).
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Following this line of reasoning, we performed 2D simulations by investigating basaltic melt
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slices with variable thickness of 0.1, 0.5, 1 and 2 m, by assuming an infinite extension of the other two dimensions. According to the output rate and terrain slope constraints in nature, the selected
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thicknesses are reliable for aphyric tholeiites erupted on Earth from superliquidus to liquidus conditions (Griffiths, 2000). The numerical procedure follows the modelling approach already presented in Vetere et al. (2015). The simulated basaltic liquid layers are sandwiched between an overlying layer of air and an underlying rock substrate with temperature of 20 and 25 °C, respectively. The models start from an initial homogeneous temperature of the liquid of 1300°C. Each point within a silicate liquid slice cools from the initial temperature following a parabolic trend with ΔT/Δt that progressively decreases with T (Fig. 2). Notwithstanding, the simulated ΔT/Δt are approximated to be linear within the temperature interval 1300 °C - 600 °C. The heat release was simulated using the finite elements code implemented in the Heat 3D software (Wohletz et al., 1999), and the selected initial parameters, such as thermal conductivity (K), density (ρ) and specific heat (cp), are listed in Table 2S.
Journal Pre-proof This simplified approach accounts only for heat conduction, by ignoring any mechanism of irradiation and convection, as well as the latent heat of crystallization. Therefore, results from simulations and modeled ΔT/Δt (Fig. 2) are conservative with respect to natural solidifying melts, corroborating the numerical cooling dynamics reported by previous authors (Neri, 1998; Patrick et al., 2004, 2005; Petitjean et al., 2006). It is also important to note that cooling rates of lavas erupted beneath the surface of seawater are even larger than those simulated at air conditions (Fink and Griffiths 1990; Griffiths and Fink, 1992; Gregg, 2017 and references therein).
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Results of the thermal models shows that a basaltic slice of 0.1 m cools with rates in the order
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of 103 and 102 °C/h in the outer and inner portions, respectively (Fig. 2). In a basaltic layer with a
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thickness of 0.5 m, the cooling rates progressively decrease from 103 to ~101 °C/h (Fig. 2). For a
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layer of 1 m, only the outermost millimetric-to-centimetric lava crust cools from 1300 °C with ΔT/Δt between 101 and 103 °C/h, whereas the innermost portion undergoes cooling rates of 101 <
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ΔT/Δt < 100 °C/h (Fig. 2). The model for a lava with thickness of 2 m yields ΔT/Δt of ~101 °C/h in
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the outermost part, whereas the innermost part cools at rates of 100 °C/h or even lower (Fig. 2). These data indicate that most of the dynamic solidification experiments adopting high to moderate
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ΔT/Δt (Tab. 1S) can be considered representative of the crystallization conditions only for the outermost portions of basaltic liquid slices with thicknesses larger than 0.5 m or for those with thicknesses lower than ~ 0.5 m (Figs. 1 and 2).
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Fig. 2. 2D numerical simulations of conductive cooling of virtual basaltic liquid layers with different thicknesses. Data were obtained through the Heat 3D code (Wholetz et al., 1999). The layers were cooled from 1300 to 600 °C, keeping the overlying air at 20 °C and the initial temperature of the underlying rock substrate at 25 °C. The thermodynamic parameters used for the simulations are listed in Table 2S.
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Table 2S (to be deposited as supplementary) Physical parameters employed for cooling simulations overlying silicate liquid underlying parameter unit air layer rock 0.02 0.9 2.1 thermal conductivity K (W/mK) 1.1 2750 2600 density ρ (Kg/m3) 1004 1300 1019 specific heat cp (J/K kg) Ti °C 20 1300 25 Tf °C by simulation 600 by simulation
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Footnotes: boundary conditions for 2D numerical model of basaltic lava flows.The thickness mesh has size of 100 x 100, corresponding to a total area (m2) of 10 x 10 m (grid size = 0.1),with exception for the thickness of 0.1 m where the total area is 1 m x 1 m (grid size = 0.01).
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4. Experiments and analytical methods
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The bulk composition (B100) and experimental conditions reappraised in this study are those reported in Vetere et al., (2013, 2015). The starting glass was prepared from the tholeiitic basalt
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cropping out in Iceland and named BIR 1A as USGS standard. The natural powdered rock was
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melted in air at temperature of 1600 °C for 4 h. After a rapid quench, the obtained glass was crushed and re-melted at the same condition to assure homogeneity. The bulk composition of B100
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measured by microprobe is reported in Table 1S and plotted in Fig. 1. Using FTIR spectroscopy and
determined.
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a modified Wilson's method, the residual H2O content (~ 50 ppm) and Fe2+/Fetot ratio (0.39) were
For each experiment, ~50 mg of the B100 starting glass fragments were loaded into Pt tubes successively welded shut. The Pt-charges were heated with a rate of 420 °C/h from ambient temperature to 1300 °C, then kept for 2 h at this T to homogenize the melt and remove potential heterogeneous sites of nucleation (Iezzi et al., 2008; Vetere et al., 2013, 2015). This temperature is 67 °C above the melting (liquidus) temperature Tm of 1233 °C. Then, the experiments were quenched at 800 °C close to the solidus temperature. The imposed linear cooling rates were 1, 7, 60, 180, 1800 and 9000 °C/h (Fig. 3S) at 1 atm and fO2 of air (Table 3S), thus approaching to the solidification conditions on Earth and beneath the surface of seawater. A further experiment was performed with the same heating rate and dwell treatment, but it was quenched directly in water
Journal Pre-proof providing a cooling rate ≥ 105 °C/h (E-quench in Table 3S). This run-product allow us to investigate the chemical features before the onset of cooling (Table 3S). The experiments are labeled with the value of the applied cooling rate: E9000, E1800, E180, E60, E7 and E1; experiments with cooling rates of 1 and 180 °C/h were duplicated to verify their reproducibility by Vetere et al. (2013, 2015). One of the two run products at 180 °C/h (E180a) was also subjected to a different heat treatment before cooling below 1300 °C. This charge was heated up to 1400 °C, cooled to 1300°C after 0.5 h, and maintained at this temperature for a dwell time of 40 h before cooling at 180 °C/h (Table 3S
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and Fig. 3S). The total cooling time of the experiments (Table 3S) ranges from hours to three weeks
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(Fig. 3S).
Fig. 3S (to be deposited as supplementary). Thermal paths of solidification experiments conducted on B100. Redrawn from Vetere et al. (2015).
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Table 3S (to be deposited as supplementary). Experimental conditions and phase proportions (area%) determined by image analysis. exp.
heating rate (°C/h)
Ti (°C)
dwell time (h)
ΔT/Δt (°C/h)
Tf (°C)
experimental time (h)
selected BS-SEM photos for image analysis
magnifications
glass
plg
cpx
sp
crystals
1.9 (0.0)
65.6 (6.6) 68.5 (7.7) 60.7 (4.3) 46.1 (2.3)
30.6 (5.6) 28.8 (7.9) 36.0 (4.5) 47.0 (3.2) 55.6 (3.5) 50.6 (1.5) 51.0 (1.3)
3.4 (0.9) 2.8 (0.3) 3.3 (0.4) 4.5 (0.6) 4.5 (1.3) 4.6 (0.8) 1.2 (0.0) 1.8 (0.8)
99.6 (0.8) 100.0 (0.0) 100.0 (0.0) 97.6 (0.8) 60.1 (2.6) 55.3 (1.1) 52.2 (1.3) 1.8 (0.8)
-
-
E1b
1
510.7
5
150 ÷ 400x
E1a
1
510.7
3
150x ÷ 400x
E7
7
72.6
6
150x ÷ 400x
E60
60
13.4
5
1300
E180a
420
1400°C for 0.5h 1300°C for 40h
800 180
47.8
E180b
180
8.8
E1800
1800
5.3
1300
rn
2
E9000
9000
E-quench
≥ 105
u o
1300
J
l a
5.1 3.2
e
r P 3 3
f o
o r p
2
400 ÷ 800x
400 ÷ 800x 400 ÷ 800x
2
800 ÷1500x
2
800 ÷1500x
-
-
-
2.4 (0.8) 39.9 (2.6) 44.7 (1.1) 47.9 (1.3) 98.3 (0.7) 100
-
-
-
-
textural features
faceted
dendritic
glassy
Footnotes: Data in bold are those relative to the most reliable run products that were selected according to the criteria reported in Vetere et al. (2013, 2015).
Journal Pre-proof 5. Analytical methods and textural quantification A more comprehensive description of the analytical methods is reported in Vetere et al. (2013, 2015). In summary, the recovered run products were mounted in epoxy, ground flat and polished. Micro-chemical characterization was carried out with an electron probe micro-analyzer (EPMAWDS) and field-emission scanning electron microscopy (SEM-EDS), while the textural characteristics were obtained by back-scattered (BS) SEM microphotographs. Following previous textural measurements (Higgins, 2006; Iezzi et al., 2008, 2011, 2014;
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Lanzafame et al., 2013, 2017; Vetere et al., 2013, 2015), we collected 11 to 21 BS-SEM images for
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each run product, corresponding to a total of 115 BS-SEM microphotographs with magnifications
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of 150×, 400×, 800×, 1500× and 3000×. In addition, some run-products (E1b and E1a) were
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characterized by BS-SEM semi-quantitative maps with 30 and 10 images for a total area of 0.4 and 0.1 mm2, respectively. The most representative microphotographs were selected for quantitative
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textural analysis at three different magnifications (Table 3S) to obtain reliable results on both large
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and small crystals. The image analysis was performed on 30 BSE microphotographs using the Image-ProPlus software (http://www.mediacy.com/imageproplus). Each BS-SEM microphotograph
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was first digitalized, calibrated and converted into grey scale. We did not apply filters nor stereological correction. Crystalline and non-crystalline phases were segmented as a function of their grey level range and then binarized with different false colors. An example of image treatment is reported in Fig. 4. Following Higgins (2006) and Iezzi et al. (2011), each crystal is described by a best-fitting equal-area ellipse. Dendritic phases were considered as individual crystals (Hammer et al., 2010; Shea and Hammer, 2013). Touching crystals and dendrites with different orientations were manually identified and separated. We determined for each measured crystalline phase the phase abundance (area%), area (μm2), perimeter (μm), length and width (μm) as major and minor axes of the equal-area ellipse, and the aspect ratio (major axis/minor axis). The number of crystals per area (#/A) was also computed. Starting from the 2D crystal size habit, the 3D crystal habit was calculated though the CSD-slice
Journal Pre-proof program of Morgan and Jerram (2006). The computed 3D shapes and size dimensions were used as input for the CSD calculations implemented in the CSD-Corrections 1.53 package (Higgins, 2000). Each CSD curve was constructed using 2 to 4 BS-SEM microphotographs and 7 bins per decade (Higgins 2000, 2006; Armienti 2008). Run-products with crystal content < 5 area% were not considered for measurements of 2D size, shape, aspect ratio and CSD plot. All textural data are
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reported in the excel spreadsheet as supplementary material.
Fig. 4. Example of image analysis on two representative run-products with faceted (E60) and dendritic (E1800) textural features. The false colors blue, red, green and yellow correspond to plg, cpx, sp and glass, respectively. The blue bar corresponds to10µm.
6. Results The main crystalline phases solidified from B100 are plg, cpx and sp (Fig. 5). Run-products cooled at the same rate, E1a-E1b and E180a-E180b (experiencing variable heat-treatments), show reproducible textures for all the crystalline phases (Fig. 5 and Table 3S). The preferential crystallization on the walls of Pt capsules involves is extremely limited in space without altering the overall textural features of the run-products (Fig. 5). This agrees with results from Mollo et al.
Journal Pre-proof (2012), which evidence that at high fO2 conditions, the effect of Pt on the crystal nucleation is negligible. The most reliable run-product between two duplicate sample charges cooled at the same ΔT/Δt has been considered the one with less crystals (Vetere et al., 2015), i.e. E1b and E180b (Tab. 3S). In the following, only these two run-products are considered together with E7, E60, E1800 and E9000. The quantitative evolution of crystal content is shown in Fig. 6 and listed in Table 3S. The total crystal content decreases as ΔT/Δt increases without a monotonic trend. The run products are
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fully crystallized at ΔT/Δt ≤ 60 °C/h. Conversely, at ΔT/Δt ≥ 60 °C/h, a marked decrease of total
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crystal content occurs. The trend flattens at 180 and 1800 °C/h with a further decrease to ~ 2 area%
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when approaching 9000 °C/h (Fig. 6). The run-products cooled at rates of 1, 7 and 60 °C/h show
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prevailing faceted crystals, while those produced at rates of 180 to 9000 °C/h are dendritic with fern leaf shapes (Fig. 5). The formation of sp and cpx takes place from 1 to 9000 °C/h, while plg occurs
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only at ΔT/Δt ≤ 60 °C/h. At low cooling rates, cpx and plg exhibit an extensive intergrowth feature
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(Fig. 5). A very low amount (≤ 1 area%) of melilite, with the typical H-shaped habit, also occurs at the intermediate cooling rates of experiments E180a-E180b (Fig. 5). Because of its paucity, melilite
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is not considered in the textural analysis.
The plg content linearly and slightly decreases from 66 to 46 area% as ΔT/Δt varies from 1 to 60 °C/h, while the cpx content progressively increases from 31 to 51 area% as ΔT/Δt varies from 1 to 1800 °C/h and then cpx is barely detectable at 9000 °C/h (Fig. 6 and Table 3S). The content of sp is always ≤ 5 area%. The glass increases from 0 to 98 area% as ΔT/Δt varies from 7 to 9000 °C/h (Fig. 6, Table 3S). The matrix glass (i.e. at ≥ 50 µm from crystal rims) is present only in E9000, whereas intra-crystalline glass (i.e. at < 50 µm from crystal rims) occurs in E1800 and E180 (Fig. 5).
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Fig. 5. BS-SEM microphotographs of the run-products from Vetere et al. (2013, 2015); run products in bold are the more reliable ones (Table 3S). The magnification per run-product increases from left to right. From black to white grey levels the phases are glass (black), plg, cpx and sp. The blue bars correspond to 10 μm and the red ones to 100 μm. Some of these BSE microphotographs have been already shown in Vetere et al. (2015) and are re-plotted to have a complete picture of the sample evolution.
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Fig. 6. Crystal content of run-products (area%) as a function of cooling rate (°C/h). The 2D size of plg changes from 101-103 to 100-102 µm at cooling rates from 1 to 60 °C/h, respectively (Fig. 7). The maximum 2D size decreases from 949 to 109 µm, while the average size (calculated as the whole range of crystal size) decreases from 70 to 10 µm (Table 4S). The 2D size of cpx is in the range of 101-102 µm at 1 and 7 °C/h, 100-102 µm at 60 and 180 °C/h, and 100-101 µm at 1800 °C/h (Fig. 7), with maximum and average sizes progressively decreasing from 398 to 25 µm and from 36 to 4 µm, respectively (Table 4S). The 2D size of sp is in the range of 100-102 µm at 1 and 7 °C/h, 100-101 µm at 60 °C/h, 100-102 µm at 180 °C/h, and 100-101 µm at 1800 °C/h. The
Journal Pre-proof maximum 2D size of sp changes from 89 to 6 µm and the average size varies from 28 to 0.8 µm as
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ΔT/Δt increases from 1 to 9000 °C/h (Table 4S).
Fig. 7. 2D sizes of major and minor axes from the equal-area ellipse (see data in Table 4S). Only linear regression analyses with R2 ≥ 0.6 and run-products with a crystal content ≥ 5 area% were considered.
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Table 4S (to be deposited as supplementary). Maximum (Lmax), minimum (Lmin) and average (Lav) 2D crystals size (µm). plg cpx sp ΔT/Δt crystal exp. (°C/h) Lmax Lmin Lav Lmax Lmin Lav Lmax Lmin Lav shape E1b
1
949
10
70
398
8
36
89
6
28
E7
7
751
5
41
264
5
21
76
4
13
E60
60
109
1
10
104
1
7
20
1
4
E180b
180
-
-
-
104
2
13
65
1
5
E1800
1800
-
-
-
25
0.5
4
6
0.5
2
E9000*
9000
-
-
-
-
-
-
7
0.2
0.8
faceted
dendritic
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Footnote: cpx and sp from E900 were considered together due to their very small size.
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The relative crystal size variability of plg, cpx and sp is displayed in Fig. 8. For each runproduct, the frequency of the long size was calculated using 100 bins with dimension of 10, 4 and 1
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μm, in the maximum range of 0-1000, 0-400 and 0-100 μm for plg, cpx and sp, respectively. With
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increasing ΔT/Δt, the size frequency of plg, cpx and sp generally shifts from large to small crystal
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dimensions. The decreasing number of large sizes is counterbalanced by an increase of tiny crystals. As the cooling rate increases from 1 to 60 °C/h, the plg major axis is characterized by: at 1 and 7 °C/h, there is a major peak at ~ 20 μm; a few percent of crystals have size < 10
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μm and more than 35 % of crystals have size ≥ 25 μm; at 60 °C/h, an exponential decrease in size starts from the major peak at 5 μm and is followed by a few percent of crystals with size > 10 and < 50 μm. Note also the lack of plg crystals with size > 50 μm.
As ΔT/Δt increases from 1 to 1800 °C/h, the size of cpx changes as follow:
at 1 and 7 °C/h there is an overall absence of crystals with size < 5 μm, with major peaks at ~ 10 μm and a significant amount of crystals with size between 10 and 50 μm;
at 60 and 180 °C/h, the two cpx populations have size > 2.5 and ≤ 10 μm, respectively;
at 1800 °C/h, ~ 75 % of dendritic crystals show size of ~ 2.5 μm.
The textural change of sp can be described as:
at 1 °C/h, an almost constant distribution of size between 5 and 50 μm;
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at 7 °C/h, the crystal population peak is between 5 and 10 μm;
at 60 and 180 °C/h, the crystal population peak corresponds to 2.5 μm and the maximum size is ~ 5-7 μm; at 1800 °C/h, most crystals are equal to or lower than 2.5 μm and they do not exceed 5 μm
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Fig. 8. Relative variation (%) of the major axis for the equal-area ellipse of plg, cpx and sp (Table 4S). 100 classes of 10, 4 and 1 μm were considered for plg, cpx and sp, respectively. The plot refers to classes with frequency > 2 % and run-products with crystal content ≥ 5 area%.
The 2D crystal habit, computed as aspect ratio between major and minor axes of equal area ellipses (Higgins, 1994), displays weak trends as a function of ΔT/Δt for plg and cpx (Fig. 9). The maximum range of 2D crystal habit varies from 1 to 7 for all the crystalline phases. Plg shows a low
Journal Pre-proof variability of the 2D habit, with the majority of crystals with values between 1 and 3 at 1, 7 and 60 °C/h (Fig. 9). For faceted cpx crystals, the 2D habit becomes less equant as the cooling rate increases from 1 to 60 °C/h. Conversely, dendritic cpx crystals obtained at 180 and 1800 °C/h show prevalent equant shapes (Fig. 9). Faceted sp has a 2D habit more equant from 1 to 60 °C/h, while dendritic sp at 180 and 1800 °C/h shows a significant number of crystals with high 2D ratio;
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overall, sp crystals become less equant as the cooling rate increases (Fig. 9).
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Fig. 9. Relative variation (%) of 2D aspect ratio for plg, cpx and sp. We considered 40 classes with 2D aspect ratio from 1:1 to 1:40. The plot refers to classes with frequency > 2 % and run-products with crystal content ≥ 5 area%. The number of crystals per area (#/A) of each crystalline phase is reported in Table 5 and plotted in Fig. 10. The value of #/A for plg increases from 10-4 to 10-2 µm-2 as the cooling rate increases from 1 to 60 °C/h. Faceted cpx crystals show increase of #/A from 10-4 to ~ 10-2 µm-2 as
Journal Pre-proof the cooling rate increases from 1 to 60 °C/h. At higher cooling rates, #/A significantly decreases to an intermediate value between 10-3 and 10-2 µm-2. Then, #/A of cpx increases up to 10-1 µm-2 at ΔT/Δt of 180 and 1800 °C/h (Fig. 10 and Table 5). The value of #/A for sp increases from above 105
to ~ 10-1 µm-2 as the cooling rate increases from 1 to 1800 °C/h, with an almost constant value at
ΔT/Δt from 60 (faceted) to 180 °C/h (dendritic) (Fig. 10 and Table 5). In summary, the number of faceted crystals per area increases of about two orders of magnitude as the cooling rate increases from 1 to 60 °C/h, according to the sequence cpx > plg > sp. At ΔT/Δt of 60 and 180 °C/h, the
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number of crystals per area is nearly constant or decreases when faceted shapes are substituted by
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dendrites and the crystallization of plg is suppressed. As the cooling rate increases, the value of #/A
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recovered at 180 and 1800 °C/h is similar for cpx and sp, while the crystal abundance (area%)
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significantly decreases (Figs. 6 and 10, Tabs. 3S and 5).
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Fig. 10. Variation of the number of crystals per area as a function of cooling rate. The plot refers to run products with a crystal content ≥ 5 area% (see data in Table 5).
exp.
Table 5: number of crystals (#) per area (µm-2); standard deviations are reported in parenthesis. ΔT/Δt crystal plg cpx sp (°C/h) shape
E1b
1
1*10-4 (4*10-5)
2*10-4 (1*10-4)
5*10-5 (3*10-5)
E7
7
3*10-4 (2*10-4)
1*10-3 (1*10-3)
2*10-4 (1*10-4)
E60
60
1*10-2 (1*10-2)
3*10-2 (2*10-2)
4*10-3 (2*10-3)
E180b
180
-
4*10-3 (1*10-3)
4*10-3 (2*10-3)
E1800
1800
-
1*10-1 (4*10-2)
5*10-2 (2*10-2)
E9000*
9000
-
faceted
dendritic
5*10-3 (1*10-3)
Footnote: cpx and sp from E9000 were considered together due to their very small size.
Journal Pre-proof 7. Discussion 7.1 MORB textural similarities between experimental and natural products The qualitative effects of cooling conditions on texture, composition and mineralogy of solid phases growing in basaltic lava flows/lakes, pillow lavas and dikes have been recognized since long time (Lofgren, 1980; Kirckpatrick, 1981; Cashman, 1989; Lasaga, 1998; Hammer, 2008; Zhang, 2008; Iezzi et al., 2009; Mollo and Hammer, 2017). The outermost and rapidly cooled portions of lavas are typically glassy and host tiny and dendritic minerals, while the innermost parts are
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characterized by abundant crystalline phases with faceted shapes and larger size (Muir and Tilley,
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1966; Bryan, 1972; Mazullo and Bence, 1976; Coish and Taylor, 1979; Schiffman and Lofgren,
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1982; Batiza and Vanko, 1984). The evolution of textures displayed in Fig. 5 mirrors those reported
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in these studies.
Two studies analyzed in detail the textures of natural MORBs. Lesher et al. (1999)
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investigated a tholeiitic drilling section composed of a series of aphyric and poorly vesiculated
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MORB flow units with thickness between 0.1 and 10 m. The top of these flows is characterized by dendritic cpx intergrowth with plg forming bands of tiny crystals. The innermost portions of the
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meter-thick flows were composed by mm-sized plg intergrown with cpx, with a typical intersertal texture (see Figs. 5a and 5b of Lesher et al., 1999). These textures of natural MORBs are very similar to our experimental runs (i.e., E7, E1a and E1b) at the lowest cooling rates (Fig. 5). In addition, Lesher et al. (1999) complemented their investigations with equilibrium plus kinetic cooling experiments adopting rates from 10 to 2000 °C/h and ΔT from 20 to 190 °C (see Table 3 in Lesher et al., 1999). An increase of ΔT/Δt resulted in the reduction of their plg size while faceted crystals formed only at cooling rates < 50 °C/h, thus resembling the textual features showed in Figs. 5, 6, 7 and 11. On the other hand, Zhou et al. (2000) provided very accurate investigations at extremely high cooling conditions to reproduce textural features in the outermost portions of a MORB-type pillow lava. The authors estimated ΔT/Δt in the order of 102 to 103 °C/h, which are values similar or higher
Journal Pre-proof than those experimentally discussed in this study (Figs. 2 and 3S). Zhou et al. (2000) reported that the crystal phases within 0.3 cm of the pillow rim (i.e., glassy margin) mainly consists of plg and cpx, with minor proportions of small sp grains. Conversely, within the crusty, glassy patches outside the pillow rim, only tiny titanomagnetite grains develop with a maximum size of a few tens of nanometers. The glassy crust above the pillow rim consists of massive basaltic glass resulting from an extremely rapid cooling rate. It is worth stressing that these textural features match with those observed in the experiments solidified at 1800 and 9000 °C/h (E9000 showed in Fig. 5), thus
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validating the overall textural parameters obtained in laboratory at very fast cooling conditions.
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Overall, the similarities between run-products reported in Fig. 5 with natural MORB rocks
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investigated by Zhou et al. (2000) and Lesher et al. (1999) at high and sluggish cooling conditions,
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7.2. Crystal size and 3D shape
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respectively, corroborate the quantitative evolution of textural attributes discussed below.
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The number (expressed in percentage) of plg, cpx and sp crystals has been divided into three classes as a function of the 2D crystal size (Fig. 11 and Table 6): long (> 100μm), medium (100-10
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μm) and short (< 10 μm) classes. At 1 and 7 °C/h, ~ 10 %, ~ 80-90 % and 0-10 % of plg crystals show long, medium and short sizes, respectively. At 60 °C/h, ~ 30 % of plg crystals have medium size, whereas ~70 % of plg crystals have short size (Fig. 11 and Table 6). At 1 and 7 °C/h, ~ 6-1 %, ~ 86-68 % and 8-31 % of cpx crystals show long, medium and short sizes, respectively (Fig. 11 and Table 6). At higher cooling rates, cpx crystals have percentages of 17, 37 and 5 % (medium size) and 83, 63 and 95 % (short size) at 60, 180 and 1800 °C/h, respectively (Fig. 11 and Table 6). The suppression of plg at ΔT/Δt between 60 and 180 °C/h is accompanied by the formation of a large amount of cpx dendrites with medium size at 180 °C/h, whilst almost faceted cpx crystals are present at 60 °C/h (Fig. 11). Sp crystals have size < 100 µm (i.e., only medium and/or short classes). The amount of sp with medium size decreases from 91 to 8 % as the cooling rate increases from 1 to 180 °C/h (Fig. 11 and Table 6). This trend is counterbalanced by the increasing percentage of sp
Journal Pre-proof crystals with short sizes, varying from 9 to 100 % with increasing cooling rate (Fig. 11 and Table 6). The evolution of sizes as a function of ΔT/Δt reported in Fig. 11 enlarges the previous literature data. These data cover the variable solidification conditions suffered inside MORB liquid sheets of
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different thicknesses (Fig. 2).
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Fig. 11. Size frequency as a function of cooling rate (see data in Table 4S and 6).
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Table 6. 2D size frequency and 3D aspect ratio computed by CSDslice (Morgan and Jerram, 2006)
E180b
180
1800
aspect
R2
sp
aspect
R2
crystals (%)
aspect
R2
>100
12.7
0.2:0.8:1
0.5
6.2
0.1:0.3:1
0.8
-
-
-
100÷10
87.3
0.2:0.3:1
0.9
86.3
0.3:0.5:1
0.8
90.7
0.4:0.5:1
0.6
<10
-
-
-
7.5
0.5:0.9:1
0.9
9.3
0.6:0.9:1
0.7
100
0.1:0.2:1
0.6
100
0.3:0.6:1
0.9
100
0.1:0.2:1
0.6
9.6
1:1:1
0.4
1.2
0.3:0.6:1
0.6
-
-
-
100÷10
79.8
0.1:0.2:1
0.9
67.9
0.3:0.7:1
0.8
52.3
0.5:0.7:1
0.9
<10
10.6
0.4:0.6:1
0.9
30.9
0.4:0.7:1
0.9
47.7
0.5:0.6:1
0.9
all
100
0.1:0.2:1
0.9
100
0.2:0.4:1
0.9
100
0.5:0.7:1
0.9
>100
-
0.1:0.9:1
0.4
-
-
-
-
-
100÷10
31.2
0.2:0.5:1
0.8
17.1
0.1:0.3:1
0.7
6.4
0.7:0.8:1
0.7
<10
68.7
0.3:0.6:1
0.9
82.9
0.3:0.6:1
0.8
93.6
0.6:0.7:1
0.8
all
100
0.2:0.4:1
0.9
100
0.2:0.5:1
0.8
100
0.6:0.7:1
0.8
of
all >100
-
>100
-
-
-
-
-
-
-
-
-
100÷10
-
-
-
36.9
0.3:0.8:1
0.8
8.3
0.10:0.6:1
0.8
<10
-
-
-
63
0.4:0.7:1
0.8
91.7
0.3:0.4:1
0.8
all
-
-
-
>100
-
-
-
100
0.4:0.8:1
0.8
100
0.2:0.4:1
0.8
-
-
-
-
-
-
100÷10
-
-
-
5.3
0.3:0.6:1
0.8
-
-
-
<10
-
-
all
-
-
-
94.7
0.4:0.7:1
0.9
100
0.1:0.3:1
0.8
-
100
0.4:0.7:1
0.9
100
0.4:0.5:1
0.8
crystal shape
faceted
dendrites
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E1800
60
cpx crystals (%)
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E60
7
plg crystals (%)
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E7
1
ΔL (μm)
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E1b
ΔT/Δt (°C/h)
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exp.
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Footnote: S = short axis, I = intermediate axis, L = long axis (S ≤ I ≤ L); the aspect ratio is expressed as S : I : L. E9000 is not reported due to its very low crystal content (< 2 area%) and very small crystal size (cpx and sp sizes are < 10μm).
An important aspect of kinetic studies conducted under variable solidification conditions concerns the shape of crystals (Higgins, 2006). The 2D crystal size is by far the most analyzed parameter, corresponding to the areal intersection of a 3D crystal with the cutting plane of the section. On the other hand, only few direct 3D tomographic investigations have effectively related the crystal shape and kinetic effects (Higgins, 2006; Lanzafame et al., 2017; Polacci et al., 2018). The 3D shape of crystals can be derived from 2D measurements by adopting different stereological approaches (Underwood, 1970; Higgins, 2006; Morgan and Jerram, 2006). In this study, we have calculated the textural parameters L, I and S for the long, intermediate and short crystal axes (L > I > S) following the method of Morgan and Jerram (2006). These data are listed in Table 6 and displayed in Fig. 12 as Zingg plots (Zingg, 1935). Faceted plg crystals exhibit a simple trend in
Journal Pre-proof which the 3D shape evolves from prismatic to tabular as ΔT/Δt increases from 1 to 60 °C/h. By contrast, faceted cpx crystals are never related to ΔT/Δt, while dendritic cpx exhibit shapes clustered in the tabular field. Faceted sp crystals are either prismatic or cubic, whereas dendritic sp are gathered between tabular and prismatic areas (Fig. 12). By considering the large variation of ΔT/Δt and ΔTc of the experiments (Fig. 1 and Table 1S), we can conclude that 2D and 3D crystal shapes of cpx and sp are poorly sensitive to the kinetic effects at least at high fO2, with the only
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qualitative exception of faceted plg crystals.
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Fig. 12. Calculated 3D shape of mineral phases (see data in Table 6). L, I and S correspond to the long, intermediate and short axes, respectively (L > I > S).
Journal Pre-proof 7.3. Crystal size distribution analysis The crystal size distribution analysis is routinely used for describing either experimental or natural solidification processes involving the crystallization of plg, cpx and sp (Cashman and Marsh, 1988; Cashman, 1993; Marsh, 1998; Higgins, 2000, 2006; Zieg and Marsh, 2002; Jerram et al., 2003; Armienti, 2008; Pupier et al., 2008;Hammer, 2009; Iezzi et al., 2011; Vona et al., 2011; 2017; Lanzafame et al., 2013; Ni et al., 2014). CSD plots (size vs log of population density per size) obtained in this study under variable ΔT/Δt conditions (Figs. 1, 2,3 and 5) are displayed in Figs. 13
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and 14. As a general rule, CSDs of plg, cpx and sp show progressive increase in slope (m) and
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population density per size (n0 in ln corresponding to the y axis intercept) with increasing ΔT/Δt.
at 1 and 7 °C/h, the trend is composed of two log-linear segments with similar values for
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For the CSD of plg (Figs. 13 and 14Sa):
the intercept; at 7 °C/h the size range is slight smaller than that at 1 °C/h, exhibiting a
at 60 °C/h, the trajectory depicts three log-linear segments with an increasing m and a
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downturn at the smaller crystal size;
decreasing of the crystal size range.
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For the CSD of cpx (Figs. 13 and 14Sb): at 1 °C/h, the trend is composed of two log-linear segments with only a moderate difference in slope, a large size range, and the lowest m and n0 values (Table 7). There is a downturn at the smaller crystal sizes.
at 7, 60 and 180 °C/h, the trends are composed of three (7 °C/h and 180 °C/h) and two (60 °C/h) log-linear segments with almost similar slopes. The CSD at 7 °C/h shows the largest crystal size range and the lowest intercept value. There is a downturn (7 and 180 °C/h) at the smaller crystal sizes;
at 1800 °C/h, the almost linear trend shows the steepest behavior and the regression analysis of the log-linear CSD yields the maximum n0 intercept. As concerns the CSD of sp (Figs. 13 and 14Sc):
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at 1 and 7 °C/h, the trends are composed of three (1 °C/h) and two (7 °C/h) log-linear segments. The slopes and intercepts are low to moderate and the size range decreases.
at 60 °C/h, the almost linear trend shows m and n0 higher than those measured at lower cooling rates;
at 180 °C/h, the trend is composed of two log-linear segments, with almost the same slope measured at 60 °C/h, but with a larger crystal size range. at 1800 °C/h, the almost linear trend is very steep and n0 reaches the maximum value; the
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maximum crystal size decreases of two order of magnitude from 1 to 1800 °C/h.
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The downward kinking of CSD curves measured for plg (E7), cpx (E1b, E7 and E180b) and sp (E7,
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E60 and E180b) at the small sizes is consistent with a coarsening effect and, possibly, an Ostwald
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ripening process. This phenomenon can be ascribed to actual crystallization processes or to an
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artefact due to limited spatial resolution not allowing the accurate counting of tiny crystals (Hammer et al., 1999; Mock et al., 2002; Higgins, 2006; Armienti, 2008; Vona et al., 2017). Since
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SEM images have a high spatial resolution, we exclude artifacts due to an inaccurate counting of tiny crystals. This interpretation is corroborated by ex-situ observations of Pupier et al. (2008) under
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either rapid or slow ΔT/Δt conditions, as well as by in-situ observations of Ni et al. (2014) under relative rapid ΔT/Δt conditions (Table 1). The sluggish solidification conditions presented in this study significantly enlarge the field of CSD investigation and show that as ΔT/Δt increases: 1) the crystal size range decreases (Fig. 13), 2) the population density n0 moves upward (Fig. 13 and Table 7) in line with the increasing of number of crystals per area (#/A) (Fig. 10), 3) the CSD curve derived at low cooling rates is mainly composed of two or three log-linear segments (Figs. 14), and 4) the CSD slope m increases (Fig. 13 and Table 7). At low ΔT/Δt, the crystals have enough time to aggregate according to a coarsening process which, in turn, produces flattening and low intercept for the CSD curve (Pupier et al., 2008; Ni et al., 2014). The coarsening is also supported by the faceted cpx in the verge of a complete aggregation observed in Figs. 4 and 5 for the E60 experiment. Moreover, it is likely that tiny
Journal Pre-proof crystals dissolve under the effect of low cooling rates in order to feed the growth of larger crystals by Ostwald ripening (Mills et al., 2011). According to Pupier et al. (2008) and Ni et al. (2014), different log-linear slopes and CSD trends (Fig. 14) indicate multiple events or pulses of nucleation. The CSD curve with lowest slope and largest crystal size corresponds to the early event of nucleation occurred below the liquidus and followed by further pulses of nucleation at lower temperatures. Since the CSD slope decreases with increasing crystal size (Figs. 13 and 14), it is plausible that the growth of early-formed nuclei
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proceeds by agglomeration and attachment (Iezzi et al., 2008, 2011, 2014). Successive pulses of
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nucleation in the residual melt are also responsible for CSD segments with moderate and/or high
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slopes, as well as for the high population density observed at smaller crystal sizes (Figs. 13 and 14).
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Fig. 13. Crystal size distribution (CSD) of plg, cpx and sp, computed by CSDCorrection (Higgins, 2000) and relative area% (see data in Table 3); n0 corresponds to the population density per size.
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4
3
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7
E180b
E1800
60
180
1800
4
3
2
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E60
na
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-p
E7
(n0) (µm-4)
crystal shape
-1*10-8 -3*10-7 -7*10-11 -2*10-7 -2*10-6 -7*10-8 -4*10-8 -3*10-8 -6*10-7 -1*10-8 -5*10-8 -5*10-7 -2*10-9 -5*10-6 -2*10-5 -2*10-6 -3*10-8
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E1b
#images
Table 7. Crystal Size Distribution (CSD) data calculated by CSDCorrection (Higgins, 2000) area crystalline size range slope, m y-intercept, magnification #crystals (μm2) phase (µm) (μm-1) ln(n0) (µm-4) 25 – 956 -1*10-2 -18.2 (full range) plg -2 25 – 256 -5*10 -15.0 100 plg 256 - 956 -1*10-3 -23.4 859353 277 cpx 19 - 365 -2 61 sp -3*10 -15.5 (full range) cpx -2 19 -70 -7*10 -13.3 150 x 70 - 365 -2*10-2 -16.5 20 – 104 -2 -3*10 -17.0 (full range) 79 plg -2 20 - 54 -1*10 -17.3 961083 184 cpx sp 31 sp 54 - 75 -7*10-2 -14.4 75 - 104 -2*10-2 -18.4 39 – 752 -2 -1*10 -16.9 (full range) plg -2 39 – 280 -3*10 -14.6 160 plg 200 x 270420 746 cpx 280 – 752 -1*10-2 -20.2 89 sp 17 – 235 -2 - 4*10 -12.2 (full range) -2 17 - 88 - 7*10 - 10.9 cpx 88 - 170 - 4*10-2 - 13.2 170 - 235 - 2*10-2 -17.4 129 plg 11 – 78 -1 150 x 945323 1631 cpx -1*10 -12.3 (full range) 212 sp sp -1 11 - 56 - 2*10 -11.1 56 - 78 - 3*10-2 -19.2 2 – 112 -2 -9*10 - 9.0 (full range) 962 plg -1 2 - 22 - 2*10 -7.1 400 x 138897 1464 cpx plg 295 sp 22 – 81 - 9*10-2 - 9.9 81 - 112 - 9*10-2 - 16.3 2 – 98 -1 -1*10 -7.8 (full range) cpx 539 plg -1 2 - 37 - 2*10 -6,2 800 x 34762 1172 cpx 37 – 98 - 7*10-2 -11.2 144 sp 2 – 23 sp - 3*10-1 - 7.9 (full range) 4 – 108 - 8*10-2 -10.3 (full range) 695 cpx -1 4 – 11 - 4*10 - 7.4 400 x 145934 cpx 736 sp 11 – 40 - 1*10-1 - 10.6 40 – 108 - 5*10-2 -12.6 3 – 54 -1 - 2*10 - 9.5 (full range) 419 cpx 300 x 119680 sp -1 276 sp 3 - 22 - 4*10 - 7.5 22 - 54 - 5*10-2 - 13.9 214 cpx 2 – 23 4798 cpx - 4*10-1 - 3.6 138 sp (full range) 1500 x 318 cpx 1–6 4790 sp - 1.9*100 - 1.2 271 sp (full range)
-4*10-6 -2*10-5 -5*10-9 -1*10-4 -9*10-4 -5*10-5 -8*10-8 -4*10-4 -2*10-3 -1*10-5 -4*10-4 -3*10-5 -6*10-4 -2*10-5 -3*10-6 -8*10-5 -6*10-4 -9*10-7 -3*10-2 -3*10-1
Footnote: E9000 is not reported due to the very low crystal content (< 2 area%) and very small crystal size (cpx and sp sizes are < 10μm).
dendritic
exp.
ΔT/Δt (°C/h)
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Fig. 14aS (to be deposited as supplementary). CSD data of plg; n0 correspond to the population density per size. The black dashed lines are the regressions of all data (full range in Table 7), whereas the red lines correspond to regressions of different range of sizes (Table 7).
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Fig. 14bS (to be deposited as supplementary). CSD data of cpx; n0 corresponds to the population density per size. The black dashed lines are the regressions of all data (full range in Table 7), whereas the red lines correspond to regressions of different range of sizes (Table 7).
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Fig. 14cS (to be deposited as supplementary). CSD data of sp; n0 corresponds to the population density per size. The black dashed lines are the regressions of all data (full range in Table 7), whereas the red lines correspond to regressions of different range of sizes (Table 7).
Journal Pre-proof Finally, it is possible to models the CSD parameters as a function of kinetic conditions. The average m and n0 (full range in Table 7) for plg, cpx and sp have high liner correlations like those displayed in Fig. 15. In turn, the increase of ΔT/Δt induces a progressive counterclockwise rotation (Fig. 13) during the cooling of a complete molten MORB from liquidus to solidus. Moreover, the very high linear trends between ΔT/Δt versus m and n0 for plg, cpx and sp allow us to calculate the cooling rates by CSD parameters obtained on MORB and basaltic rocks (Fig. 16). Consequently, the equations retrieved in this study can be used to calculate kinetic conditions of solidification
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from the outer to the innermost parts of MORB lavas with thickness ≤ 2 m (Fig. 2).
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Fig. 15. Relationships between average CSD slopes (m) and population density (n0) (Table 7).
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Fig. 16. Relationships between ΔT/Δt versus average CSD slopes m (left column) and population density n0 (right column) (Table 7).
Journal Pre-proof 7.4. Maximum and average growth rates A central role in the reconstruction of volcanic processes is the time of crystal growth (Marsh, 1988; Costa et al., 2008; Cooper and Kent, 2014; Cooper, 2017; Putirka, 2017; Polacci et al., 2018). Several approaches can be used but the most frequent and simple one consists in the measure of the crystal size divided by growth rate (G) determined in laboratory (Cashman, 1993; Larsen, 2005; Pupier et al., 2008; Brugger and Hammer, 2010; Shea and Hammer, 2013; Cooper and Kent, 2014; Mollo and Hammer, 2017). In the last decades, many experimental studies retrieved the values of G
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for the most common crystalline phases in silicate liquids (Armienti et al., 1994; Lasaga, 1998;
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Zhang, 2008; Hammer, 2008). Here, we enlarge these datasets by determining the maximum and
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average growth rates (Gmax and GCSD, respectively) for plg, cpx and sp. We recall that Tliquidus of
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B100 is 1233 °C (Tm in Table 1), while the time intervals (t) between Tliquidus and the final solidification temperature of 800 °C are 433, 61.9, 7.2, 2.4 and 0.2 hours for ΔT/Δt of 1, 7, 60, 180
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and 1800 °C/h, respectively. In our calculations, it is assumed that crystals nucleate instantaneously
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at Tliquidus and then continuously grow down to 800 °C. The resulting growth rates are listed in Table 8. They represent the minimum of the actual growth rates since the crystallization can initiate below
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Tliquidus and terminate above Tsolidus.
The maximum growth rate (Table 8) is calculated by averaging the five longest crystals (Lmax) and dividing them by the cooling time, as Gmax = Lmax/t (Burkhard, 2002; Hammer and Rutherford, 2002; Couch, 2003; Iezzi et al., 2011). The average growth rate (Table 8) is calculated by interpolating the CSD slope (m) and the cooling time, as GCSD = -1/mt (Zieg and Marsh, 2002). The slope derived by global linear regression analysis of the same mineral phase and experiment yields the value of GCSD. Conversely, the different slopes derived by log-linear segments of the same mineral phase and experiment (i.e., plg at 7 °C/h, cpx at 1, 7 and 180 °C/h, andsp at 7, 60 and 180 °C/h) are referred as G*CSD (Table 8). Overall, GCSD and G*CSD for the same mineral phase and experiment are comparable within the same order of magnitude.
Journal Pre-proof Gmax and GCSD are plotted in Fig. 17 as a function of ΔT/Δt. As expected, Gmax is higher than GCSD and both parameters increase with increasing ΔT/Δt. Gmax and GCSD of plg increases remarkably from 1 to 7 °C/h and then do not change substantially; the relation of ΔT/Δt versus Gmax can be well modeled by a linear regression, whereas that of ΔT/Δt versus GCSD is only qualitative for plg (Fig. 17). Gmax and GCSD of cpx follow an increasing monotonic, linear trend as a function of cooling rate (Fig. 17). Gmax and GCSD of sp increase from 1 to 180 °C/h and then do not substantially change.
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The overall ranges of Gmax and GCSD measured with increasing cooling rate are: 4.7*10-8 –
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3.5*10-7 and 6.4*10-9 – 4.3*10-8 cm/s for plg, 1.9*10-8 – 2.5*10-6 and 2.1*10-9 – 2.7*10-7 cm/s for
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cpx, and 5.1*10-9 – 6.2*10-7 and 2.1*10-9 – 6.8*10-8 cm/s for sp, respectively (Fig. 17and Table 8).
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At 1 °C/h, Gmax is the highest for plg and the lowest for sp, with an intermediate value for cpx. At this cooling rate, GCSD shows similar values for all the mineral phases. At 7 °C/h, Gmax and GCSD are
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the highest for plg and the lowest for sp, with an intermediate value for cpx. At 60 °C/h, Gmax and
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GCSD of plg and cpx are comparable and higher than those calculated for sp. At 180 and 1800 °C/h, both Gmax and GCSD of cpx are higher than those calculated for sp (Fig. 17).
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Gmax and GCSD from this study enlarge the data set measured on basaltic melts. It is noteworthy, that our data are similar to those measured in previous studies for similar cooling rates, so demonstrating the consistency of the approaches. Pupier et al. (2008) found Gmax and GCSD of plg in the range of 3.3*10-8 – 9.4*10-8 cm/s and 6*10-9 – 1.4*10-8 cm/s for melts experimentally cooled at 0.2 and 3 °C/h, but obtained with ΔT between 20 and 80 °C (see Tables 1 and 3 reported in Pupier et al., 2008). Among the time series experiments conducted on a primitive basalt from Mt. Etna volcano (Sicily, Italy), Pontesilli et al. (2019) calculated Gmax of cpx and sp for the anhydrous run products in the range of 15*10-7 – 0.5*10-7 cm/s and 1.5*10-7 – 0.1*10-7 cm/s, respectively, at ΔT/Δt of 1.3 °C/h and low to moderate ΔT between 80 and 120 °C. Dunbar et al. (1995) quantified Gmax of cpx in the range of 1*10-8 – 1*10-7cm/s in the interior part of a large (4.8 m3) artificial mafic melt generated cooled from 1500 to 500 °C in ~ 6 days. Ni et al. (2014) measured Gmax of cpx
Journal Pre-proof between 6*10-7 and 17*10-7 cm/s by in-situ experiments conducted at ΔT/Δt of 100 °C/h. Moreover, Gmax and GCSD have been observed to increase with ΔT/Δt in naturally cooling basaltic lavas where minerals were sampled at progressive depth from the top of the flow (Kirkpatrick, 1977; Cashman and Marsh, 1988; Burkard, 2002; Oze and Winter, 2005). At 2 cm from the top of the lava flow, Burkhard (2002) found Gmax in the ranges of 1.3*10-9– 1.7*10-8 cm/s, 3.2*10-9 – 8.6*10-7 cm/s and 3.5*10-9 – 2.2*10-8 cm/s for plg, cpx and sp, respectively. Similarly, Oze and Winter (2005) estimated Gmax in the range of 1.6*10-4 - 2.6*10-7 cm/s for plg, 3.5*10-5- 2.4*10-7 cm/s for cpx and
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3.5*10-6 – 2*10-7 cm/s for sp (see Table 5 reported in Oze and Winter, 2005).
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Fig. 17. Variation of maximum growth rate (Gmax, full symbols) and average growth rate (GCSD, empty symbols) as a function of ΔT/Δt (Table 8). Only linear regressions with R2 ≥ 0.6 have been reported.
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E60
E180b
5.1*10-9
2.1*10-9
plg
2.4*10-7
4.5*10-8
cpx
1.0*10-7
1.1*10-8
sp
2.8*10-8
3.4*10-9
plg
3.5*10-7
4.3*10-8
cpx
2.7*10-7
3.2*10-8
sp
6.4*10-8
1.1*10-8
cpx
1.0*10-6
1.4*10-7
6.3*10-7
7.2*10-8
60
180
1800
cpx sp
2.5*10-6 6.2*10-7
faceted
2.7*10-7 6.8*10-8
dendritic
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E1800
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sp
3.2*10-9 (70 to 365 µm) 6.4*10-9 (20 to 54 µm) 9.2*10-10 (54 to 75 µm) 3.2*10-9 (75 to 104 µm) 1.5*10-8 (36 to 280 µm) 4.5*10-8 (280 to 752 µm) 6.4*10-9 (17 to 88 µm) 1.1*10-8 (88 to 170 µm) 2.2*10-8 (170 to 235 µm) 2.6*10-9 (11 to 56 µm) 1.5*10-8 (56 to 78 µm) 1.7*10-8 (2 to 22 µm) 4.3*10-8 (22 to 81 µm) 4.3*10-8 (81 to 112 µm) 1.8*10-8 (2 to 37 µm) 5.5*10-8 (37 to 98 µm) 1.1*10-8 (2 to 23 µm) 2.9*10-8 (4 to 11 µm) 1.2*10-7 (11 to 40 µm) 2.3*10-7 (40 to 108 µm) 3.3*10-8 (3 to 22 µm) 2.3*10-7 (22 to 54 µm) 2.7*10-7 (1 to 23 µm) 6.2*10-8 (1 to 6 µm)
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9.2*10-10 (19 to 70 µm)
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2.1*10-9
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1.9*10-8
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Table 8. Maximum (Gmax) and average (GCSD) growth rates (cm/s) ΔT/Δt crystalline Gmax GCSD G*CSD (°C/h) phase -9 1.3*10 (25 to 256 µm) plg 4.7*10-8 6.4*10-9 6.4*10-8 (256 to 956 µm)
Therefore, the quantified Gmax and GCSD obtained in our study can be used to retrieve timing of growth of crystalline phases of aphyric MORB, according to the fact that ΔT/Δt changes over a broad range and the thermal path of cooling is comprised between liquidus and solidus temperatures. The cpx is the silicate phase that crystallize with a remarkable abundance from 1 to 1800 °C/h (Figs. 5 and 6) and, for this reason, it can be used to reconstruct the cooling rate of MORB rocks. In Fig. 18, the values of Gmax for cpx are plotted versus Lmax, m and n0. The linear relationships between Gmax with m and n0 are highly correlated (Fig. 18), whereas that with Lmax is semi-quantitative (R2 = 0.56). Furthermore, the decreasing trends of Gmax and GCSD with the decrease of ΔT/Δt suggest that the G rate of minerals growing in anhydrous magmatic reservoirs and conduits could be lower than
Journal Pre-proof those quantified by experimental data, even for cooling rate as low as of 1 °C/h. New investigations at ΔT/Δt lower than 1 °C/h under anhydrous and hydrous conditions are, indeed, required to enlarge the trends depicted in Figs. 17 and 18, as well as to quantify the role of H2O on the crystallization of magmas. On the other hand, G rate of minerals growing in anhydrous magmatic systems could also be enhanced by dynamic crystallization process (cooling + shearing) allowing continuous “feed growth ingredients [..] on individual crystal surfaces that facilitates crystal growth at high shear
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rate” as reported in Vetere and Holtz (2019).
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Fig. 18. From left to right: variation of Gmax (Table 8) versus CSD slope (m) and population density (n0) (Table 7), plus the maximum length (Lmax) of cpx (Table 4S).
Journal Pre-proof 7.5. Volcanological and Petrological implications The textural features of B100 indicate that as ΔT/Δt increases from 60 and 180 °C/h, several first order effects take place: 1) plg crystals disappear, 2) faceted crystals are replaced by dendritic cpx and sp (Fig. 5), 3) the matrix glass content increases significantly (Fig. 6) and 4) the #/A (µm-2) of both cpx and sp sharply decreases (Fig. 10). The plg is unable to nucleate at high cooling rates since it contains a high number of connected tetrahedral sites, requiring the breaking and formation of high energy bonds; also, the stability of plg is extremely sensitive in Al and Si, as cations with
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the slowest chemical diffusivity (Walker et al., 1976; Grove and Walker, 1977; Grove and
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Raudsepp, 1978; Iezzi et al., 2008, 2011, 2014; Vetere et al., 2013, 2015). There is a close
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relationship between the melt composition (evolving during crystallization and cooling) and the
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formation of crystal nuclei with a critical size that may potentially grow over time (Gonzalez et al., 2017, 2019). On the other hand, the nucleation of cpx from the melt requires a low energetic barrier
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because of the lower number of Si-Al tetrahedra in the crystal structure and the preferential
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incorporation of cations (i.e., Ca, Mg and Fe) that more easily diffuse in the melt phase. This applies also to the greater stability of sp as Si-free phase (Kirckpatrick, 1983; Vetere et al., 2015).
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Moreover, at ΔT/Δt between 60 and 180 °C/h, crystals show dendritic shapes, attesting disequilibrium crystallization conditions (Lofgren et al., 1974; Walker et al., 1976; Shiffman and Lofgren, 1982; Ohnenstetter and Brown, 1992; Oze and Winter, 2005; Blundy and Cashman, 2008) and in concert with lower values of #/A for cpx and sp (Figs. 10 and 13). By contrast, at ΔT/Δt<60 °C/h, plg, cpx and sp are faceted and relatively large, as well as CSD curves exhibit moderate to low slopes and intercepts reflecting equilibrium crystallization conditions (Fig. 13). These observations indicate that, between 60 and 180 °C/h, the MORB melt strongly changes its solidification behavior from crystallization occurring prevalently by growth at low ΔT/Δt to nucleation-dominated at high ΔT/Δt (Blundy and Cashman, 2008). The rapid solidification of MORB melts indicate that the rheology of basaltic lavas, dikes and pillows is deeply controlled by crystallization under the effect of temperature. Moreover, the
Journal Pre-proof convective flow of magma further enhances the nucleation and crystal growth (Vona et al. 2011, 2017; Kolzenburg et al., 2016, 2018), accelerating the attainment of a significant amount of crystals able to arrest their mobility. In turn, the increasing viscosity of basaltic liquids is mainly governed by the onset and successive crystallization steps, which is dominantly controlled by the cooling rate. A further result unveiled by our experiments concerns the sizes and abundances of crystal in the ranges > 0.5 (or 1) mm, between 0.1 and 0.5 (or 1) mm and < 0.01 mm, that are commonly considered phenocrysts, microphenocrysts and microlites, respectively (Lanzafame et al., 2013;
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Vona et al., 2017 and references therein). Phenocrysts are generally interpreted as minerals formed
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in magmatic reservoirs, whereas microphenocrysts and microlites are addressed to magma ascent
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within the conduit and late solidification on the Earth surface during lava flow. However, the
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textural analysis from this study (Fig. 7) points out that it is possible to grow plg crystals with size up to 0.5-1 mm at cooling rates of 1-7 °C/h, as well as a significant number of plg and cpx crystals
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> 0.1 mm or 100 µm (Fig. 11), starting from a complete molten and anhydrous MORB liquid
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cooling on Earth. This demonstrates that, at low ΔT/Δt conditions, MORB liquids solidifying as lava flows, pillow lavas and dikes can rapidly and easily attain phenocryst and microphenocryst
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textures. Faceted to dendritic plg and cpx crystals may also grow at subaerial conditions after the eruption of magma at the surface. The effects of crystallization kinetics in lavas are not the same from the base towards the uppermost crust, due to the different cooling conditions (Fig. 2). Consequently, aphyric MORBs will show different CSD features (Fig. 13) moving from innermost to outermost parts of the flow. Similarly, plg or cpx with variable sizes and shapes (faceted to dendritic) may be partially or fully formed after eruption of lava flows/pillows in response to cooling-induced solidification phenomena. The main implication is that a single rock sample collected from a naturally cooled lava flow cannot be representative of the whole flow, thus resulting misleading for the reconstruction of solidification histories of basaltic rocks, especially for discriminating minerals formed at pre-, syn- and post-eruptive conditions.
Journal Pre-proof 12. Conclusions The quantitative evolution of textural attributes of experimental plg, cpx and sp obtained at liquidus-solidus temperature ranges and slow-fast cooling rates ΔT/Δt, can be used to constrain in full the dynamic and kinetic conditions driving the solidification of natural MORB, with thickness variable from 0.1 to 1/2 m (Fig. 2). Moving from the innermost to the outermost portions of lava flows, pillow lavas and dikes, the kinetic effects will progressively increase as a function of ΔT/Δt and heat release. This increase determines: a decreasing amount of crystals accompanied by a transition from faceted to dendritic
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1)
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crystals and the suppression of plg between 60 and 180 °C/h (Figs. 5 and 6); in turn,
a strong variation of the crystal size with significant differences among plg, cpx and
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2)
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plg will not occur in the external portion of aphyric lavas, dikes and/or pillows;
sp cooled at the same rate (Figs. 7 and 8);
the lack of a clear evolution for the 2D and 3D crystal shapes (Figs. 9 and 12);
4)
a monotonic increase of two and three orders of magnitude in the number of plg and
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sp crystals per area (µm-2), respectively, whereas that of cpx first increases,
5)
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subsequently decreases, and finally increases again (Fig. 10); a progressive evolution of the CSD curves for plg, cpx and sp (Figs. 13 and 14S), with an increase in slope m and population density n0 (Fig. 15), by producing nearly continuous clockwise rotations (Fig. 13); these m and n0 parameters can be also used to determine the cooling rate conditions of MORB rocks (Fig. 16); 6)
the growth rates Gmax and GCSD decrease of several orders of magnitude from 10-9 to ~ 10-5 cm/s (Fig. 17); Gmax can be faithfully calculated by m and n0 for MORB rocks (Fig. 18);
7)
plg and cpx crystals with sizes of 102 to 103 µm may develop from nearly anhydrous MORB lava flows and pillows (Figs. 11); these relatively large dimensions are not
Journal Pre-proof unequivocally indicative of phenocryst growth in magmatic reservoirs (phenocrysts)
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or microphenocrysts growth during magma ascent in volcanic conduits.
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Whittington, A. G., Hofmeister, A. M., Nabelek, P. I., 2009.Temperature-dependent thermal diffusivity ofthe Earth’s crust and implications for magmatism. Nature, 458, 319 – 321.doi:
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Witter, J. B., Harris, J. L., 2007. Field measurements of heat loss from skylights and lava tube systems. J. Geophys. Res. 112. doi: 10.1029/2005JB003800. Wohletz, K., Civetta, L., Orsi, G., 1999.Thermal evolution of the Phlegrean magmatic system. J. Geotherm. Res., 91, 381 – 414. Zieg, M. J., Marsh, B.D., 2002. Crystal size distributions and scaling laws in the quantification of igneous texture. Journal of Petrology, 43, 85 – 101. Zieg, M. J., Lofgren, G. E., 2006. An experimental investigation of texture evolution during continuous
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Journal Pre-proof Zhang, Y., 2008. Geochemical Kinetic. Princeton University Press, New York. Zhou, W., Van der Voo, R., Peacor, D.R., Zhang, Y., 2000. Variable Ti-content and grain size of titanomagnetite as a function of cooling rate in very young MORB. Earth and Planetary Science
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Letters, 179, 9 – 20.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14A
Figure 14B
Figure 15
Figure 16
Figure 17
Figure 18
Letizia Giuliani, Gianluca Iezzi, Francesco Vetere, Harald Behrens, Silvio Mollo, Federica Cauti, Guido Ventura, Piergiorgio Scarlato PII:
S0012-8252(19)30778-0
DOI:
https://doi.org/10.1016/j.earscirev.2019.103063
Reference:
EARTH 103063
To appear in:
Earth-Science Reviews
Received date:
25 November 2019
Accepted date:
10 December 2019
Please cite this article as: L. Giuliani, G. Iezzi, F. Vetere, et al., Evolution of textures, crystal size distributions and growth rates of plagioclase, clinopyroxene and spinel solidified at variable cooling rates from a mid-ocean ridge basaltic liquid, Earth-Science Reviews(2019), https://doi.org/10.1016/j.earscirev.2019.103063
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Journal Pre-proof Evolution of textures, crystal size distributions and growth rates of plagioclase, clinopyroxene and spinel solidified at variable cooling rates from a mid-ocean ridge basaltic liquid
Letizia Giuliania (corresponding author), Gianluca Iezzia,b, Francesco Veterec-d, Harald Behrensd, Silvio Mollob-e, Federica Cautia, Guido Venturab, Piergiorgio Scarlatob
Affiliations: Dipartimento di Ingegneria & Geologia (InGeO), Università G. D’Annunzio di Chieti-Pescara,
of
a
Istituto Nazionale di Geofisica e Vulcanologia, INGV, Via di Vigna Murata 605, 00143, Roma,
-p
b
na
Institute for Mineralogy, Leibniz University of Hannover, Callinstrasse 3, Hannover, D-30167,
Germany
Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale A. Moro 5, 00185
Roma, Italy
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e
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Dipartimento di Fisica & Geologia, Università degli studi di Perugia, Piazza dell’Università 1,
06123, Perugia, Italy d
re
Italy c
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Via dei Vestini 31, 66100, Chieti, Italy
Keywords: MORB, cooling rate, solidification, texture, CSD, growth rate. Abstract Mid-ocean ridge basalts (MORB) are the most abundant eruptive tholeiitic products on Earth. Many experimental investigations reproduce and model the solidification of basalts, but under limited thermal ranges of cooling (ΔTc) and cooling rates (ΔT/Δt). Previous studies report a set of experiments run between 1300 °C and 800 °C under ΔT/Δt of 1, 7, 60, 180, 1800 and 9000 °C/h. These experiments allowed to determine the glass-forming ability (GFA) of sub-alkaline silicate
Journal Pre-proof liquids, but do not give information on their textures. Here, we quantify the evolution of sizes, shapes, number per area and CSDs of plg (plagioclase), cpx (clinopyroxene) and sp (spinel) using image analysis on thousands of crystals. These textural measurements are the most complete dataset ever obtained in laboratory, since reflects rapid, intermediate and sluggish cooled parts of MORB from liquidus to solidus. Faceted plg grows only for ΔT/Δt ≤ 60 °C/h, while cpx and sp became dendritic at ΔT/Δt between 60 and 180 °C/h. As ΔT/Δt increase, crystal size ranges decrease from 1000-10 μm (1
of
°C/h) to 100-1 (60 °C/h) µm, 400-8 µm (1°C/h) to 25-0.5 µm (1800 °C/h) µm and 90-6 µm (1 °C/h)
ro
to 6-0.5 (1800 °C/h) µm, respectively for plg, cpx and sp. The 2D aspect ratio and calculated 3D
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shapes of all phases do not evidence trends as a function of ΔT/Δt. The number of crystals per area
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(#/A) increases with increasing ΔT/Δt, except for cpx between 60 and 180 °C/h. As ΔT/Δt increases, CSDs of plg, cpx and sp increase their slopes (m) and population densities per size (n0), reduce the
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size ranges and tend to be log-linear. At low ΔT/Δt, CSDs are composed by several log-linear
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segments, which slopes are related to different pulses of nucleation, followed by the crystal growth by coarsening. The average m of CSDs linearly scale with the corresponding n0 and both m and n0
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for plg, cpx and sp are highly correlated with ΔT/Δt. The calculated maximum (Gmax) and average (GCSD) growth rates increase as ΔT/Δt increases; GCSD for plg, cpx and sp changes from 10-9 to 10-8 cm/s, from 10-9 to 10-7 cm/s and from 10-9 to 10-8 cm/s, respectively. The Gmax of cpx is highly correlated with m and n0 and semi-quantitatively to Lmax. Therefore, the CSD parameters of cpx in MORB can be used to retrieve either ΔT/Δt and Gmax. The experimental plg and cpx crystals with dimensions between 0.1 and 1 mm are abundant at low ΔT/Δt. In volcanic rocks, these crystal sizes are referred to phenocrysts and microphenocrysts formed in intra-telluric environments. Our data demonstrate that phenocrysts and microphenocrysts may grow also after eruptions. The continuous evolution of textures in response to ΔT/Δt variations implies that the solidification of MORB can be fully captured only by considering the effects of kinetics. As a result, the widely accepted assumption that phenocrysts represent the products of evolution processes, e.g. fractionation, in conduits or reservoirs could be not valid for some basaltic lavas.
Journal Pre-proof
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1. Introduction
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The effusion of magmas from mid-ocean ridges (MORs) is by far the most relevant
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mechanism of crust formation on Earth (Crisp, 1984; Soule, 2015). MORs erupt basalts (MORB)
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with a prevailing tholeiitic composition characterized by SiO2 contents ~ 50 wt.%, Na2O + K2O < 3 wt.% (Rogers, 2015) and, with some exceptions (Ligi et al., 2005), H2O mostly close to 0.1 - 0.2
lP
wt.% (Wallace et al., 2015). Although extremely abundant, the textural features of MORB rocks
na
have been investigated in detail only in few studies (Coish and Taylor, 1979; Jafri and Charan, 1992; Lesher et al., 1999; Zhou et al., 2000; Faure and Schiano, 2004). These studies highlight the
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general aphyric and sub-aphyric texture of these rocks in the more rapidly cooled outermost parts of lava flows with tiny minerals dispersed in a glassy matrix. The innermost and sluggishly cooled portions are crystal-rich (Soule, 2015). Submarine MORBs are frequently erupted under superheated or close to liquidus conditions (Wallace et al., 2015). The analysis of both solidified and still molten basaltic lavas has improved our knowledge on the crystallization of silicate liquids and magmas (Muir and Tilley, 1966; Bryan, 1972; Mevel and Velde, 1976; Coish and Taylor, 1979; Schiffman and Lofgren, 1982; Long and Wood 1986; Rowland and Walker, 1988; Degraff et al., 1989; Jafri and Charan, 1992; Cashman, 1993; Armienti et al., 1994; Crisp et al., 1994; Lesher et al., 1999; Zhou et al., 2000; Burkhard, 2005; Chistyakova and Latypov, 2009; Lanzafame et al., 2013, 2017; Robert et al., 2014; Soldati et al., 2018 and references therein). In the last five decades, the basaltic rocks on Earth and Moon have been
Journal Pre-proof extensively investigated with experiments focused on the nucleation and crystal growth processes of natural melts and magmas (Lofgren et al., 1974; Usselman et al., 1975; Walker et al., 1976, 1978; Bianco and Taylor, 1977; Grove and Bence, 1977; Lofgren, 1977; Walker et al., 1977; Grove, 1978; Lofgren et al., 1979; Kirkpatrick, 1981; Lesher, 1999). Most of these processes are summarized in several review studies (Dowty, 1980; Lofgren, 1980; Kirckpatrick, 1981; Cashman, 1991; Lasaga, 1998; Higgins, 2006; Jerram and Higgins, 2007; Marsh, 2007; Hammer, 2008; Iezzi et al., 2009; Vetere et al., 2015; Mollo and Hammer, 2017). More recently, some authors have investigated the
of
role of strain rate and oxygen fugacity on the solidification of basaltic liquids (Kolzenburg et al.,
ro
2018; Vona et al., 2011; 2017; Vetere et al., 2017; 2019a; 2019b). The majority of anhydrous
-p
solidification experiments were conducted at atmospheric pressure and melt redox conditions close
re
to air through the application of variable (mainly linear) cooling rates (Hammer, 2008; Vetere et al., 2015; Mollo and Hammer, 2017). Alternatively, the solidification of hydrous basaltic magmas was
lP
investigated by degassing-induced decompression experiments (Applegarth et al., 2013; Fiege et al.,
na
2015; Arzilli et al., 2016). Some cooling experiments were also performed at high-P under either anhydrous or hydrous conditions (Del Gaudio et al., 2010; Shea and Hammer, 2013). Nevertheless,
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the solidification process induced by cooling at atmospheric pressure appears by far the most appropriate condition for understanding the crystallization path of aphyric MORBs and H2O-poor basaltic magmas.
In this study, we first focus on the imposed thermal ranges of cooling (ΔTc) and cooling rates (ΔT/Δt) imposed on basaltic compositions from literature. Then, we extend the experimental results reported in Vetere et al. (2013, 2015) on a tholeiitic basalt (sample B100 from Iceland) cooled between 1300 and 800 °C (ΔTc) at ΔT/Δt of 1, 7, 60, 180, 1800 and 9000 °C/h. These data point out that the abundance (area%) of plagioclase (plg), clinopyroxene (cpx) and spinel (sp) phases decreases as ΔT/Δt increases, making possible to model the glass-forming ability of the melt. On this premise, we reconstruct the overall evolutionary path of plg, cpx and sp by quantifying the textural maturation of crystals at different ΔT/Δt through the following parameters:
Journal Pre-proof size and 2D-3D shape of crystals, number of crystals per area (#/A), crystal size distribution (CSD) and growth rate (G). These new data rely on the measurement of several thousands of textural data collected by image analysis of crystals formed under very rapid to intermediate and extremely sluggish ΔT/Δt applied from liquidus to solidus conditions (ΔTc). Our textual dataset constrains the crystallization of aphyric basaltic lava flows, pillow lavas, bombs, and decimeter-to-meter thick surface dikes. These experimental outcomes have implications for (1) the interpretation of MORB cooling paths, (2) a better understanding of crystal formation and growth before and after the
ro
of
eruption and (3) the reconstruction of magma solidification conditions.
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2. Previous experimental investigations
re
More than 50 basaltic compositions investigated by cooling-induced crystallization experiments are present, to our knowledge, in the literature (Fig. 1 and Table 1S). The anhydrous
lP
starting compositions of these experiments range from picro-basalt to basalt to basaltic andesite to
na
trachybasalt, and consist of glassy materials from synthetic or natural products melted and rapidly quenched at 1 atm. Cooling experiments were performed at log fO2 ranging from air (-0.68 at
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1600°C) to NNO-6.2 (where NNO refers to the nickel-nickel oxide buffer; Table 1S). The thermal range of cooling significantly changes among the different studies, with the largest applied ΔTc of 1000 °C (from 1600 to 600 °C; Fig. 1S). The most frequent ΔTc is generally initiated below 1250 °C and terminated above 900 °C (Fig. 1 and Table 1S). However, the melting (or liquidus) temperature (Tm) and the glass transition temperature (Tg) of MORB liquids are ~ 1250 and 650 °C, respectively (Vetere et al., 2015; Iezzi et al., 2017). Hence, a significant part of the crystallization path of dry basaltic liquids is not considered by most of the previous studies (Fig. 1 and Table 1S). The experimental cooling paths applied to basaltic systems are at constant ΔT/Δt rates, with only few investigations with more complex thermal paths (Table 1S). Notably, a few experiments covering a large ΔTc (above 1300 °C and down to 800 °C) explored only high ΔT/Δt rates, whereas experiments with very slow ΔT/Δt were quenched above 900 °C (Fig. 1 and Table 1S). This implies
Journal Pre-proof that the overall knowledge of the MORB solidification process requires a more systematic ΔT/Δt change with ΔTc between liquidus and solidus temperatures (Fig. 1). Consequently, the ΔT/Δt and ΔTc used in Vetere et al. (2013; 2015) are appropriate to capture the whole solidification paths of MORB units, albeit possible reheating processes by the emplacement of other successive units are
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not considered.
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f o
l a
o r p
r P
e
n r u
o J
Fig.1. Thermal paths of the most relevant experimental solidification studies from literature (Table 1S) and the related melt compositions plotted in the TAS diagram (La Maitre, 2002). The green field highlights the experimental conditions investigated in this study (Table 1S; Vetere et al., 2013, 2015).
Journal Pre-proof
authors Backer and Grove (1985) Bianco and Taylor (1977)
Bowles et al. (2009)
Brachfeld and Hammer (2006)
Conte et al. (2006)
Donaldson et al. (1975) Faure et al. (2003) Faure and Schiano (2005)
Table 1S (to be deposited as supplementary). Bulk compositions (wt.%) and experimental conditions of basaltic melts from previous solidification experiments ΔT/Δt pre-cooling SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O Cr2O3 total TAS Ti (°C) Tf (°C) (°C/h) phases basaltic 1008÷ 50, 10, liquid + 54.0 0.9 17.9 7.7 0.1 5.9 8.7 3.3 1.0 99.5 1170 andesite 1131 5, 1, 0.5 plg + ol 43.3
2.9
8.8
23.7
0.4
7.0
0.4
0.2
-
0.7
99.1
44.3
3.7
12.2
17.7
0.4
9.4
10.6
0.4
0.1
-
0.6
99.2
51.2
1.6
8.7
18.9
0.5
7.0
8.4
2.3
0.8
0.5
0.1
100.0
51.2
1.6
17.0
8.5
0.5
7.0
10.5
2.3
0.7
0.5
0.1
8.7
19.0
-
7.1
8.5
2.3
0.8
-
50.4
0.9
15.4
7.6
0.2
7.7
15.5
2.3
1.9
0.5
-
54.2
0.8
16.0
7.0
0.1
6.3
9.8
2.9
1.8
0.2
-
100.8
55.5
0.7
15.0
7.4
0.2
6.4
9.7
2.3
1.6
0.2
-
99.9
45.8
2.9
10.4
16.2
0.3
11.7
11.8
51.3
-
14.1
-
-
17.1
17.4
J
r P
99.5
1290 1200
1075 1100
2, 5, 16, 48
liquid and liquid + plg
NNO-5.3
basalt
1205
f o
400÷ 600°C
5.7, 18.7, 72.4, 231
liquid
NNO-0.8 NNO-4.8
1210
285÷ 510
3, 6, 19, 72, 231
liquid
NNO-4.8 NNO-0.8 NNO NNO+4.2
99.9
1200
1175 1150 1125 1100
1, 9000
liquid
NNO
2.7÷1430
liquid
NNO-4.8
1÷ 1890
liquid
air
liquid
air
liquid
air
liquid
NNO-3.8
liquid
NNO+1.2
liquid
-
basalt
basalt
0.3
l a -
-
0.4
99.7
basalt
1300
-
-
-
-
100.0
basalt
1466÷ 1364
1110÷ 1047 1322÷ 986 1318÷ 1166
rn
u o
-
e
1.6
basaltic andesite
16.6
17.4
-
-
-
-
100.0
basalt
1394÷ 1361
-
17.1
17.4
-
-
-
-
100.0
basalt
1394÷ 1364
1082÷ 1226
15.9
0.5
18.3
6.7
0.1
-
b.d.
0.7
100.5
basalt
1435
1115, 909
15.0
8.4
0.2
7.6
10.7
2.6
0.4
0.3
-
100.0
basalt
1198
1162.41÷ 1141
13.6
19.1
0.2
5.3
13.1
0.3
-
-
0.2
100.0
basalt
1225
1210÷ 990
-
14.3
-
-
51.3
-
14.1
-
49.8
0.6
6.0
Gibb (1974)
48.7
1.7
Grove and Bence (1977)
47.1
1.2
NNO-0.8
picro basalt
o r p
100
51.5
51.6
Faure et al. (2007) First and Hammer (2016)
11.9
log fO2
2, 1639÷1730, 1, 225, 525, 991, 1890 2, 1890, 1639÷2182 1, 5, 10, 28, 50 ÷ 1000 10, 5, 2.5, 2, 1, 0.5, 0.25, 0.1, 0.05 0.5, 1.75, 3.75, 10, 30, 60, 150
Journal Pre-proof Table 1S (continued) (to be deposited as supplementary). Bulk compositions (wt.%) and experimental conditions of basaltic melts from previous solidification experiments ΔT/Δt pre-cooling authors SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O Cr2O3 total TAS Ti (°C) Tf (°C) (°C/h) phases 0.5, 1.7, Grove 1010÷ 47.1 0.9 12.6 20.2 0.3 6.3 12.8 0.2 0.2 100.59 basalt 1200 10, 40, liquid (1978) 950 150, 600 Grove and 1204 ÷ 48.4 2.3 12.7 20.2 0.2 3.2 11.7 0.3 0.1 0.1 99.23 basalt 1045 150 liquid Raudsepp, (1978) 1039 Grove and 1207÷ 1164÷ 47.1 0.9 12.6 20.2 0.3 6.3 12.8 0.2 0.2 100.6 basalt 0.48 ÷ 689 liquid Bence (1979) 1196 953 Grove 822, 846, 50.2 1.7 16.0 9.9 7.6 11.2 2.9 0.1 0.2 99.8 basalt 1195 2, 9.5, 110 liquid (1990) 876 Hammer (2006; 2009)
Kennedy et al. (1993)
Kolzenburg et al. (2016)
Kolzenburg et al. (2018)
Lattard and Partzsch, (2001)
o r p
51.4
1.6
9.1
18.9
-
7.3
8.8
2.1
0.8
0.5
-
45.0
0.2
5.8
16.7
0.3
29.4
2.3
0.2
0.4
-
0.6
100.9
54.8
-
3.7
16.2
-
31.9
1.2
0.7
0.1
-
-
98.8
56.9
-
4.0
13.9
0.1
23.1
1.2
-
-
-
rn
55.4
-
6.6
7.1
0.1
30.4
2.6
-
54.8
-
3.7
16.2
-
21.9
1.2
0.7
56.9
-
4.0
13.9
0.1
23.1
1.2
48.9
1.9
13.3
15.0
-
6.8
49.7
1.8
13.8
12.0
0.2
7.0
48.0
47.8
1.9
2.9
17.2
13.9
10.9
12.8
0.1
-
5.0
6.5
e
1300÷ 1150
1438
1425
2
1250
1129÷ 1107
30, 60, 180, 300
liquid
1300
1122÷ 984
30, 60, 180
liquid
air NNO-2.9
1180
1146 1133 1110 1103 1098 1091
1.5, 2, 3
liquid
NNO+0.2 ÷ NNO–3.8
98.8
-
0.1
-
-
99.1
basaltic andesite
11.9
2.3
-
-
-
100.0
basalt
12.4
2.7
0.2
0.2
-
99.8
basalt
2.8
0.4
-
-
98.2
100, 933, 2191 1, 5, 100, 1000, 2000
basaltic andesite
-
10.9
NNO-5.3
1430
99.1
97.3
liquid + nuclei*
basaltic andesite
-
-
liquid
NNO-4.8 NNO-0.8 NNO NNO+4.2
1200
0.1
0.6
NNO-0.8
1620
102.3
2.1
-
basalt
-
3.5
-
1210
basalt
-
10.5
2.8, 5.7, 18.7, 72.4, 231
-
310÷ 401
-
u o
J
l a 0.1
100.4
r P
f o
log fO2
trachybasalt
basalt
air
Journal Pre-proof
authors
Lesher et al. (1999)
Lofgren et al. (2006)
Lofgren and Lainer (1991) Lofgren et al. (1974) Lofgren (1977) Lofgren et al. (1978) Lofgren et al. (1979) Lofgren and Smith (1980)
Table 1S (continued) (to be deposited as supplementary). Bulk compositions (wt.%) and experimental conditions of basaltic melts from previous solidification experiments ΔT/Δt pre-cooling SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O Cr2O3 total TAS Ti (°C) Tf (°C) (°C/h) phases 48.7
2.9
14.0
11.6
-
6.5
11.0
2.8
0.4
-
-
97.9
48.4
3.0
13.9
12.0
-
6.6
11.0
2.8
0.4
-
-
98.3
48.2
3.0
13.6
12.6
-
6.6
10.8
2.8
0.4
-
-
98.4
47.6
2.9
13.5
13.3
-
6.5
10.7
2.8
0.4
-
-
98.1
48.2
2.9
13.8
12.8
-
6.4
10.9
2.6
0.3
-
-
98.3
48.5
1.1
14.0
13.6
0.2
8.0
11.1
2.2
0.1
-
-
98.8
46.2
0.2
3.6
14.2
0.4
30.4
1.8
0.9
0.1
-
0.4
98.1
43.3
2.0
9.3
10.1
0.1
10.7
22.8
-
-
e
0.2
-
0.1
r P
99.7
picro basalt
0.1
0.1
-
0.4
99.3
basalt
0.4
-
0.1
99.5
basalt
0.4
0.2
0.2
0.3
99.9
basalt basalt
2.0
9.3
10.2
0.1
10.5
23.0
48.0
1.8
11.2
18.0
0.3
9.2
10.3
48.8
1.2
20.5
7.7
-
7.8
46.2
1.3
17.6
11.6
0.2
10.3
47.5
1.7
10.1
20.7
0.3
u o
0.6
10.2
0.2
-
-
0.4
99.8
49.1
1.4
16.9
8.1
0.2
9.3
3.4
0.5
0.2
-
100.0
J 8.7 7.2
11.6
-
l a
rn
0.2
98.7
1199÷ 1194
f o
o r p picro basalt
0.2
44.3
12.5
basalt
1265
log fO2
1173÷ 1006
10, 50, 100, 1000, 2000
liquid
1000÷ 1225
5, 250, 100, 1000, 3000
liquid
NNO-3.6
NNO-0.8
1400÷ 1250
1000
5 2÷2100
liquid liquid+nuclei
NNO-4.8÷ NNO-3.6; NNO-2.8; > NNO-0.8; NNO-4.5
1280÷ 1240 1300÷ 1280 1280÷ 1230
1085÷ 1042
1.2÷1260
liquid
NNO-5.8
1000
2, 4.5, 33.6, 5
liquid liquid+plg
935÷ 890
2.6÷1.8
liquid
NNO-5.3
1225
925÷ 980
4÷136
liquid
basalt
1215÷ 1180
1000
0.5, 2
liquid
< NNO-0.8
0.5, 2, 5, 10, 50 5, 100, 2500, 2000
liquid, liquid+ol liquid+ol+plg
NNO-0.6
liquid liquid+opx+ol
NNO-4.8
50.6
1.4
16.6
8.2
-
7.5
9.2
3.2
0.5
0.2
-
100.0
Lofgren (1983)
51.9
1.2
15.9
8.3
0.2
7.0
8.7
3.0
0.8
0.2
0.1
99.5
basalt
1300÷ 1180
1012÷ 899
Lofgren and Russell (1986)
55.2
0.1
2.8
9.6
0.3
28.9
1.5
0.7
0.1
-
0.6
99.7
basaltic andesite
1525÷ 1425
1225÷ 1125
Journal Pre-proof authors
Lofgren (1989)
Mollo et al. (2012) Mollo et al. (2013a, 2013c) Mollo et al. (2013b) Nabelek et al. (1978) Ni et al. (2014) Pupier et al. (2008) Sato (1995) Schiffman and Lofgren (1982) Sossi and Neill (2016) Toplis et al. (1994)
Toplis and Carroll (1995)
Table 1S (continued) (to be deposited as supplementary). Bulk compositions (wt.%) and experimental conditions of basaltic melts from previous solidification experiments ΔT/Δt pre-cooling SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O Cr2O3 total TAS Ti (°C) Tf(°C) (°C/h) phases 56.5
-
4.5
8.0
-
27.6
1.9
1.0
0.1
-
-
99.7
53.3
0.1
1.9
17.5
0.3
24.2
1.2
0.5
0.1
-
0.3
99.4
1600 1585 1570 1550
1300÷ 975
5, 100, 1850
liquid
NNO-4.8
1250
1100
2.5, 10, 50, 150
liquid
NNO-0.8; NNO+1.5
trachybasalt
1250
1100
2.5, 10, 50
liquid
NNO-0.8; NNO+1.5
basalt
1250
1000
30, 126,180, 564, 900
liquid
NNO+1.5
99.5
basalt
1320÷ 1265
994÷ 1001
10, 2
liquid liquid + plg
NNO-5.1
-
98.6
basalt
1240
600
50÷2000
liquid
-
-
97.2
basalt
1291÷ 1186
1179÷ 1109 1100÷ 1050 1000÷ 850 1434.5÷ 1350
1, 0.2, 3
liquid
NNO-0.8
1800÷10
liquid
NNO-0.8
2400÷0.5
liquid
NNO-0.8
30, 150, 90, 390, 360
liquid
NNO-2.4
basalt 54.8
-
3.7
16.2
-
21.9
1.2
0.7
0.1
-
-
99.0
46.9
-
5.7
12.5
0.1
28.9
2.5
0.9
0.1
-
-
97.6
42.0
2.6
9.8
10.0
0.1
11.2
22.9
n.d.
-
0.1
0.2
98.9
54.5
0.7
17.2
6.1
0.2
7.1
11.3
1.9
0.9
0.1
-
100.1
49.7
1.4
15.3
8.6
0.2
7.0
11.5
3.8
1.4
0.4
-
99.5
47.6
1.7
16.8
10.6
0.2
6.5
11.1
3.3
1.8
0.5
-
48.8
1.2
20.5
7.7
n.d.
7.9
12.5
0.6
0.4
-
0.1
49.4
0.8
15.8
7.7
0.2
8.0
12.7
2.3
48.8
-
14.9
13.1
-
6.5
10.9
2.7
52.6
1.3
14.5
12.0
0.2
4.6
10.1
53.6
1.9
13.7
9.3
0.2
4.5
45.2
0.2
3.7
10.9
0.2
49.5
4.3
11.5
14.6
-
48.8
2.9
14.9
13.1
49.5
4.3
11.5
48.7
4.3
11.5
l a
log fO2
r P
f o
o r p
e 100.0
basaltic andesite
1.9
-
0.4
-
-
97.7
8.1
rn
0.3
3.0
1.1
0.3
-
98.7
32.2
5.3
0.4
0.2
-
0.5
100.3
basalt
1500
4.8
10.0
2.9
0.5
-
-
98.1
basalt
1130
1072
3
liquid
NNO+1.2 NNO-2.8
-
6.5
10.9
2.7
0.3
-
-
100.1
14.6
-
4.8
10.0
2.9
0.5
-
-
98.1
basalt
1300
1158÷ 1050
3, 5, 10, 20
liquid
16.2
-
4.6
9.7
3.0
0.5
-
-
98.5
NNO-0.8, NNO+0.2, NNO-1.8, NNO-2.8
u o
J
2.0
basaltic andesite basaltic andesite
1200 1140
Journal Pre-proof Table 1S (continued) (to be deposited as supplementary). Bulk compositions (wt.%) and experimental conditions of basaltic melts from previous solidification experiments authors
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O
Cr2O3
total
TAS
Ti(°C)
Tf(°C)
ΔT/Δt (°C/h)
pre-cooling phases
log fO2
Usselman et al. (1975)
42.3
10.6
8.8
17.6
0.2
7.9
10.8
0.9
0.3
-
0.2
99.5
picro basalt
1250
1125÷ 920
2÷210
liquid
NNO-5.1 ÷ NNO6.2
48.0
1.0
15.6
10.2
0.2
9.4
13.2
1.8
-
0.1
-
100.7
basalt 1300
800
1, 7, 60, 180, 1800, 9000
liquid
air
1187÷ 1157
1162÷ 1101
0.16÷ 100.44
liquid
-
1162÷ 1059 1080÷ 600
0.5÷ 2000
liquid
-
NNO-5.3
NNO-5.8
This study (B100) Vetere et al. (2013, 2015) Vona and Romano (2013) Walker et al. (1976)
Walker et al. (1978)
Zieg and Lofgren (2006)
53.0
0.8
15.0
9.5
0.2
7.6
10.8
2.2
1.0
-
-
100.0
45.8
0.4
8.4
12. 2
0.2
23.0
8.7
0.9
0.2
-
0.5
100.3
f o
basaltic andesite
o r p basalt
50.4
0.9
17.2
1.7
0.2
6.3
11.6
2.3
1.9
0.5
-
99.2
48.2
1.7
17.1
2.5
0.2
5.4
10.3
3.7
1.9
0.4
-
99.4
1182÷ 1131
43.8
2.8
8.0
22.0
0.3
13.5
8.2
0.2
-
-
0.7
99.4
picro basalt
1350
43.7
2.9
8.2
21.8
0.4
12.5
8.4
0.1
-
-
0.8
98.7
43.1
2.4
8.8
22.6
-
9.9
9.3
-
-
-
-
e
trachybasalt
96.2
44.0
3.3
9.6
21.3
-
8.7
10.0
-
-
-
96.9
46.0
3.5
10.2
19.5
0.3
7.7
10.4
-
0.6
98.4
46.0
3.8
11.1
18.8
0.3
45.6
3.8
11.4
19.3
0.3
45.6
4.4
11.1
19.6
0.3
44.9
4.7
10.8
19.8
0.4
49.2
1.0
12.2
18.1
49.8
0.1
7.2
14.4
7.0
rn
l a
11.2
u o
0.2 -
-
r P -
0.5
picro basalt
98.7 basalt
6.4
11.2
0.1
-
-
0.4
98.5
6.2
11.6
0.1
-
-
0.4
99.4
6.0
11.5
0.2
-
-
0.4
98.7
picro basalt
0.5
7
10.6
0.5
0.1
0.1
-
99.8
basalt
0.1
24.5
3.2
1.1
0.1
-
-
100.5
basalt
J
1200÷ 1149
1159÷ 806
0.5÷2000
liquid liquid+px +plg+sil+sp +ilm+sulf
1545
1515÷ 1176
92
liquid+ol
Footnotes: plg = plagioclase, ol = olivine, px = pyroxene, opx = orthopyroxene, sil = silica, sp = spinel, ilm = ilmenite,sulf = sulphide; Ti is the initial temperature of cooling (regardless of the heating treatment adopted) and Tf is the quenching temperature. When the temperatures and/or the cooling rates are not clearly explained in the paper, we have used the symbol “”. All the cooling rates have been converted in °C/h, log fO2 is expressed as a function of NNO buffer and the total amount of Fe is reported as FeO. The compositions are plotted in the TAS diagram according to La Maitre (2000), with the relative thermal paths (Fig. 1). The starting material B100 has density of 2.76 g/cm3, the liquidus temperature (Tm) is 1233°C, the glass transition (at 1012 Pa/s) occurs at Tg of 651°C and Trg is 0.53 (Vetere et al. 2015).
Journal Pre-proof 3. Cooling of basaltic solidifying bodies with variable thicknesses Previous cooling rate studies on basaltic lava flows/lakes and domes quantified only the shallower thermal conditions of the cooling bodies and, thus, they principally refer to the heat released at the outermost surface (Flynn and Mouginis-Mark, 1992; Cashman, 1993; Hon et al., 1994; Cashman et al., 1999; Witter and Herris, 2007; Kolzenburg et al., 2016). On the other hand, 2D and 3D numerical models can properly simulate the heat released from a cooling basaltic body (e.g., a lava flow) once the melt composition, lava thickness/volume and the temperature of the
of
surrounding medium(s) are quantified (Brandeis et al., 1984; Brandeis and Juapart, 1987; Neri,
ro
1998; Wholetz et al., 1999; Patrick et al., 2004, 2005; Kattenjorn and Schaefer, 2008; Whittington
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et al., 2009; Nabelek et al., 2012; Blundy and Annen, 2016). We also stress that this approach has
re
been already applied to reproduce the cooling history of basaltic lava flows (Neri, 1998; Patrick et al., 2004, 2005).
lP
Following this line of reasoning, we performed 2D simulations by investigating basaltic melt
na
slices with variable thickness of 0.1, 0.5, 1 and 2 m, by assuming an infinite extension of the other two dimensions. According to the output rate and terrain slope constraints in nature, the selected
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thicknesses are reliable for aphyric tholeiites erupted on Earth from superliquidus to liquidus conditions (Griffiths, 2000). The numerical procedure follows the modelling approach already presented in Vetere et al. (2015). The simulated basaltic liquid layers are sandwiched between an overlying layer of air and an underlying rock substrate with temperature of 20 and 25 °C, respectively. The models start from an initial homogeneous temperature of the liquid of 1300°C. Each point within a silicate liquid slice cools from the initial temperature following a parabolic trend with ΔT/Δt that progressively decreases with T (Fig. 2). Notwithstanding, the simulated ΔT/Δt are approximated to be linear within the temperature interval 1300 °C - 600 °C. The heat release was simulated using the finite elements code implemented in the Heat 3D software (Wohletz et al., 1999), and the selected initial parameters, such as thermal conductivity (K), density (ρ) and specific heat (cp), are listed in Table 2S.
Journal Pre-proof This simplified approach accounts only for heat conduction, by ignoring any mechanism of irradiation and convection, as well as the latent heat of crystallization. Therefore, results from simulations and modeled ΔT/Δt (Fig. 2) are conservative with respect to natural solidifying melts, corroborating the numerical cooling dynamics reported by previous authors (Neri, 1998; Patrick et al., 2004, 2005; Petitjean et al., 2006). It is also important to note that cooling rates of lavas erupted beneath the surface of seawater are even larger than those simulated at air conditions (Fink and Griffiths 1990; Griffiths and Fink, 1992; Gregg, 2017 and references therein).
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Results of the thermal models shows that a basaltic slice of 0.1 m cools with rates in the order
ro
of 103 and 102 °C/h in the outer and inner portions, respectively (Fig. 2). In a basaltic layer with a
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thickness of 0.5 m, the cooling rates progressively decrease from 103 to ~101 °C/h (Fig. 2). For a
re
layer of 1 m, only the outermost millimetric-to-centimetric lava crust cools from 1300 °C with ΔT/Δt between 101 and 103 °C/h, whereas the innermost portion undergoes cooling rates of 101 <
lP
ΔT/Δt < 100 °C/h (Fig. 2). The model for a lava with thickness of 2 m yields ΔT/Δt of ~101 °C/h in
na
the outermost part, whereas the innermost part cools at rates of 100 °C/h or even lower (Fig. 2). These data indicate that most of the dynamic solidification experiments adopting high to moderate
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ΔT/Δt (Tab. 1S) can be considered representative of the crystallization conditions only for the outermost portions of basaltic liquid slices with thicknesses larger than 0.5 m or for those with thicknesses lower than ~ 0.5 m (Figs. 1 and 2).
Jo ur
na
lP
re
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ro
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Journal Pre-proof
Fig. 2. 2D numerical simulations of conductive cooling of virtual basaltic liquid layers with different thicknesses. Data were obtained through the Heat 3D code (Wholetz et al., 1999). The layers were cooled from 1300 to 600 °C, keeping the overlying air at 20 °C and the initial temperature of the underlying rock substrate at 25 °C. The thermodynamic parameters used for the simulations are listed in Table 2S.
Journal Pre-proof
Table 2S (to be deposited as supplementary) Physical parameters employed for cooling simulations overlying silicate liquid underlying parameter unit air layer rock 0.02 0.9 2.1 thermal conductivity K (W/mK) 1.1 2750 2600 density ρ (Kg/m3) 1004 1300 1019 specific heat cp (J/K kg) Ti °C 20 1300 25 Tf °C by simulation 600 by simulation
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Footnotes: boundary conditions for 2D numerical model of basaltic lava flows.The thickness mesh has size of 100 x 100, corresponding to a total area (m2) of 10 x 10 m (grid size = 0.1),with exception for the thickness of 0.1 m where the total area is 1 m x 1 m (grid size = 0.01).
ro
4. Experiments and analytical methods
-p
The bulk composition (B100) and experimental conditions reappraised in this study are those reported in Vetere et al., (2013, 2015). The starting glass was prepared from the tholeiitic basalt
re
cropping out in Iceland and named BIR 1A as USGS standard. The natural powdered rock was
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melted in air at temperature of 1600 °C for 4 h. After a rapid quench, the obtained glass was crushed and re-melted at the same condition to assure homogeneity. The bulk composition of B100
na
measured by microprobe is reported in Table 1S and plotted in Fig. 1. Using FTIR spectroscopy and
determined.
Jo ur
a modified Wilson's method, the residual H2O content (~ 50 ppm) and Fe2+/Fetot ratio (0.39) were
For each experiment, ~50 mg of the B100 starting glass fragments were loaded into Pt tubes successively welded shut. The Pt-charges were heated with a rate of 420 °C/h from ambient temperature to 1300 °C, then kept for 2 h at this T to homogenize the melt and remove potential heterogeneous sites of nucleation (Iezzi et al., 2008; Vetere et al., 2013, 2015). This temperature is 67 °C above the melting (liquidus) temperature Tm of 1233 °C. Then, the experiments were quenched at 800 °C close to the solidus temperature. The imposed linear cooling rates were 1, 7, 60, 180, 1800 and 9000 °C/h (Fig. 3S) at 1 atm and fO2 of air (Table 3S), thus approaching to the solidification conditions on Earth and beneath the surface of seawater. A further experiment was performed with the same heating rate and dwell treatment, but it was quenched directly in water
Journal Pre-proof providing a cooling rate ≥ 105 °C/h (E-quench in Table 3S). This run-product allow us to investigate the chemical features before the onset of cooling (Table 3S). The experiments are labeled with the value of the applied cooling rate: E9000, E1800, E180, E60, E7 and E1; experiments with cooling rates of 1 and 180 °C/h were duplicated to verify their reproducibility by Vetere et al. (2013, 2015). One of the two run products at 180 °C/h (E180a) was also subjected to a different heat treatment before cooling below 1300 °C. This charge was heated up to 1400 °C, cooled to 1300°C after 0.5 h, and maintained at this temperature for a dwell time of 40 h before cooling at 180 °C/h (Table 3S
of
and Fig. 3S). The total cooling time of the experiments (Table 3S) ranges from hours to three weeks
Jo ur
na
lP
re
-p
ro
(Fig. 3S).
Fig. 3S (to be deposited as supplementary). Thermal paths of solidification experiments conducted on B100. Redrawn from Vetere et al. (2015).
Journal Pre-proof
Table 3S (to be deposited as supplementary). Experimental conditions and phase proportions (area%) determined by image analysis. exp.
heating rate (°C/h)
Ti (°C)
dwell time (h)
ΔT/Δt (°C/h)
Tf (°C)
experimental time (h)
selected BS-SEM photos for image analysis
magnifications
glass
plg
cpx
sp
crystals
1.9 (0.0)
65.6 (6.6) 68.5 (7.7) 60.7 (4.3) 46.1 (2.3)
30.6 (5.6) 28.8 (7.9) 36.0 (4.5) 47.0 (3.2) 55.6 (3.5) 50.6 (1.5) 51.0 (1.3)
3.4 (0.9) 2.8 (0.3) 3.3 (0.4) 4.5 (0.6) 4.5 (1.3) 4.6 (0.8) 1.2 (0.0) 1.8 (0.8)
99.6 (0.8) 100.0 (0.0) 100.0 (0.0) 97.6 (0.8) 60.1 (2.6) 55.3 (1.1) 52.2 (1.3) 1.8 (0.8)
-
-
E1b
1
510.7
5
150 ÷ 400x
E1a
1
510.7
3
150x ÷ 400x
E7
7
72.6
6
150x ÷ 400x
E60
60
13.4
5
1300
E180a
420
1400°C for 0.5h 1300°C for 40h
800 180
47.8
E180b
180
8.8
E1800
1800
5.3
1300
rn
2
E9000
9000
E-quench
≥ 105
u o
1300
J
l a
5.1 3.2
e
r P 3 3
f o
o r p
2
400 ÷ 800x
400 ÷ 800x 400 ÷ 800x
2
800 ÷1500x
2
800 ÷1500x
-
-
-
2.4 (0.8) 39.9 (2.6) 44.7 (1.1) 47.9 (1.3) 98.3 (0.7) 100
-
-
-
-
textural features
faceted
dendritic
glassy
Footnotes: Data in bold are those relative to the most reliable run products that were selected according to the criteria reported in Vetere et al. (2013, 2015).
Journal Pre-proof 5. Analytical methods and textural quantification A more comprehensive description of the analytical methods is reported in Vetere et al. (2013, 2015). In summary, the recovered run products were mounted in epoxy, ground flat and polished. Micro-chemical characterization was carried out with an electron probe micro-analyzer (EPMAWDS) and field-emission scanning electron microscopy (SEM-EDS), while the textural characteristics were obtained by back-scattered (BS) SEM microphotographs. Following previous textural measurements (Higgins, 2006; Iezzi et al., 2008, 2011, 2014;
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Lanzafame et al., 2013, 2017; Vetere et al., 2013, 2015), we collected 11 to 21 BS-SEM images for
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each run product, corresponding to a total of 115 BS-SEM microphotographs with magnifications
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of 150×, 400×, 800×, 1500× and 3000×. In addition, some run-products (E1b and E1a) were
re
characterized by BS-SEM semi-quantitative maps with 30 and 10 images for a total area of 0.4 and 0.1 mm2, respectively. The most representative microphotographs were selected for quantitative
lP
textural analysis at three different magnifications (Table 3S) to obtain reliable results on both large
na
and small crystals. The image analysis was performed on 30 BSE microphotographs using the Image-ProPlus software (http://www.mediacy.com/imageproplus). Each BS-SEM microphotograph
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was first digitalized, calibrated and converted into grey scale. We did not apply filters nor stereological correction. Crystalline and non-crystalline phases were segmented as a function of their grey level range and then binarized with different false colors. An example of image treatment is reported in Fig. 4. Following Higgins (2006) and Iezzi et al. (2011), each crystal is described by a best-fitting equal-area ellipse. Dendritic phases were considered as individual crystals (Hammer et al., 2010; Shea and Hammer, 2013). Touching crystals and dendrites with different orientations were manually identified and separated. We determined for each measured crystalline phase the phase abundance (area%), area (μm2), perimeter (μm), length and width (μm) as major and minor axes of the equal-area ellipse, and the aspect ratio (major axis/minor axis). The number of crystals per area (#/A) was also computed. Starting from the 2D crystal size habit, the 3D crystal habit was calculated though the CSD-slice
Journal Pre-proof program of Morgan and Jerram (2006). The computed 3D shapes and size dimensions were used as input for the CSD calculations implemented in the CSD-Corrections 1.53 package (Higgins, 2000). Each CSD curve was constructed using 2 to 4 BS-SEM microphotographs and 7 bins per decade (Higgins 2000, 2006; Armienti 2008). Run-products with crystal content < 5 area% were not considered for measurements of 2D size, shape, aspect ratio and CSD plot. All textural data are
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na
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re
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reported in the excel spreadsheet as supplementary material.
Fig. 4. Example of image analysis on two representative run-products with faceted (E60) and dendritic (E1800) textural features. The false colors blue, red, green and yellow correspond to plg, cpx, sp and glass, respectively. The blue bar corresponds to10µm.
6. Results The main crystalline phases solidified from B100 are plg, cpx and sp (Fig. 5). Run-products cooled at the same rate, E1a-E1b and E180a-E180b (experiencing variable heat-treatments), show reproducible textures for all the crystalline phases (Fig. 5 and Table 3S). The preferential crystallization on the walls of Pt capsules involves is extremely limited in space without altering the overall textural features of the run-products (Fig. 5). This agrees with results from Mollo et al.
Journal Pre-proof (2012), which evidence that at high fO2 conditions, the effect of Pt on the crystal nucleation is negligible. The most reliable run-product between two duplicate sample charges cooled at the same ΔT/Δt has been considered the one with less crystals (Vetere et al., 2015), i.e. E1b and E180b (Tab. 3S). In the following, only these two run-products are considered together with E7, E60, E1800 and E9000. The quantitative evolution of crystal content is shown in Fig. 6 and listed in Table 3S. The total crystal content decreases as ΔT/Δt increases without a monotonic trend. The run products are
of
fully crystallized at ΔT/Δt ≤ 60 °C/h. Conversely, at ΔT/Δt ≥ 60 °C/h, a marked decrease of total
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crystal content occurs. The trend flattens at 180 and 1800 °C/h with a further decrease to ~ 2 area%
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when approaching 9000 °C/h (Fig. 6). The run-products cooled at rates of 1, 7 and 60 °C/h show
re
prevailing faceted crystals, while those produced at rates of 180 to 9000 °C/h are dendritic with fern leaf shapes (Fig. 5). The formation of sp and cpx takes place from 1 to 9000 °C/h, while plg occurs
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only at ΔT/Δt ≤ 60 °C/h. At low cooling rates, cpx and plg exhibit an extensive intergrowth feature
na
(Fig. 5). A very low amount (≤ 1 area%) of melilite, with the typical H-shaped habit, also occurs at the intermediate cooling rates of experiments E180a-E180b (Fig. 5). Because of its paucity, melilite
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is not considered in the textural analysis.
The plg content linearly and slightly decreases from 66 to 46 area% as ΔT/Δt varies from 1 to 60 °C/h, while the cpx content progressively increases from 31 to 51 area% as ΔT/Δt varies from 1 to 1800 °C/h and then cpx is barely detectable at 9000 °C/h (Fig. 6 and Table 3S). The content of sp is always ≤ 5 area%. The glass increases from 0 to 98 area% as ΔT/Δt varies from 7 to 9000 °C/h (Fig. 6, Table 3S). The matrix glass (i.e. at ≥ 50 µm from crystal rims) is present only in E9000, whereas intra-crystalline glass (i.e. at < 50 µm from crystal rims) occurs in E1800 and E180 (Fig. 5).
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Fig. 5. BS-SEM microphotographs of the run-products from Vetere et al. (2013, 2015); run products in bold are the more reliable ones (Table 3S). The magnification per run-product increases from left to right. From black to white grey levels the phases are glass (black), plg, cpx and sp. The blue bars correspond to 10 μm and the red ones to 100 μm. Some of these BSE microphotographs have been already shown in Vetere et al. (2015) and are re-plotted to have a complete picture of the sample evolution.
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Fig. 6. Crystal content of run-products (area%) as a function of cooling rate (°C/h). The 2D size of plg changes from 101-103 to 100-102 µm at cooling rates from 1 to 60 °C/h, respectively (Fig. 7). The maximum 2D size decreases from 949 to 109 µm, while the average size (calculated as the whole range of crystal size) decreases from 70 to 10 µm (Table 4S). The 2D size of cpx is in the range of 101-102 µm at 1 and 7 °C/h, 100-102 µm at 60 and 180 °C/h, and 100-101 µm at 1800 °C/h (Fig. 7), with maximum and average sizes progressively decreasing from 398 to 25 µm and from 36 to 4 µm, respectively (Table 4S). The 2D size of sp is in the range of 100-102 µm at 1 and 7 °C/h, 100-101 µm at 60 °C/h, 100-102 µm at 180 °C/h, and 100-101 µm at 1800 °C/h. The
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ΔT/Δt increases from 1 to 9000 °C/h (Table 4S).
Fig. 7. 2D sizes of major and minor axes from the equal-area ellipse (see data in Table 4S). Only linear regression analyses with R2 ≥ 0.6 and run-products with a crystal content ≥ 5 area% were considered.
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Table 4S (to be deposited as supplementary). Maximum (Lmax), minimum (Lmin) and average (Lav) 2D crystals size (µm). plg cpx sp ΔT/Δt crystal exp. (°C/h) Lmax Lmin Lav Lmax Lmin Lav Lmax Lmin Lav shape E1b
1
949
10
70
398
8
36
89
6
28
E7
7
751
5
41
264
5
21
76
4
13
E60
60
109
1
10
104
1
7
20
1
4
E180b
180
-
-
-
104
2
13
65
1
5
E1800
1800
-
-
-
25
0.5
4
6
0.5
2
E9000*
9000
-
-
-
-
-
-
7
0.2
0.8
faceted
dendritic
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Footnote: cpx and sp from E900 were considered together due to their very small size.
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The relative crystal size variability of plg, cpx and sp is displayed in Fig. 8. For each runproduct, the frequency of the long size was calculated using 100 bins with dimension of 10, 4 and 1
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μm, in the maximum range of 0-1000, 0-400 and 0-100 μm for plg, cpx and sp, respectively. With
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increasing ΔT/Δt, the size frequency of plg, cpx and sp generally shifts from large to small crystal
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dimensions. The decreasing number of large sizes is counterbalanced by an increase of tiny crystals. As the cooling rate increases from 1 to 60 °C/h, the plg major axis is characterized by: at 1 and 7 °C/h, there is a major peak at ~ 20 μm; a few percent of crystals have size < 10
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μm and more than 35 % of crystals have size ≥ 25 μm; at 60 °C/h, an exponential decrease in size starts from the major peak at 5 μm and is followed by a few percent of crystals with size > 10 and < 50 μm. Note also the lack of plg crystals with size > 50 μm.
As ΔT/Δt increases from 1 to 1800 °C/h, the size of cpx changes as follow:
at 1 and 7 °C/h there is an overall absence of crystals with size < 5 μm, with major peaks at ~ 10 μm and a significant amount of crystals with size between 10 and 50 μm;
at 60 and 180 °C/h, the two cpx populations have size > 2.5 and ≤ 10 μm, respectively;
at 1800 °C/h, ~ 75 % of dendritic crystals show size of ~ 2.5 μm.
The textural change of sp can be described as:
at 1 °C/h, an almost constant distribution of size between 5 and 50 μm;
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at 7 °C/h, the crystal population peak is between 5 and 10 μm;
at 60 and 180 °C/h, the crystal population peak corresponds to 2.5 μm and the maximum size is ~ 5-7 μm; at 1800 °C/h, most crystals are equal to or lower than 2.5 μm and they do not exceed 5 μm
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Fig. 8. Relative variation (%) of the major axis for the equal-area ellipse of plg, cpx and sp (Table 4S). 100 classes of 10, 4 and 1 μm were considered for plg, cpx and sp, respectively. The plot refers to classes with frequency > 2 % and run-products with crystal content ≥ 5 area%.
The 2D crystal habit, computed as aspect ratio between major and minor axes of equal area ellipses (Higgins, 1994), displays weak trends as a function of ΔT/Δt for plg and cpx (Fig. 9). The maximum range of 2D crystal habit varies from 1 to 7 for all the crystalline phases. Plg shows a low
Journal Pre-proof variability of the 2D habit, with the majority of crystals with values between 1 and 3 at 1, 7 and 60 °C/h (Fig. 9). For faceted cpx crystals, the 2D habit becomes less equant as the cooling rate increases from 1 to 60 °C/h. Conversely, dendritic cpx crystals obtained at 180 and 1800 °C/h show prevalent equant shapes (Fig. 9). Faceted sp has a 2D habit more equant from 1 to 60 °C/h, while dendritic sp at 180 and 1800 °C/h shows a significant number of crystals with high 2D ratio;
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overall, sp crystals become less equant as the cooling rate increases (Fig. 9).
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Fig. 9. Relative variation (%) of 2D aspect ratio for plg, cpx and sp. We considered 40 classes with 2D aspect ratio from 1:1 to 1:40. The plot refers to classes with frequency > 2 % and run-products with crystal content ≥ 5 area%. The number of crystals per area (#/A) of each crystalline phase is reported in Table 5 and plotted in Fig. 10. The value of #/A for plg increases from 10-4 to 10-2 µm-2 as the cooling rate increases from 1 to 60 °C/h. Faceted cpx crystals show increase of #/A from 10-4 to ~ 10-2 µm-2 as
Journal Pre-proof the cooling rate increases from 1 to 60 °C/h. At higher cooling rates, #/A significantly decreases to an intermediate value between 10-3 and 10-2 µm-2. Then, #/A of cpx increases up to 10-1 µm-2 at ΔT/Δt of 180 and 1800 °C/h (Fig. 10 and Table 5). The value of #/A for sp increases from above 105
to ~ 10-1 µm-2 as the cooling rate increases from 1 to 1800 °C/h, with an almost constant value at
ΔT/Δt from 60 (faceted) to 180 °C/h (dendritic) (Fig. 10 and Table 5). In summary, the number of faceted crystals per area increases of about two orders of magnitude as the cooling rate increases from 1 to 60 °C/h, according to the sequence cpx > plg > sp. At ΔT/Δt of 60 and 180 °C/h, the
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number of crystals per area is nearly constant or decreases when faceted shapes are substituted by
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dendrites and the crystallization of plg is suppressed. As the cooling rate increases, the value of #/A
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recovered at 180 and 1800 °C/h is similar for cpx and sp, while the crystal abundance (area%)
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significantly decreases (Figs. 6 and 10, Tabs. 3S and 5).
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Fig. 10. Variation of the number of crystals per area as a function of cooling rate. The plot refers to run products with a crystal content ≥ 5 area% (see data in Table 5).
exp.
Table 5: number of crystals (#) per area (µm-2); standard deviations are reported in parenthesis. ΔT/Δt crystal plg cpx sp (°C/h) shape
E1b
1
1*10-4 (4*10-5)
2*10-4 (1*10-4)
5*10-5 (3*10-5)
E7
7
3*10-4 (2*10-4)
1*10-3 (1*10-3)
2*10-4 (1*10-4)
E60
60
1*10-2 (1*10-2)
3*10-2 (2*10-2)
4*10-3 (2*10-3)
E180b
180
-
4*10-3 (1*10-3)
4*10-3 (2*10-3)
E1800
1800
-
1*10-1 (4*10-2)
5*10-2 (2*10-2)
E9000*
9000
-
faceted
dendritic
5*10-3 (1*10-3)
Footnote: cpx and sp from E9000 were considered together due to their very small size.
Journal Pre-proof 7. Discussion 7.1 MORB textural similarities between experimental and natural products The qualitative effects of cooling conditions on texture, composition and mineralogy of solid phases growing in basaltic lava flows/lakes, pillow lavas and dikes have been recognized since long time (Lofgren, 1980; Kirckpatrick, 1981; Cashman, 1989; Lasaga, 1998; Hammer, 2008; Zhang, 2008; Iezzi et al., 2009; Mollo and Hammer, 2017). The outermost and rapidly cooled portions of lavas are typically glassy and host tiny and dendritic minerals, while the innermost parts are
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characterized by abundant crystalline phases with faceted shapes and larger size (Muir and Tilley,
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1966; Bryan, 1972; Mazullo and Bence, 1976; Coish and Taylor, 1979; Schiffman and Lofgren,
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1982; Batiza and Vanko, 1984). The evolution of textures displayed in Fig. 5 mirrors those reported
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in these studies.
Two studies analyzed in detail the textures of natural MORBs. Lesher et al. (1999)
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investigated a tholeiitic drilling section composed of a series of aphyric and poorly vesiculated
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MORB flow units with thickness between 0.1 and 10 m. The top of these flows is characterized by dendritic cpx intergrowth with plg forming bands of tiny crystals. The innermost portions of the
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meter-thick flows were composed by mm-sized plg intergrown with cpx, with a typical intersertal texture (see Figs. 5a and 5b of Lesher et al., 1999). These textures of natural MORBs are very similar to our experimental runs (i.e., E7, E1a and E1b) at the lowest cooling rates (Fig. 5). In addition, Lesher et al. (1999) complemented their investigations with equilibrium plus kinetic cooling experiments adopting rates from 10 to 2000 °C/h and ΔT from 20 to 190 °C (see Table 3 in Lesher et al., 1999). An increase of ΔT/Δt resulted in the reduction of their plg size while faceted crystals formed only at cooling rates < 50 °C/h, thus resembling the textual features showed in Figs. 5, 6, 7 and 11. On the other hand, Zhou et al. (2000) provided very accurate investigations at extremely high cooling conditions to reproduce textural features in the outermost portions of a MORB-type pillow lava. The authors estimated ΔT/Δt in the order of 102 to 103 °C/h, which are values similar or higher
Journal Pre-proof than those experimentally discussed in this study (Figs. 2 and 3S). Zhou et al. (2000) reported that the crystal phases within 0.3 cm of the pillow rim (i.e., glassy margin) mainly consists of plg and cpx, with minor proportions of small sp grains. Conversely, within the crusty, glassy patches outside the pillow rim, only tiny titanomagnetite grains develop with a maximum size of a few tens of nanometers. The glassy crust above the pillow rim consists of massive basaltic glass resulting from an extremely rapid cooling rate. It is worth stressing that these textural features match with those observed in the experiments solidified at 1800 and 9000 °C/h (E9000 showed in Fig. 5), thus
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validating the overall textural parameters obtained in laboratory at very fast cooling conditions.
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Overall, the similarities between run-products reported in Fig. 5 with natural MORB rocks
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investigated by Zhou et al. (2000) and Lesher et al. (1999) at high and sluggish cooling conditions,
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7.2. Crystal size and 3D shape
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respectively, corroborate the quantitative evolution of textural attributes discussed below.
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The number (expressed in percentage) of plg, cpx and sp crystals has been divided into three classes as a function of the 2D crystal size (Fig. 11 and Table 6): long (> 100μm), medium (100-10
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μm) and short (< 10 μm) classes. At 1 and 7 °C/h, ~ 10 %, ~ 80-90 % and 0-10 % of plg crystals show long, medium and short sizes, respectively. At 60 °C/h, ~ 30 % of plg crystals have medium size, whereas ~70 % of plg crystals have short size (Fig. 11 and Table 6). At 1 and 7 °C/h, ~ 6-1 %, ~ 86-68 % and 8-31 % of cpx crystals show long, medium and short sizes, respectively (Fig. 11 and Table 6). At higher cooling rates, cpx crystals have percentages of 17, 37 and 5 % (medium size) and 83, 63 and 95 % (short size) at 60, 180 and 1800 °C/h, respectively (Fig. 11 and Table 6). The suppression of plg at ΔT/Δt between 60 and 180 °C/h is accompanied by the formation of a large amount of cpx dendrites with medium size at 180 °C/h, whilst almost faceted cpx crystals are present at 60 °C/h (Fig. 11). Sp crystals have size < 100 µm (i.e., only medium and/or short classes). The amount of sp with medium size decreases from 91 to 8 % as the cooling rate increases from 1 to 180 °C/h (Fig. 11 and Table 6). This trend is counterbalanced by the increasing percentage of sp
Journal Pre-proof crystals with short sizes, varying from 9 to 100 % with increasing cooling rate (Fig. 11 and Table 6). The evolution of sizes as a function of ΔT/Δt reported in Fig. 11 enlarges the previous literature data. These data cover the variable solidification conditions suffered inside MORB liquid sheets of
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Fig. 11. Size frequency as a function of cooling rate (see data in Table 4S and 6).
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Table 6. 2D size frequency and 3D aspect ratio computed by CSDslice (Morgan and Jerram, 2006)
E180b
180
1800
aspect
R2
sp
aspect
R2
crystals (%)
aspect
R2
>100
12.7
0.2:0.8:1
0.5
6.2
0.1:0.3:1
0.8
-
-
-
100÷10
87.3
0.2:0.3:1
0.9
86.3
0.3:0.5:1
0.8
90.7
0.4:0.5:1
0.6
<10
-
-
-
7.5
0.5:0.9:1
0.9
9.3
0.6:0.9:1
0.7
100
0.1:0.2:1
0.6
100
0.3:0.6:1
0.9
100
0.1:0.2:1
0.6
9.6
1:1:1
0.4
1.2
0.3:0.6:1
0.6
-
-
-
100÷10
79.8
0.1:0.2:1
0.9
67.9
0.3:0.7:1
0.8
52.3
0.5:0.7:1
0.9
<10
10.6
0.4:0.6:1
0.9
30.9
0.4:0.7:1
0.9
47.7
0.5:0.6:1
0.9
all
100
0.1:0.2:1
0.9
100
0.2:0.4:1
0.9
100
0.5:0.7:1
0.9
>100
-
0.1:0.9:1
0.4
-
-
-
-
-
100÷10
31.2
0.2:0.5:1
0.8
17.1
0.1:0.3:1
0.7
6.4
0.7:0.8:1
0.7
<10
68.7
0.3:0.6:1
0.9
82.9
0.3:0.6:1
0.8
93.6
0.6:0.7:1
0.8
all
100
0.2:0.4:1
0.9
100
0.2:0.5:1
0.8
100
0.6:0.7:1
0.8
of
all >100
-
>100
-
-
-
-
-
-
-
-
-
100÷10
-
-
-
36.9
0.3:0.8:1
0.8
8.3
0.10:0.6:1
0.8
<10
-
-
-
63
0.4:0.7:1
0.8
91.7
0.3:0.4:1
0.8
all
-
-
-
>100
-
-
-
100
0.4:0.8:1
0.8
100
0.2:0.4:1
0.8
-
-
-
-
-
-
100÷10
-
-
-
5.3
0.3:0.6:1
0.8
-
-
-
<10
-
-
all
-
-
-
94.7
0.4:0.7:1
0.9
100
0.1:0.3:1
0.8
-
100
0.4:0.7:1
0.9
100
0.4:0.5:1
0.8
crystal shape
faceted
dendrites
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E1800
60
cpx crystals (%)
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E60
7
plg crystals (%)
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E7
1
ΔL (μm)
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E1b
ΔT/Δt (°C/h)
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exp.
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Footnote: S = short axis, I = intermediate axis, L = long axis (S ≤ I ≤ L); the aspect ratio is expressed as S : I : L. E9000 is not reported due to its very low crystal content (< 2 area%) and very small crystal size (cpx and sp sizes are < 10μm).
An important aspect of kinetic studies conducted under variable solidification conditions concerns the shape of crystals (Higgins, 2006). The 2D crystal size is by far the most analyzed parameter, corresponding to the areal intersection of a 3D crystal with the cutting plane of the section. On the other hand, only few direct 3D tomographic investigations have effectively related the crystal shape and kinetic effects (Higgins, 2006; Lanzafame et al., 2017; Polacci et al., 2018). The 3D shape of crystals can be derived from 2D measurements by adopting different stereological approaches (Underwood, 1970; Higgins, 2006; Morgan and Jerram, 2006). In this study, we have calculated the textural parameters L, I and S for the long, intermediate and short crystal axes (L > I > S) following the method of Morgan and Jerram (2006). These data are listed in Table 6 and displayed in Fig. 12 as Zingg plots (Zingg, 1935). Faceted plg crystals exhibit a simple trend in
Journal Pre-proof which the 3D shape evolves from prismatic to tabular as ΔT/Δt increases from 1 to 60 °C/h. By contrast, faceted cpx crystals are never related to ΔT/Δt, while dendritic cpx exhibit shapes clustered in the tabular field. Faceted sp crystals are either prismatic or cubic, whereas dendritic sp are gathered between tabular and prismatic areas (Fig. 12). By considering the large variation of ΔT/Δt and ΔTc of the experiments (Fig. 1 and Table 1S), we can conclude that 2D and 3D crystal shapes of cpx and sp are poorly sensitive to the kinetic effects at least at high fO2, with the only
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Fig. 12. Calculated 3D shape of mineral phases (see data in Table 6). L, I and S correspond to the long, intermediate and short axes, respectively (L > I > S).
Journal Pre-proof 7.3. Crystal size distribution analysis The crystal size distribution analysis is routinely used for describing either experimental or natural solidification processes involving the crystallization of plg, cpx and sp (Cashman and Marsh, 1988; Cashman, 1993; Marsh, 1998; Higgins, 2000, 2006; Zieg and Marsh, 2002; Jerram et al., 2003; Armienti, 2008; Pupier et al., 2008;Hammer, 2009; Iezzi et al., 2011; Vona et al., 2011; 2017; Lanzafame et al., 2013; Ni et al., 2014). CSD plots (size vs log of population density per size) obtained in this study under variable ΔT/Δt conditions (Figs. 1, 2,3 and 5) are displayed in Figs. 13
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and 14. As a general rule, CSDs of plg, cpx and sp show progressive increase in slope (m) and
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population density per size (n0 in ln corresponding to the y axis intercept) with increasing ΔT/Δt.
at 1 and 7 °C/h, the trend is composed of two log-linear segments with similar values for
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For the CSD of plg (Figs. 13 and 14Sa):
the intercept; at 7 °C/h the size range is slight smaller than that at 1 °C/h, exhibiting a
at 60 °C/h, the trajectory depicts three log-linear segments with an increasing m and a
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downturn at the smaller crystal size;
decreasing of the crystal size range.
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For the CSD of cpx (Figs. 13 and 14Sb): at 1 °C/h, the trend is composed of two log-linear segments with only a moderate difference in slope, a large size range, and the lowest m and n0 values (Table 7). There is a downturn at the smaller crystal sizes.
at 7, 60 and 180 °C/h, the trends are composed of three (7 °C/h and 180 °C/h) and two (60 °C/h) log-linear segments with almost similar slopes. The CSD at 7 °C/h shows the largest crystal size range and the lowest intercept value. There is a downturn (7 and 180 °C/h) at the smaller crystal sizes;
at 1800 °C/h, the almost linear trend shows the steepest behavior and the regression analysis of the log-linear CSD yields the maximum n0 intercept. As concerns the CSD of sp (Figs. 13 and 14Sc):
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at 1 and 7 °C/h, the trends are composed of three (1 °C/h) and two (7 °C/h) log-linear segments. The slopes and intercepts are low to moderate and the size range decreases.
at 60 °C/h, the almost linear trend shows m and n0 higher than those measured at lower cooling rates;
at 180 °C/h, the trend is composed of two log-linear segments, with almost the same slope measured at 60 °C/h, but with a larger crystal size range. at 1800 °C/h, the almost linear trend is very steep and n0 reaches the maximum value; the
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maximum crystal size decreases of two order of magnitude from 1 to 1800 °C/h.
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The downward kinking of CSD curves measured for plg (E7), cpx (E1b, E7 and E180b) and sp (E7,
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E60 and E180b) at the small sizes is consistent with a coarsening effect and, possibly, an Ostwald
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ripening process. This phenomenon can be ascribed to actual crystallization processes or to an
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artefact due to limited spatial resolution not allowing the accurate counting of tiny crystals (Hammer et al., 1999; Mock et al., 2002; Higgins, 2006; Armienti, 2008; Vona et al., 2017). Since
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SEM images have a high spatial resolution, we exclude artifacts due to an inaccurate counting of tiny crystals. This interpretation is corroborated by ex-situ observations of Pupier et al. (2008) under
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either rapid or slow ΔT/Δt conditions, as well as by in-situ observations of Ni et al. (2014) under relative rapid ΔT/Δt conditions (Table 1). The sluggish solidification conditions presented in this study significantly enlarge the field of CSD investigation and show that as ΔT/Δt increases: 1) the crystal size range decreases (Fig. 13), 2) the population density n0 moves upward (Fig. 13 and Table 7) in line with the increasing of number of crystals per area (#/A) (Fig. 10), 3) the CSD curve derived at low cooling rates is mainly composed of two or three log-linear segments (Figs. 14), and 4) the CSD slope m increases (Fig. 13 and Table 7). At low ΔT/Δt, the crystals have enough time to aggregate according to a coarsening process which, in turn, produces flattening and low intercept for the CSD curve (Pupier et al., 2008; Ni et al., 2014). The coarsening is also supported by the faceted cpx in the verge of a complete aggregation observed in Figs. 4 and 5 for the E60 experiment. Moreover, it is likely that tiny
Journal Pre-proof crystals dissolve under the effect of low cooling rates in order to feed the growth of larger crystals by Ostwald ripening (Mills et al., 2011). According to Pupier et al. (2008) and Ni et al. (2014), different log-linear slopes and CSD trends (Fig. 14) indicate multiple events or pulses of nucleation. The CSD curve with lowest slope and largest crystal size corresponds to the early event of nucleation occurred below the liquidus and followed by further pulses of nucleation at lower temperatures. Since the CSD slope decreases with increasing crystal size (Figs. 13 and 14), it is plausible that the growth of early-formed nuclei
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proceeds by agglomeration and attachment (Iezzi et al., 2008, 2011, 2014). Successive pulses of
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nucleation in the residual melt are also responsible for CSD segments with moderate and/or high
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slopes, as well as for the high population density observed at smaller crystal sizes (Figs. 13 and 14).
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Fig. 13. Crystal size distribution (CSD) of plg, cpx and sp, computed by CSDCorrection (Higgins, 2000) and relative area% (see data in Table 3); n0 corresponds to the population density per size.
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4
3
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7
E180b
E1800
60
180
1800
4
3
2
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E60
na
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E7
(n0) (µm-4)
crystal shape
-1*10-8 -3*10-7 -7*10-11 -2*10-7 -2*10-6 -7*10-8 -4*10-8 -3*10-8 -6*10-7 -1*10-8 -5*10-8 -5*10-7 -2*10-9 -5*10-6 -2*10-5 -2*10-6 -3*10-8
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E1b
#images
Table 7. Crystal Size Distribution (CSD) data calculated by CSDCorrection (Higgins, 2000) area crystalline size range slope, m y-intercept, magnification #crystals (μm2) phase (µm) (μm-1) ln(n0) (µm-4) 25 – 956 -1*10-2 -18.2 (full range) plg -2 25 – 256 -5*10 -15.0 100 plg 256 - 956 -1*10-3 -23.4 859353 277 cpx 19 - 365 -2 61 sp -3*10 -15.5 (full range) cpx -2 19 -70 -7*10 -13.3 150 x 70 - 365 -2*10-2 -16.5 20 – 104 -2 -3*10 -17.0 (full range) 79 plg -2 20 - 54 -1*10 -17.3 961083 184 cpx sp 31 sp 54 - 75 -7*10-2 -14.4 75 - 104 -2*10-2 -18.4 39 – 752 -2 -1*10 -16.9 (full range) plg -2 39 – 280 -3*10 -14.6 160 plg 200 x 270420 746 cpx 280 – 752 -1*10-2 -20.2 89 sp 17 – 235 -2 - 4*10 -12.2 (full range) -2 17 - 88 - 7*10 - 10.9 cpx 88 - 170 - 4*10-2 - 13.2 170 - 235 - 2*10-2 -17.4 129 plg 11 – 78 -1 150 x 945323 1631 cpx -1*10 -12.3 (full range) 212 sp sp -1 11 - 56 - 2*10 -11.1 56 - 78 - 3*10-2 -19.2 2 – 112 -2 -9*10 - 9.0 (full range) 962 plg -1 2 - 22 - 2*10 -7.1 400 x 138897 1464 cpx plg 295 sp 22 – 81 - 9*10-2 - 9.9 81 - 112 - 9*10-2 - 16.3 2 – 98 -1 -1*10 -7.8 (full range) cpx 539 plg -1 2 - 37 - 2*10 -6,2 800 x 34762 1172 cpx 37 – 98 - 7*10-2 -11.2 144 sp 2 – 23 sp - 3*10-1 - 7.9 (full range) 4 – 108 - 8*10-2 -10.3 (full range) 695 cpx -1 4 – 11 - 4*10 - 7.4 400 x 145934 cpx 736 sp 11 – 40 - 1*10-1 - 10.6 40 – 108 - 5*10-2 -12.6 3 – 54 -1 - 2*10 - 9.5 (full range) 419 cpx 300 x 119680 sp -1 276 sp 3 - 22 - 4*10 - 7.5 22 - 54 - 5*10-2 - 13.9 214 cpx 2 – 23 4798 cpx - 4*10-1 - 3.6 138 sp (full range) 1500 x 318 cpx 1–6 4790 sp - 1.9*100 - 1.2 271 sp (full range)
-4*10-6 -2*10-5 -5*10-9 -1*10-4 -9*10-4 -5*10-5 -8*10-8 -4*10-4 -2*10-3 -1*10-5 -4*10-4 -3*10-5 -6*10-4 -2*10-5 -3*10-6 -8*10-5 -6*10-4 -9*10-7 -3*10-2 -3*10-1
Footnote: E9000 is not reported due to the very low crystal content (< 2 area%) and very small crystal size (cpx and sp sizes are < 10μm).
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ΔT/Δt (°C/h)
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Fig. 14aS (to be deposited as supplementary). CSD data of plg; n0 correspond to the population density per size. The black dashed lines are the regressions of all data (full range in Table 7), whereas the red lines correspond to regressions of different range of sizes (Table 7).
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Fig. 14bS (to be deposited as supplementary). CSD data of cpx; n0 corresponds to the population density per size. The black dashed lines are the regressions of all data (full range in Table 7), whereas the red lines correspond to regressions of different range of sizes (Table 7).
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Fig. 14cS (to be deposited as supplementary). CSD data of sp; n0 corresponds to the population density per size. The black dashed lines are the regressions of all data (full range in Table 7), whereas the red lines correspond to regressions of different range of sizes (Table 7).
Journal Pre-proof Finally, it is possible to models the CSD parameters as a function of kinetic conditions. The average m and n0 (full range in Table 7) for plg, cpx and sp have high liner correlations like those displayed in Fig. 15. In turn, the increase of ΔT/Δt induces a progressive counterclockwise rotation (Fig. 13) during the cooling of a complete molten MORB from liquidus to solidus. Moreover, the very high linear trends between ΔT/Δt versus m and n0 for plg, cpx and sp allow us to calculate the cooling rates by CSD parameters obtained on MORB and basaltic rocks (Fig. 16). Consequently, the equations retrieved in this study can be used to calculate kinetic conditions of solidification
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from the outer to the innermost parts of MORB lavas with thickness ≤ 2 m (Fig. 2).
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Fig. 15. Relationships between average CSD slopes (m) and population density (n0) (Table 7).
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Fig. 16. Relationships between ΔT/Δt versus average CSD slopes m (left column) and population density n0 (right column) (Table 7).
Journal Pre-proof 7.4. Maximum and average growth rates A central role in the reconstruction of volcanic processes is the time of crystal growth (Marsh, 1988; Costa et al., 2008; Cooper and Kent, 2014; Cooper, 2017; Putirka, 2017; Polacci et al., 2018). Several approaches can be used but the most frequent and simple one consists in the measure of the crystal size divided by growth rate (G) determined in laboratory (Cashman, 1993; Larsen, 2005; Pupier et al., 2008; Brugger and Hammer, 2010; Shea and Hammer, 2013; Cooper and Kent, 2014; Mollo and Hammer, 2017). In the last decades, many experimental studies retrieved the values of G
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for the most common crystalline phases in silicate liquids (Armienti et al., 1994; Lasaga, 1998;
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Zhang, 2008; Hammer, 2008). Here, we enlarge these datasets by determining the maximum and
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average growth rates (Gmax and GCSD, respectively) for plg, cpx and sp. We recall that Tliquidus of
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B100 is 1233 °C (Tm in Table 1), while the time intervals (t) between Tliquidus and the final solidification temperature of 800 °C are 433, 61.9, 7.2, 2.4 and 0.2 hours for ΔT/Δt of 1, 7, 60, 180
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and 1800 °C/h, respectively. In our calculations, it is assumed that crystals nucleate instantaneously
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at Tliquidus and then continuously grow down to 800 °C. The resulting growth rates are listed in Table 8. They represent the minimum of the actual growth rates since the crystallization can initiate below
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Tliquidus and terminate above Tsolidus.
The maximum growth rate (Table 8) is calculated by averaging the five longest crystals (Lmax) and dividing them by the cooling time, as Gmax = Lmax/t (Burkhard, 2002; Hammer and Rutherford, 2002; Couch, 2003; Iezzi et al., 2011). The average growth rate (Table 8) is calculated by interpolating the CSD slope (m) and the cooling time, as GCSD = -1/mt (Zieg and Marsh, 2002). The slope derived by global linear regression analysis of the same mineral phase and experiment yields the value of GCSD. Conversely, the different slopes derived by log-linear segments of the same mineral phase and experiment (i.e., plg at 7 °C/h, cpx at 1, 7 and 180 °C/h, andsp at 7, 60 and 180 °C/h) are referred as G*CSD (Table 8). Overall, GCSD and G*CSD for the same mineral phase and experiment are comparable within the same order of magnitude.
Journal Pre-proof Gmax and GCSD are plotted in Fig. 17 as a function of ΔT/Δt. As expected, Gmax is higher than GCSD and both parameters increase with increasing ΔT/Δt. Gmax and GCSD of plg increases remarkably from 1 to 7 °C/h and then do not change substantially; the relation of ΔT/Δt versus Gmax can be well modeled by a linear regression, whereas that of ΔT/Δt versus GCSD is only qualitative for plg (Fig. 17). Gmax and GCSD of cpx follow an increasing monotonic, linear trend as a function of cooling rate (Fig. 17). Gmax and GCSD of sp increase from 1 to 180 °C/h and then do not substantially change.
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The overall ranges of Gmax and GCSD measured with increasing cooling rate are: 4.7*10-8 –
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3.5*10-7 and 6.4*10-9 – 4.3*10-8 cm/s for plg, 1.9*10-8 – 2.5*10-6 and 2.1*10-9 – 2.7*10-7 cm/s for
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cpx, and 5.1*10-9 – 6.2*10-7 and 2.1*10-9 – 6.8*10-8 cm/s for sp, respectively (Fig. 17and Table 8).
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At 1 °C/h, Gmax is the highest for plg and the lowest for sp, with an intermediate value for cpx. At this cooling rate, GCSD shows similar values for all the mineral phases. At 7 °C/h, Gmax and GCSD are
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the highest for plg and the lowest for sp, with an intermediate value for cpx. At 60 °C/h, Gmax and
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GCSD of plg and cpx are comparable and higher than those calculated for sp. At 180 and 1800 °C/h, both Gmax and GCSD of cpx are higher than those calculated for sp (Fig. 17).
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Gmax and GCSD from this study enlarge the data set measured on basaltic melts. It is noteworthy, that our data are similar to those measured in previous studies for similar cooling rates, so demonstrating the consistency of the approaches. Pupier et al. (2008) found Gmax and GCSD of plg in the range of 3.3*10-8 – 9.4*10-8 cm/s and 6*10-9 – 1.4*10-8 cm/s for melts experimentally cooled at 0.2 and 3 °C/h, but obtained with ΔT between 20 and 80 °C (see Tables 1 and 3 reported in Pupier et al., 2008). Among the time series experiments conducted on a primitive basalt from Mt. Etna volcano (Sicily, Italy), Pontesilli et al. (2019) calculated Gmax of cpx and sp for the anhydrous run products in the range of 15*10-7 – 0.5*10-7 cm/s and 1.5*10-7 – 0.1*10-7 cm/s, respectively, at ΔT/Δt of 1.3 °C/h and low to moderate ΔT between 80 and 120 °C. Dunbar et al. (1995) quantified Gmax of cpx in the range of 1*10-8 – 1*10-7cm/s in the interior part of a large (4.8 m3) artificial mafic melt generated cooled from 1500 to 500 °C in ~ 6 days. Ni et al. (2014) measured Gmax of cpx
Journal Pre-proof between 6*10-7 and 17*10-7 cm/s by in-situ experiments conducted at ΔT/Δt of 100 °C/h. Moreover, Gmax and GCSD have been observed to increase with ΔT/Δt in naturally cooling basaltic lavas where minerals were sampled at progressive depth from the top of the flow (Kirkpatrick, 1977; Cashman and Marsh, 1988; Burkard, 2002; Oze and Winter, 2005). At 2 cm from the top of the lava flow, Burkhard (2002) found Gmax in the ranges of 1.3*10-9– 1.7*10-8 cm/s, 3.2*10-9 – 8.6*10-7 cm/s and 3.5*10-9 – 2.2*10-8 cm/s for plg, cpx and sp, respectively. Similarly, Oze and Winter (2005) estimated Gmax in the range of 1.6*10-4 - 2.6*10-7 cm/s for plg, 3.5*10-5- 2.4*10-7 cm/s for cpx and
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3.5*10-6 – 2*10-7 cm/s for sp (see Table 5 reported in Oze and Winter, 2005).
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Fig. 17. Variation of maximum growth rate (Gmax, full symbols) and average growth rate (GCSD, empty symbols) as a function of ΔT/Δt (Table 8). Only linear regressions with R2 ≥ 0.6 have been reported.
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E60
E180b
5.1*10-9
2.1*10-9
plg
2.4*10-7
4.5*10-8
cpx
1.0*10-7
1.1*10-8
sp
2.8*10-8
3.4*10-9
plg
3.5*10-7
4.3*10-8
cpx
2.7*10-7
3.2*10-8
sp
6.4*10-8
1.1*10-8
cpx
1.0*10-6
1.4*10-7
6.3*10-7
7.2*10-8
60
180
1800
cpx sp
2.5*10-6 6.2*10-7
faceted
2.7*10-7 6.8*10-8
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E1800
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3.2*10-9 (70 to 365 µm) 6.4*10-9 (20 to 54 µm) 9.2*10-10 (54 to 75 µm) 3.2*10-9 (75 to 104 µm) 1.5*10-8 (36 to 280 µm) 4.5*10-8 (280 to 752 µm) 6.4*10-9 (17 to 88 µm) 1.1*10-8 (88 to 170 µm) 2.2*10-8 (170 to 235 µm) 2.6*10-9 (11 to 56 µm) 1.5*10-8 (56 to 78 µm) 1.7*10-8 (2 to 22 µm) 4.3*10-8 (22 to 81 µm) 4.3*10-8 (81 to 112 µm) 1.8*10-8 (2 to 37 µm) 5.5*10-8 (37 to 98 µm) 1.1*10-8 (2 to 23 µm) 2.9*10-8 (4 to 11 µm) 1.2*10-7 (11 to 40 µm) 2.3*10-7 (40 to 108 µm) 3.3*10-8 (3 to 22 µm) 2.3*10-7 (22 to 54 µm) 2.7*10-7 (1 to 23 µm) 6.2*10-8 (1 to 6 µm)
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Table 8. Maximum (Gmax) and average (GCSD) growth rates (cm/s) ΔT/Δt crystalline Gmax GCSD G*CSD (°C/h) phase -9 1.3*10 (25 to 256 µm) plg 4.7*10-8 6.4*10-9 6.4*10-8 (256 to 956 µm)
Therefore, the quantified Gmax and GCSD obtained in our study can be used to retrieve timing of growth of crystalline phases of aphyric MORB, according to the fact that ΔT/Δt changes over a broad range and the thermal path of cooling is comprised between liquidus and solidus temperatures. The cpx is the silicate phase that crystallize with a remarkable abundance from 1 to 1800 °C/h (Figs. 5 and 6) and, for this reason, it can be used to reconstruct the cooling rate of MORB rocks. In Fig. 18, the values of Gmax for cpx are plotted versus Lmax, m and n0. The linear relationships between Gmax with m and n0 are highly correlated (Fig. 18), whereas that with Lmax is semi-quantitative (R2 = 0.56). Furthermore, the decreasing trends of Gmax and GCSD with the decrease of ΔT/Δt suggest that the G rate of minerals growing in anhydrous magmatic reservoirs and conduits could be lower than
Journal Pre-proof those quantified by experimental data, even for cooling rate as low as of 1 °C/h. New investigations at ΔT/Δt lower than 1 °C/h under anhydrous and hydrous conditions are, indeed, required to enlarge the trends depicted in Figs. 17 and 18, as well as to quantify the role of H2O on the crystallization of magmas. On the other hand, G rate of minerals growing in anhydrous magmatic systems could also be enhanced by dynamic crystallization process (cooling + shearing) allowing continuous “feed growth ingredients [..] on individual crystal surfaces that facilitates crystal growth at high shear
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rate” as reported in Vetere and Holtz (2019).
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Fig. 18. From left to right: variation of Gmax (Table 8) versus CSD slope (m) and population density (n0) (Table 7), plus the maximum length (Lmax) of cpx (Table 4S).
Journal Pre-proof 7.5. Volcanological and Petrological implications The textural features of B100 indicate that as ΔT/Δt increases from 60 and 180 °C/h, several first order effects take place: 1) plg crystals disappear, 2) faceted crystals are replaced by dendritic cpx and sp (Fig. 5), 3) the matrix glass content increases significantly (Fig. 6) and 4) the #/A (µm-2) of both cpx and sp sharply decreases (Fig. 10). The plg is unable to nucleate at high cooling rates since it contains a high number of connected tetrahedral sites, requiring the breaking and formation of high energy bonds; also, the stability of plg is extremely sensitive in Al and Si, as cations with
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the slowest chemical diffusivity (Walker et al., 1976; Grove and Walker, 1977; Grove and
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Raudsepp, 1978; Iezzi et al., 2008, 2011, 2014; Vetere et al., 2013, 2015). There is a close
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relationship between the melt composition (evolving during crystallization and cooling) and the
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formation of crystal nuclei with a critical size that may potentially grow over time (Gonzalez et al., 2017, 2019). On the other hand, the nucleation of cpx from the melt requires a low energetic barrier
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because of the lower number of Si-Al tetrahedra in the crystal structure and the preferential
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incorporation of cations (i.e., Ca, Mg and Fe) that more easily diffuse in the melt phase. This applies also to the greater stability of sp as Si-free phase (Kirckpatrick, 1983; Vetere et al., 2015).
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Moreover, at ΔT/Δt between 60 and 180 °C/h, crystals show dendritic shapes, attesting disequilibrium crystallization conditions (Lofgren et al., 1974; Walker et al., 1976; Shiffman and Lofgren, 1982; Ohnenstetter and Brown, 1992; Oze and Winter, 2005; Blundy and Cashman, 2008) and in concert with lower values of #/A for cpx and sp (Figs. 10 and 13). By contrast, at ΔT/Δt<60 °C/h, plg, cpx and sp are faceted and relatively large, as well as CSD curves exhibit moderate to low slopes and intercepts reflecting equilibrium crystallization conditions (Fig. 13). These observations indicate that, between 60 and 180 °C/h, the MORB melt strongly changes its solidification behavior from crystallization occurring prevalently by growth at low ΔT/Δt to nucleation-dominated at high ΔT/Δt (Blundy and Cashman, 2008). The rapid solidification of MORB melts indicate that the rheology of basaltic lavas, dikes and pillows is deeply controlled by crystallization under the effect of temperature. Moreover, the
Journal Pre-proof convective flow of magma further enhances the nucleation and crystal growth (Vona et al. 2011, 2017; Kolzenburg et al., 2016, 2018), accelerating the attainment of a significant amount of crystals able to arrest their mobility. In turn, the increasing viscosity of basaltic liquids is mainly governed by the onset and successive crystallization steps, which is dominantly controlled by the cooling rate. A further result unveiled by our experiments concerns the sizes and abundances of crystal in the ranges > 0.5 (or 1) mm, between 0.1 and 0.5 (or 1) mm and < 0.01 mm, that are commonly considered phenocrysts, microphenocrysts and microlites, respectively (Lanzafame et al., 2013;
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Vona et al., 2017 and references therein). Phenocrysts are generally interpreted as minerals formed
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in magmatic reservoirs, whereas microphenocrysts and microlites are addressed to magma ascent
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within the conduit and late solidification on the Earth surface during lava flow. However, the
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textural analysis from this study (Fig. 7) points out that it is possible to grow plg crystals with size up to 0.5-1 mm at cooling rates of 1-7 °C/h, as well as a significant number of plg and cpx crystals
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> 0.1 mm or 100 µm (Fig. 11), starting from a complete molten and anhydrous MORB liquid
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cooling on Earth. This demonstrates that, at low ΔT/Δt conditions, MORB liquids solidifying as lava flows, pillow lavas and dikes can rapidly and easily attain phenocryst and microphenocryst
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textures. Faceted to dendritic plg and cpx crystals may also grow at subaerial conditions after the eruption of magma at the surface. The effects of crystallization kinetics in lavas are not the same from the base towards the uppermost crust, due to the different cooling conditions (Fig. 2). Consequently, aphyric MORBs will show different CSD features (Fig. 13) moving from innermost to outermost parts of the flow. Similarly, plg or cpx with variable sizes and shapes (faceted to dendritic) may be partially or fully formed after eruption of lava flows/pillows in response to cooling-induced solidification phenomena. The main implication is that a single rock sample collected from a naturally cooled lava flow cannot be representative of the whole flow, thus resulting misleading for the reconstruction of solidification histories of basaltic rocks, especially for discriminating minerals formed at pre-, syn- and post-eruptive conditions.
Journal Pre-proof 12. Conclusions The quantitative evolution of textural attributes of experimental plg, cpx and sp obtained at liquidus-solidus temperature ranges and slow-fast cooling rates ΔT/Δt, can be used to constrain in full the dynamic and kinetic conditions driving the solidification of natural MORB, with thickness variable from 0.1 to 1/2 m (Fig. 2). Moving from the innermost to the outermost portions of lava flows, pillow lavas and dikes, the kinetic effects will progressively increase as a function of ΔT/Δt and heat release. This increase determines: a decreasing amount of crystals accompanied by a transition from faceted to dendritic
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1)
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crystals and the suppression of plg between 60 and 180 °C/h (Figs. 5 and 6); in turn,
a strong variation of the crystal size with significant differences among plg, cpx and
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2)
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plg will not occur in the external portion of aphyric lavas, dikes and/or pillows;
sp cooled at the same rate (Figs. 7 and 8);
the lack of a clear evolution for the 2D and 3D crystal shapes (Figs. 9 and 12);
4)
a monotonic increase of two and three orders of magnitude in the number of plg and
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sp crystals per area (µm-2), respectively, whereas that of cpx first increases,
5)
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subsequently decreases, and finally increases again (Fig. 10); a progressive evolution of the CSD curves for plg, cpx and sp (Figs. 13 and 14S), with an increase in slope m and population density n0 (Fig. 15), by producing nearly continuous clockwise rotations (Fig. 13); these m and n0 parameters can be also used to determine the cooling rate conditions of MORB rocks (Fig. 16); 6)
the growth rates Gmax and GCSD decrease of several orders of magnitude from 10-9 to ~ 10-5 cm/s (Fig. 17); Gmax can be faithfully calculated by m and n0 for MORB rocks (Fig. 18);
7)
plg and cpx crystals with sizes of 102 to 103 µm may develop from nearly anhydrous MORB lava flows and pillows (Figs. 11); these relatively large dimensions are not
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or microphenocrysts growth during magma ascent in volcanic conduits.
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Azilli, F., Piochi, M., Mormone, A., Agostini, C., Carroll, M.R., 2016.Constraining pre-
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trachy-phonolites. Bull. Volcanol. 78: 72. doi: 10.1007/s00445-016-1062-z. Bakee, M.B., Grove, T.L., 1985. Kinetic controls on pyroxene nucleation and metastable
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Batiza, R., Vanko, D., 1984. Petrology of young pacific seamounts. J. Geophys. Res., 89, 235 – 260.
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Bianco, A.S., Taylor, L.A., 1977. Applications of dynamic crystallization studies: lunar olivine-normative basalts. Proc. Lunar Sci. Conf. 8th, 1593 – 1610. Blundy, J. D., Cashman, K., 2008. Petrologic reconstruction of magmatic system variables and processes. In: Putirka, K. D., Tepley, F. J., (Eds.). Rev. Mineral. Geochem., 26, 179 – 239.doi: 10.2138/rmg.2008.69.6. Blundy, J. D., Annen, C. J., 2016. Crustal magmatic systems from the perspective of heat transfer. Elements. 12, 115 – 120. doi: 10.2113/gselements.12.2.115. Bowles, J.A., Hammer, J.E., Brachfeld, S.A., 2009. Magnetic and petrologic characterization of synthetic Martian basalts and implications for the surface magnetization of Mars. J. Geophys. Res., 114, E10003. doi: 10.1029/2009JE003378
Journal Pre-proof Brachfeld, S.A., Hammer, J.E., 2006. Rock-magnetic and remanence properties of synthetic Fe-rich basalts: implications for Mars crustal anomalies. Earth and Planetary Science Letters, 248, 599 – 617. doi: 10.1016/j.epsl.2006.04.015 Brandeis, G., Juapart, C., Allègre, C. J., 1984. Nucleation, crystal growth and thermal ragime of cooling magmas. J. Geophys. Res., 89, 10, 161 – 10, 177. Brandeis, G., Juapart, C., 1987. The kinetics of nucleation and crystal growth and scaling laws for magmatic crystallization. Contrib. Mineral. Petrol., 96, 24 – 34.
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Burkhard, D. J. M., 2002. Kinetics of crystallization: example of micro-crystallization in
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basalt lava. Contrib. Mineral. Petrol. 142, 724 – 737. DOI 10.1007/s00410-001-0321-x. Burkhard, D.J., 2005. Nucleation and growth rates of pyroxene, plagioclase and Fe-Ti oxides
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Cashman, K. V., Marsh, B. D., 1988.Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization. Contrib. Mineral. Petrol. 99, 292 – 305. Cashman, K. V., 1989. Relationship between plagioclase crystallization and cooling rate in basaltic melts. Contrib. Mineral. Petrol., 113, 126 – 142. Cashman, K.V., 1991. Textural constraints on the kinetics of crystallization on igneous rocks. In: Nicholls, J., Russell, J.K., (Eds.), Rev. Mineral, 24, 259 – 309. Cashman, K.V., 1993. Relationship between plagioclase crystallization and cooling rate in basaltic melts. Contrib. Mineral. Petrol. 113, 126 – 142.
Journal Pre-proof Cashman, K.V., Thornber, C., Kauahikaua, J. P., 1999.Cooling and crystallization of lava in open channels, and the transition of Pahoehoe lava to ‘a‘a. Bull. Volanol. 61, 306 – 323. Chistyakova, S.Y., Latypov, R.M., 2009. Two independent process responsible for compositional zonation in mafic dykes of the Åland-Åboland Dyke Swarm, Kestiö Island, SW Finland. Lithos, 112, 382 – 396.doi: 10.1016/j.lithos.2009.03.037. Coish, R.A., and Taylor, L.A., 1979.The effects of cooling rate on texture and pyroxene chemistry in DSDP LEG 34 basalt: a microprobe study. Earth and Planetary Science Letters, 42,
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Cooper, K. M., Kent, A. J. R., 2014.Rapid remobilization of magmatic crystal kept in cold
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storage. Nature. 506, doi:10.1038/nature12991.
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