Mantle–crust interactions in the oceanic lithosphere: Constraints from minor and trace elements in olivine

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ScienceDirect Geochimica et Cosmochimica Acta 141 (2014) 423–439 www.elsevier.com/locate/gca

Mantle–crust interactions in the oceanic lithosphere: Constraints from minor and trace elements in olivine Alessio Sanfilippo a,⇑,1, Riccardo Tribuzio a,b, Massimo Tiepolo b a

Dipartimento di Scienze della Terra e dell’Ambiente, Universita` di Pavia, Via Ferrata 1, 27100 Pavia, Italy b CNR – Istituto di Geoscienze e Georisorse, Unita` Operativa di Pavia, Via Ferrata 1, 27100 Pavia, Italy Received 7 December 2013; accepted in revised form 12 June 2014; Available online 24 June 2014

Abstract Minor and trace element compositions of olivines are used as probes into the melt-rock reaction processes occurring at the mantle–crust transition in the oceanic lithosphere. We studied mantle and lower crustal sections from the Alpine Jurassic ophiolites, where lithospheric remnants of a fossil slow-spreading ocean are exposed. Olivines from plagioclase-harzburgites and replacive dunites (Fo = 91–89 mol%) and from olivine-rich troctolites and troctolites (Fo = 88–84 mol%) were considered. Positive correlations among the concentrations of Mn, Ni, Co, Sc and V characterize the olivines from the dunites. These chemical variations are reconciled with formation by melts produced by a mixed source consisting of a depleted peridotite and a pyroxene-rich, garnet-bearing component melted under different pressure conditions. We thereby infer that the melts extracted through these dunites channels were not fully aggregated after their formation into the asthenospheric mantle. Olivines from the olivine-rich troctolites and the troctolites are distinct by those in the dunites by lower Ni and higher concentrations of Mn and incompatible trace elements (Ti, Zr, Y and HREE). Fractional crystallization cannot reproduce the chemical variations of the olivines from the olivine-rich troctolites and the troctolites. In these rock-types, the olivines commonly show heterogeneous Ti, Zr, Y and HREE compositions, which produce variable Ti/Y and Zr/Y values. We correlate these olivine characteristics with events of reactive melt migration occurred during the formation of the primitive lower oceanic crust. We propose that the migrating melts formed at the mantle–crust transition via interaction with mantle peridotites during periods of low melt supply. Ó 2014 Elsevier Ltd. All rights reserved.

1. INTRODUCTION There is a general consensus that reactions between primitive MORB and peridotites extensively occur in the shallow mantle (Stolper, 1980; Elthon and Scarfe, 1984; ⇑ Corresponding author. Tel.: +39 0382985899; fax: +39 0382985890. E-mail addresses: alessio.sanfi[email protected] (A. Sanfilippo), [email protected] (R. Tribuzio), [email protected] (M. Tiepolo). 1 Present address: School of Natural System, College of Science and Engineering, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan.

http://dx.doi.org/10.1016/j.gca.2014.06.012 0016-7037/Ó 2014 Elsevier Ltd. All rights reserved.

Johnson et al., 1990; Kelemen et al., 1990). One example for this is the formation of replacive dunites, considered as products of melt-rock reactions between migrating primitive MORB and the host harzburgite/lherzolite through a process of pyroxene dissolution and precipitation of new olivine (Dick, 1977; Hopson et al., 1981; Quick, 1981). Once these replacive dunites are formed, they act as porous flow channels that transport melts extracted from the melting region preventing further interaction with the shallow mantle (see also Kelemen et al., 1995). Dunites are also expected to form the mantle–crust transition, where they may originate either by fractionation of primitive melts or by melt–peridotite interactions (e.g., Coleman, 1977; Nicolas and Prinzhofer, 1983; Kelemen et al., 1997; Dick

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et al., 2008, 2010). It has been recently proposed that the olivine-rich troctolites enclosed in the lower oceanic crust represent products of interaction between dunites at the mantle–crust transition and MORB-melts reactively migrating through the olivine-rich matrix (Drouin et al., 2009; Suhr et al., 2008; Renna and Tribuzio, 2011; Sanfilippo et al., 2013). Furthermore, evidence has been emerging that melt-rock reaction may also occur within the gabbros, when ascending magmas react chemically with a forming crystal mush, thereby modifying the compositions of the minerals and of the transient melts (Coogan et al., 2000; Dick et al., 2000; Gao et al., 2007; Lissenberg and Dick, 2008; Lissenberg et al., 2013). Taken as a whole, these melt-rock reaction processes questioned the idea that the lower oceanic crust exclusively represents the product of fractional crystallization of primitive melts (e.g., O’Hara, 1968) and that information about the mantle sources may be directly addressed from the composition of MORB after correction for fractional crystallization only (e.g. Klein and Langmuir, 1987). Recent studies showed that the chemistry of olivine is a powerful tool to inspect mantle processes. For instance, the minor and trace element compositions of olivine were used to constrain the metasomatic and/or subsolidus history of lithospheric subcontinental mantle peridotites (Mallmann et al., 2009; De Hoog et al., 2010; Foley et al., 2013). Minor and trace element compositions of olivine phenocrysts in basalts were also used to obtain information about the compositions and/or the melting conditions of the mantle sources (Sobolev et al., 2005, 2007; Herzberg, 2011; Putirka et al., 2011). Olivine is the major constituent of the mantle peridotites and dunites and is the first phase to crystallize from a primitive MORB (Grove et al., 1992 and references therein). For these reasons, the olivine chemistry has great potential to investigate the melt-rock reaction processes occurring below an oceanic ridge (e.g. Drouin et al., 2009; Lissenberg et al., 2013). However, a thorough characterization of the minor and trace element compositions of olivines from the oceanic lithosphere has not been ascertained yet. This is partly due to the inability to recover abyssal sections in which olivine of the mantle peridotites and the lower crustal rocks are unaffected by the low temperature alteration. In this study, we selected fresh mantle harzburgites, replacive dunites and primitive lower crustal rocks (olivine-rich troctolites to troctolites) from the Jurassic ophiolites exposed along the Alpine–Apennine belt. These ophiolites are considered to represent remnants of the oceanic lithosphere formed in an embryonic slow-spreading basin (Lagabrielle and Cannat, 1990; Tribuzio et al., 2004; Manatschal and Mu¨ntener, 2009; Sanfilippo and Tribuzio, 2011), which developed in the Middle to Upper Jurassic in conjunction with the opening of the Central Atlantic Ocean (Schettino and Turco, 2011; Vissers et al., 2013). We carried out in situ minor and trace elements analyses on olivines from these mantle and lower crustal rocks using Laser Ablation Inductively Coupled Plasma Mass Spectrometry. These data allow us to inspect the relationships between the melts extracted from the mantle and those crystallizing the primitive crust. Hence, we examine

the early magmatic processes occurring under an oceanic ridge and the role of melt-rock reaction processes on the composition of the lower oceanic crust. 2. GEOLOGICAL FRAMEWORK AND SELECTED SAMPLES The studied ophiolites include lower crustal sequences considered to be fossil analogues of oceanic core complexes from slow and ultraslow spreading ridges (Sanfilippo and Tribuzio, 2011, 2013a; Alt et al., 2012). These sequences are represented by the gabbroic sections from the Internal Ligurian and Pineto (Corsica) ophiolites (Fig. 1) and are associated with mantle sequences essentially consisting of depleted spinel-peridotites. These peridotites commonly record melt impregnation event and re-equilibration under plagioclase-facies conditions (Rampone et al., 1996, 1997). The mantle sequences from the Jurassic ophiolites of the Alpine–Apennine belt locally contain MOR-type replacive dunites (Piccardo et al., 2007; Sanfilippo and Tribuzio, 2011). The best examples for these replacive dunites are exposed in the Lanzo South massif (Fig. 1), where they are hosted by melt-impregnated plagioclase-peridotites (see also Kaczmarek and Mu¨ntener, 2009). We selected three mantle harzburgites and three olivinerich troctolites from the gabbro-peridotite sequences of the Internal Ligurian ophiolite. Two olivine-rich troctolites, four troctolites and two mantle harzburgites were considered for the Pineto gabbroic sequence and the associated mantle sequence from Serra Debbione. Five dunites and

Fig. 1. Geological sketch map of the Alpine–Appenine system and location of the studied mantle and crustal sequences. Location and petrographic description of the samples are in the Supplementary material.

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three host harzburgites were selected from the Lanzo South massif. The location, the main petrographic features and the modal compositions of the selected samples are reported as Supplementary material (Table EM1). The major element compositions of minerals (Tables EM2–EM6) and whole-rocks (Table EM7), and the clinopyroxene trace element compositions (Table EM8) are also included as Supplementary material together with details on the analytical procedures. 2.1. Lower crustal sequences The gabbroic sections from the Internal Ligurian ophiolites mostly consist of clinopyroxene-rich gabbros to troctolites locally interlayered with lenses of olivine-rich troctolites. The Pineto gabbroic sequence exposes a 1.5 km-thick section consisting of moderately evolved gabbros near its stratigraphic top, and of troctolites to olivinegabbros in its deeper sector (Sanfilippo and Tribuzio, 2013a). The lithological heterogeneity and the occurrence of mantle peridotites slices within the Internal Ligurian and Pineto gabbroic sections suggest formation by multiple melt intrusions (Renna and Tribuzio, 2011; Sanfilippo and Tribuzio, 2011). This process was corroborated by cooling rate determinations on olivines, which furnished cooling rates of 2.2 to 1.7 °C/year log units (Sanfilippo and Tribuzio, 2013a), in agreement with other thermochronological studies of gabbros from modern oceanic core complexes (John et al., 2004; Grimes et al., 2011). The olivine-rich troctolites form bodies at different stratigraphic heights within the gabbroic sequences of the Internal Ligurian and Pineto ophiolites. They consist of euhedral olivine (Fo = 88–87 mol%), minor subhedral to oikocrystic plagioclase (An = 73–69 mol%), and accessory clinopyroxene and spinel (Fig. 2). The clinopyroxenes locally form large oikocrysts, with MORB-type incompatible elements signature (Fig. 1 of Supplementary material). These rocks were interpreted to represent products of multiple interactions between an olivine-rich matrix and MORBtype melts crystallizing plagioclase and clinopyroxene (i.e., Renna and Tribuzio, 2011). The olivine-rich troctolites may represent portions of the mantle–crust transition dissected by the multiple melt intrusions and incorporated into the gabbroic sequence (see also Sanfilippo and Tribuzio, 2013b). The troctolites from the Pineto gabbroic sequence have euhedral to subhedral olivine (Fo = 89–84 mol%) and plagioclase (An = 73–66 mol%), and accessory clinopyroxene and spinel (Fig. 2). Similar to the olivine-rich troctolites, the troctolites locally contain oikocrystic clinopyroxenes, having a MORB-type incompatible elements signature (Fig. 1 of Supplementary material). These rocks also locally contain accessory orthopyroxene, which is locally associated with Ti-pargasite and ilmenite, thereby forming patches interstitial to olivine and plagioclase. The troctolites show a positive correlation between the forsterite in olivine and the anorthite in coexisting plagioclase, which indicates an origin mostly ruled by fractional crystallization (Sanfilippo and Tribuzio, 2013a). We selected troctolites with variably evolved compositions.

Fig. 2. (a) Forsterite proportion (mol%) in olivine versus anorthite in plagioclase (mol%) and (b) forsterite proportion (mol%) in olivine versus Cr# = Cr/Cr + Al in spinel (mol%). Data are averaged per sample; the error bars represent one standard deviation of the mean value. Are also reported other peridotites from Alpine Jurassic ophiolites (Beccaluva et al., 1984; Rampone et al., 1996, 1997; Mu¨ntener and Piccardo, 2003; Piccardo et al., 2007). Olivine-rich troctolites and troctolites are from the Ligurian ophiolites (Renna and Tribuzio, 2011) and from the Pineto Gabbroic sequence (Sanfilippo and Tribuzio, 2013a). The compositions of abyssal peridotites are from (Dick et al., 1984; Dick and Bullen, 1984; Tartarotti et al., 2002; Kelemen et al., 2007). The dunites, olivine-rich troctolites and troctolites from slow-spreading ridges are from Suhr et al. (2008), Drouin et al. (2009) and Dick et al. (2010).

2.2. Replacive dunites Replacive dunites from Lanzo South massif form bodies up to tens of meters in size (Boudier and Nicolas, 1972). Samples were collected in the central portions of five different replacive dunite bodies, and consist of up to 30 mm olivine grains (Fo = 90–89 mol%) associated with accessory amounts of euhedral to subhedral spinel. Accessory clinopyroxene is rare and occurs as anhedral grains associated with the spinel. Note that the selected dunites do not contain any mineral relict (i.e., orthopyroxene or plagioclase) of the host harzburgites. The clinopyroxene incompatible element compositions and the relatively high Cr# = [Cr/ (Cr + Al)] and TiO2 concentrations of the spinel (Fig. 2) indicate that the Lanzo South replacive dunites formed by infiltration of melts with a MORB-type geochemical signature (Mu¨ntener and Piccardo, 2003; Piccardo et al., 2007).

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The clinopyroxenes from the Lanzo South dunites are characterized by variable LREE fractionations (LaN/ SmN = 1.5–0.2) with respect to MREE and HREE, which are nearly flat at 10 times chondrite (Fig. 1 of Supplementary material). These variable LREE fractionations were attributed to a chromatographic effect during the final stages of crystallization of the percolating melt (Piccardo et al., 2007). Similar LREE fractionations were documented for the clinopyroxenes from the replacive MORB-type dunites of the Oman ophiolite, and interpreted as derived by differences in the melt–peridotite ratios (Kelemen et al., 1995; Akizawa et al., 2013). 2.3. Mantle peridotites The selected mantle peridotites have clinopyroxene and plagioclase ranging from 5 to 2 vol% and from 11 to 2 vol%, respectively, and will be hereafter referred as plagioclase-harzburgites. One peridotite sample without evidence for plagioclase crystallization by an impregnating melt was selected and hereafter referred to as spinel-harzburgite. The transition from spinel- to the plagioclase-harzburgites is associated with chemical modification of the original spinel facies minerals. The olivine from the plagioclase harzburgites shows a slight decrease in forsterite component with respect to the spinel harzburgite. The forsterite decrease is associated with an increase in Cr# and TiO2 contents of the spinel (Fig. 2). Chemical equilibration with impregnating melts is also documented by the clinopyroxene from the plagioclase-harzburgites, as shown by the low concentrations of Na2O and Sr (Mu¨ntener et al., 2010; Sanfilippo and Tribuzio, 2011). The LREE-depleted chondrite-normalized patterns of the clinopyroxenes from the plagioclase-harzburgites of Liguria and Corsica ophiolites (Fig. 1a of Supplementary material) suggest an event of impregnation by melts depleted in incompatible elements with respect to typical MORB compositions (Piccardo et al., 2004; Sanfilippo and Tribuzio, 2011). The clinopyroxenes from the plagioclase-harzburgites from Lanzo South massif show typical MORB-type REE patterns. Conversely, the clinopyroxenes from the associated plagioclase-free harzburgites have patterns markedly depleted in LREE (Piccardo et al., 2007). The REE variability of the clinopyroxenes from the Lanzo South mantle peridotites was attributed to an event of impregnation by melts with a normal MORB geochemical signature (see also Mu¨ntener et al., 2010). 3. MINOR AND TRACE ELEMENT COMPOSITIONS OF OLIVINE 3.1. Analytical methods Trace element compositions of olivine were obtained using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at C.N.R., Istituto di Geoscienze e Georisorse (Unita` di Pavia), and are given in Tables 1 and 2. The instrument couples a Nd:YAG laser source (Brilliant, Quantel) operating at 266 nm with a quadrupole ICPMS (Drc-e, Perkin Elmer). Analyses were

carried out with a spot diameter of 80–40 lm. In each sample, we performed 2–3 spot analyses in 3 to 4 olivine grains. Data reduction was carried out using the “Glitter” software package (Van Achterbergh et al., 2001). Ablation signal and integration intervals were selected by inspecting the time-resolved signal to ensure that no inclusions were present in the analyzed volume. NIST SRM 612 and 29Si were used as external and internal standards, respectively. Accuracy was tested on the BCR2-g (USGS) reference glass and is estimated to be <4% for Mn, Ni and Co, <5% for Al, Cr and Ca, <7% for Sc, V and Ti, <15% for Y, and <20% for Zr and HREE. The concentrations of minor and trace elements obtained in this study for the San Carlos olivine standard (Table 3) are consistent with those obtained by previous LA-ICP-MS and SIMS studies (De Hoog et al., 2010; Spandler and O’Neill, 2010). 3.2. Compatible to moderately incompatible elements (Ni, Mn and Co) The whole spectrum of analyzed olivines shows rough correlations between the forsterite component and Ni and Mn contents. Starting from the harzburgites, to the dunites, the olivine-rich troctolites and the troctolites, the Ni and Mn contents overall decrease and increase, respectively, with decreasing forsterite component (Fig. 3). The concentrations of Co in the olivines are not correlated with forsterite content. The olivines from the harzburgites have nearly constant Ni contents, and Mn and Co concentrations displaying significant variations. The olivines from the plagioclaseharzburgites from the Internal Ligurian and Corsica ophiolites have lower Mn and Co than those from the Lanzo South plagioclase harzburgites. The concentrations of Ni, Mn and Co in the olivines from the dunites produce positive correlations. The olivines from the olivine-rich troctolites do not exhibit a significant Ni–Mn correlation, whereas a negative Ni–Mn correlation exists for the olivines from the troctolites. The olivines from the olivine-rich troctolites and the troctolites have similar Mn and Co contents displaying positive correlations. Taken as a whole, the olivines from the dunites, the olivine-rich troctolites and the troctolites have Ni, Mn and Co contents similar to those of primitive olivine phenocrysts (Fo = 91–89 mol%) in MORB (Sobolev et al., 2005, 2007). 3.3. Highly incompatible elements (Ti, Zr, Y and HREE) The incompatible elements of the olivines overall increase with increasing Mn. The olivines from the plagioclase-harzburgites have higher concentrations of incompatible elements than those from the spinel harzburgite, most likely in response to the equilibration with the impregnating melts (Figs. 3 and 4). In particular, the olivines from Lanzo South plagioclase-harzburgites have the highest concentrations of incompatible trace elements (Table 1), which are most likely related to the typical MORB-type pattern geochemical signature of the impregnating melts (Piccardo et al., 2007; Mu¨ntener et al., 2010). The relatively low amounts of incompatible trace elements in the olivines from

Table 1 Trace element olivine compositions (average values, ppm). Unit

Serra Debbione P34b Plharzburgite

Internal Ligurides BR4 Plharzburgite

Internal Ligurides IN52 Plharzburgite

Lanzo

Lanzo

Lanzo

Lanzo

Lanzo

Lanzo

Lanzo

Lanzo

LZ61 Plharzburgite

LZ7 Plharzburgite

LZ11 Plharzburgite

LZ31 Dunite

LZ66 Dunite

LZ14 Dunite

LZ21 Dunite

LZ60 Dunite

n.

8

std

8

std

8

std

8

std

8

std

8

std

8

std

8

std

10

std

10

std

10

std

8

std

Al P Ca Sc Ti V Cr Mn Co Ni Cu Zn Y Zr Dy Er Yb

83.2 22.1 798 4.14 9.9 3.23 98.2 965 140 3000 0.36 57.8 0.031 0.011 0.008 0.008 0.021

11.8 4.74 76.9 0.40 2.24 0.32 12.5 15.9 2.29 46.2 0.12 3.20 0.010 0.004 0.004 0.001 0.006

63.8 17.3 503 7.36 21.5 3.60 79.5 971 137 2905 0.35 63.6 0.044 0.016 0.011 0.018 0.048

11.8 2.33 182 0.48 2.62 0.45 7.68 15.3 2.85 21.6 0.19 3.20 0.013 0.014 0.001 0.010 0.021

63.8 24.3 556 6.39 22.0 3.31 56.0 994 137 2896 0.40 56.5 0.038 0.012 0.021 0.018 0.072

10.3 6.86 129 0.58 2.46 0.51 6.34 7.5 1.85 26.8 0.10 3.10 0.014 0.001 0.010 0.007 0.038

48.0 12.8 469 3.31 15.6 2.62 50.3 1009 140 2884 0.25 56.5 0.027 0.012 0.011 0.014 0.041

18.1 3.14 77.2 0.41 3.86 0.64 20.0 15.0 1.25 31.0 0.07 2.00 0.010 0.006 0.006 0.007 0.026

73.6 16.2 618 3.51 26.4 3.59 79.0 1076 143 2842 0.25 55.5 0.045 0.018 0.008 0.027 0.029

24.9 3.07 143 0.53 6.01 0.83 20.2 14.9 1.37 23.9 0.06 5.95 0.009 0.003 0.003 0.010 0.018

71.8 33.8 554 4.53 44.3 4.04 75.0 1111 147 2895 0.31 66.7 0.063 0.034 0.016 0.011 0.036

6.5 5.95 93.8 0.19 3.70 0.39 11.0 18.8 1.92 47.6 0.09 3.15 0.015 0.016 0.008 0.003 0.020

43.5 20.7 334 4.91 28.7 2.92 44.1 1061 146 2985 0.19 107.7 0.033 0.010 0.016 0.013 0.039

13.0 4.34 67.4 0.24 4.65 0.45 6.06 5.9 1.81 35.2 0.1 5.03 0.004 0.004 0.006 0.009 0.013

66.7 23.4 1303 5.24 27.3 3.90 117.8 1095 136 2676 0.35 54.4 0.074 0.013 0.014 0.022 0.054

16.4 3.76 74.7 0.32 4.78 0.62 20.01 14.2 1.58 23.3 0.32 1.85 0.010 0.006 0.004 0.005 0.027

73.0 20.5 1121 4.75 26.9 3.16 78.8 1041 131 2486 0.09 59.9 0.075 0.021 0.015 0.024 0.042

22.0 3.41 131 0.42 4.11 0.18 4.07 12.9 1.47 76.0 0.01 2.61 0.019 0.004 0.013 0.014 0.024

53.1 24.1 1407 7.10 26.1 3.59 102.9 1117 140 2670 0.17 87.3 0.088 0.024 0.013 0.017 0.032

9.36 5.82 100 0.71 3.33 0.50 15.2 9.9 1.93 27.1 0.11 7.89 0.013 0.008 0.006 0.011 0.032

89.9 35.1 1603 7.00 32.9 4.92 135.7 1209 146 2826 0.13 61.9 0.112 0.024 0.018 0.026 0.042

18.0 6.84 224 0.53 4.03 0.60 22.8 21.2 2.63 21.9 0.06 8.54 0.018 0.009 0.010 0.015 0.030

108 24.5 1652 5.13 31.4 3.44 122.9 1071 132 2529 0.11 79.7 0.124 0.039 0.010 0.026 0.041

53.7 2.96 105 0.26 5.29 0.54 46.3 14.0 1.66 35.3 0.02 9.28 0.009 0.019 0.006 0.010 0.028

Unit

Pineto

Pineto

Internal Ligurides ST3 Ol-rich troctolite

Internal Ligurides SC7 Ol-rich troctolite

Internal Ligurides MA5 Ol-rich troctolite

Pineto

Pineto

Pineto

Pineto

Sample Rock-type

PI85 Ol-rich troctolite

PI41a Ol-rich troctolite

PI16 Troctolite

PI33 Troctolite

PI54 Troctolite

PI19 Troctolite

n.

6

std

7

std

8

std

8

std

8

std

6

std

8

std

8

std

Al P Ca Sc Ti* V Cr Mn Co Ni Cu

35.5 50.3 462 6.79 63.2 2.34 48.2 1341 139 1996 0.21

6.87 27.0 37.2 0.61 7.83 0.21 5.73 8.72 0.99 12.1 0.05

59.3 57.7 353 7.57 103 5.10 71.2 1443 148 2058 0.16

35.3 19.3 91.6 1.79 68.5 2.76 22.7 21.0 2.16 38.0 0.06

72.0 33.9 912 6.00 50.1 3.01 81.2 1274 139 2234 0.19

11.9 22.5 79.5 0.50 7.53 0.58 25.8 27.3 2.05 34.7 0.09

62.7 35.0 523 7.57 65.1 2.90 65.2 1407 142 2096 0.22

18.1 22.2 156 0.49 9.78 0.65 26.9 16.7 2.28 13.5 0.07

39.9 127 455 7.93 70.8 3.45 85.6 1450 143 2186 0.20

17.5 57.9 118 0.72 22.3 1.17 55.8 21.7 1.81 21.3 0.03

33.7 56.0 288 5.85 63.9 3.91 57.3 1300 142 2019 0.12

9.73 51.1 75.1 1.87 21.3 1.49 15.0 29.4 1.40 54.2 0.06

50.5 33.6 488 7.08 146 5.95 60.1 1561 148 1752 0.21

9.79 13.5 133 0.89 49.1 1.40 11.9 17.5 2.59 23.3 0.03

40.3 35.3 499 7.08 95.4 5.65 43.6 1717 152 1464 0.38

7.6 33.6 2.90 12.9 41.3 14.5 183 380 77.9 0.55 8.55 0.40 42.9 83.4 11.1 1.29 8.28 1.59 6.29 31.8 5.92 23.3 1831 27.5 2.84 152 1.25 16.5 1196 13.4 0.07 0.24 0.09 (continued on next page)

6

std

A. Sanfilippo et al. / Geochimica et Cosmochimica Acta 141 (2014) 423–439

Serra Debbione Sample P3 Rock-type Splharzburgite

Line missing 427

Highly variable within each olivine-rich troctolite and troctolite (see Table 2).

70.3 0.103 0.070 0.006 0.018 0.070 Zn Y* Zr* Dy Er Yb

*

std 6

120 0.185 0.301 0.006 0.040 0.147 13.71 0.048 0.128 <0.01 0.020 0.010

std 8

159 0.114 0.148 0.030 0.052 0.116

8

120 0.141 0.362 0.023 0.045 0.152

std

3.49 0.037 0.059 0.007 0.041

std

6

132.0 0.112 0.187 0.015 0.024 0.101

8

3.4. Elements controlled by subsolidus exchange (Cr, Al, Ca, Sc and V)

2.61 0.041 0.026 <0.01 0.021 0.031

std 8

69.5 0.113 0.044 0.017 0.035 0.099 2.67 0.022 0.018 0.007 0.010 0.049

std 8

89.0 0.095 0.069 0.012 0.027 0.067 3.79 0.036 0.157 0.005 0.010 0.067

std 7

73.9 0.162 0.205 0.016 0.018 0.145

6 n.

std

the plagioclase harzburgites of the Internal Ligurian ophiolites are correlated with the highly depleted geochemical signature of the infiltrating melts (Rampone et al., 1997; Piccardo et al., 2004; Sanfilippo and Tribuzio, 2011). The olivines from the olivine-rich troctolites and troctolites have similar incompatible trace elements concentrations, which are commonly higher than those of the olivines from the dunites (Figs. 3 and 4). Within a single sample of olivine-rich troctolites and troctolites, olivines have heterogeneous Ti, Zr, Y and HREE compositions. In each sample, the spot analyses commonly show Ti and Zr varying up to a factor of 5, and Y and HREE varying up to a factor of 3 (Fig. 5). In addition, we observed that the concentration of these elements show significant variations in a single olivine grain. These chemical heterogeneities commonly produce variable Ti/Y and Zr/Y values in the olivines from the olivine-rich troctolites and troctolites (Fig. 6).

5.17 0.060 0.096 0.014 0.010 0.034

74.7 0.084 0.139 <0.001 0.022 0.083

std

PI54 Troctolite PI33 Troctolite PI16 Troctolite PI41a Ol-rich troctolite PI85 Ol-rich troctolite Sample Rock-type

1.91 0.022 0.047 0.002 0.006 0.031

PI19 Troctolite

Pineto Pineto Pineto Pineto

Internal Ligurides MA5 Ol-rich troctolite Internal Ligurides SC7 Ol-rich troctolite Internal Ligurides ST3 Ol-rich troctolite Pineto Pineto Unit

Table 1 (continued)

1.45 0.027 0.110 <0.01 0.009 0.054

A. Sanfilippo et al. / Geochimica et Cosmochimica Acta 141 (2014) 423–439

6.98 0.053 0.237 0.010 0.026 0.071

428

The Cr, Al, Ca, Sc and V contents of the olivines from the mantle harzburgites are nearly constant and do not show any correlation with Mn (Fig. 3 and Table 1). The concentrations of these elements in olivines from mantle peridotites are believed to be partitioned into the co-existing minerals during the subsolidus evolution (Witt-Eickschen and O’Neill, 2005). On the basis of the Cr-, Al- and Ca-in olivine geothermometers (De Hoog et al., 2010), we obtained equilibration temperatures ranging between 950 °C and 840 °C for the harzburgites (Table 4). These temperature evaluations are lower by 200 °C than those obtained by the Ca-in orthopyroxene geothermometer (Brey and Ko¨hler, 1990) for the same sample. The differences between the olivine and the orthopyroxene equilibration temperatures are most likely related to the slow cooling history experienced by these harzburgites. The olivines from the olivine-rich troctolites and troctolites have Cr, Al, Ca, Sc and V contents partly overlapping those of the harzburgites (Fig. 3 and Table 1). Note that the concentrations of Cr and Ca in the olivines from the olivine-rich troctolites and the troctolites are lower than those of primitive olivine phenocrysts in MORB (Sobolev et al., 2005, 2007). The Ca contents of the olivines from the troctolites and olivine-rich troctolites were shown to reflect subsolidus diffusion with coexisting clinopyroxene during cooling (Coogan et al., 2002, 2005; Sanfilippo and Tribuzio, 2013a). Similarly, Cr, Al, Sc and V in these olivines are most likely related to subsolidus exchange with coexisting minerals (clinopyroxene and spinel) during the cooling evolution. Similar to the olivines from the olivine-rich troctolites and troctolites, the olivines from the dunites have low Cr contents (Fig. 3), which are attributed to subsolidus diffusion of Cr into the coexisting spinel (see also MilmanBarris et al., 2008). The olivines from the dunites have Ca contents partly overlapping those of the primitive olivine phenocrysts in MORB (Sobolev et al., 2005, 2007). Because clinopyroxene is absent or occurs in very low amounts within the dunites, we propose that the olivine from the

91.9 3.61 0.201 0.014 0.331 0.025 457 1.65 69.6 2.74 0.189 0.013 0.225 0.020 368 1.19 85.6 3.26 0.176 0.012 0.258 0.021 487 1.47 74.2 2.86 0.160 0.012 0.241 0.020 464 1.51

1

79.8 3.17 0.155 0.013 0.240 0.023 515 1.55

2

4.1. The replacive dunites: evidence for formation by not fully aggregated mantle-derived melts

103 6.9 0.187 0.024 0.128 0.027 551 0.68

4

101 6.2 0.132 0.014 0.079 0.014 764 0.60 63.9 3.95 0.062 0.011 0.081 0.014 1030 1.31

3

1r Zr 1r Ti/Y Zr/Y

Y

1r Ti

The values refer to single spot analyses and anaytical error based on external satndard (1r). Nd, value not determined.

71.3 4.25 0.053 0.009 0.094 0.017 1345 1.77 77.6 4.46 0.149 0.014 n.d 0.012 521 –

2

58.3 3.27 0.069 0.010 0.097 0.013 844 1.41 3 2 4 3 2 1

92.5 3.36 0.113 0.011 0.221 0.023 818 1.96

Grain #

66.0 2.53 0.087 0.010 0.106 0.016 761 1.22

56.3 2.21 0.043 0.006 0.111 0.016 1322 2.61

34.0 1.51 0.040 0.005 0.066 0.014 849 1.65

70.6 2.75 0.134 0.013 0.198 0.022 526 1.48

52.3 2.31 0.087 0.010 0.132 0.020 688 1.56

114 6.03 0.124 0.015 0.313 0.045 922 2.52

159 8.4 0.146 0.013 0.509 0.062 1278 4.10

129 7.3 0.093 0.013 0.284 0.045 1392 3.05

1

136 7.8 0.223 0.019 0.329 0.048 612 1.48

100 6.13 0.155 0.016 0.235 0.040 648 1.52

81.1 5.17 0.085 0.012 0.192 0.035 954 2.26

45.0 2.81 0.093 0.011 0.078 0.031 484 1.61

238 15.5 0.207 0.019 0.882 0.100 1150 4.26

71.1 3.82 0.116 0.012 n.d – 613 –

1

Pineto Troctolite PI54 Pineto Troctolite PI33 Pineto Troctolite PI16

1r Zr 1r Ti/Y Zr/Y

Y

1r

Unit Rock Sample

74.8 3.19 0.137 0.012 0.150 0.018 546 1.09 61.9 2.64 0.105 0.011 0.023 0.005 589 0.22

4. DISCUSSION

185 11.8 0.143 0.016 0.406 0.041 1297 2.84

Pineto Troctolite PI19a

47.5 2.62 0.080 0.015 0.026 0.011 594 0.33 58.6 3.07 0.090 0.016 0.058 0.017 651 0.64

3

72.8 3.58 0.120 0.018 0.028 0.011 607 0.23 73.2 3.63 0.141 0.020 0.084 0.020 519 0.60

2

64.9 3.28 0.096 0.015 0.014 0.008 676 0.15 1

61.8 3.15 0.125 0.019 n.d – 494 – 56.3 4.40 0.099 0.014 0.089 0.017 569 0.90 47.4 3.97 0.119 0.022 nd – 398 – 35.9 2.83 0.107 0.016 nd – 336 – 51.1 3.87 nd – nd – – – 56.2 2.58 0.154 0.017 0.116 0.021 365 0.75 62.1 2.83 0.156 0.018 0.163 0.028 398 1.04 58.1 2.81 0.130 0.020 0.048 0.009 447 0.37 193 7.58 0.194 0.022 0.401 0.048 996 2.07 56.6 2.60 0.082 0.010 0.064 0.014 689 0.78

2

Ti

54.0 2.33 0.106 0.010 0.053 0.010 511 0.50 Grain # 1

69.6 3.02 0.115 0.011 0.096 0.014 605 0.83

3

62.5 2.85 0.075 0.009 0.035 0.008 830 0.47

1

212 8.27 0.214 0.023 0.455 0.052 990 2.13

2

3

69.5 3.15 0.068 0.009 0.134 0.021 1022 1.97

4

68.7 3.01 0.122 0.015 0.116 0.020 563 0.95

1

48.1 3.39 0.062 0.009 nd – 781 –

2

49.5 3.60 0.092 0.011 0.056 0.011 538 0.61

3

50.3 3.83 0.070 0.010 0.061 0.015 718 0.87

4

62.1 4.86 0.114 0.013 nd – 545 –

Internal Ligurides Ol-rich troctolite SC7 Internal Ligurides Ol-rich troctolite ST3 Pineto Ol-rich troctolite PI41 Pineto Ol-rich troctolite PI85 Unit Rock Sample

Table 2 In-sample variations of Ti, Y and Zr concentrations of olivine from olivine rich-troctolites and troctolites (ppm).

429

dunites preserved the Ca, Sc and V contents acquired during the equilibration with the transient melts.

99.4 3.84 0.228 0.016 0.512 0.033 436 2.25

3

71.0 2.66 0.078 0.015 0.076 0.018 916 0.98

3

64.3 3.03 0.208 0.009 0.169 0.019 309 0.81 93.8 6.06 0.132 0.015 0.236 0.043 711 1.79 68.0 5.01 0.046 0.009 n.d – 1466 –

1

45.8 5.51 0.072 0.019 n.d – 636 – 63.7 3.23 0.062 0.014 0.052 0.015 1028 0.84

4

78.5 5.38 0.191 0.021 n.d – 411 –

Internal Ligurides Ol-rich troctolite MA5

113 3.07 0.073 0.018 0.308 0.038 1544 4.22 2

59.8 2.61 0.194 0.013 0.255 0.020 308 1.31

4

51.2 2.57 0.097 0.014 0.079 0.019 527 0.81

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The replacive dunites bodies from Lanzo were proposed to have formed by reactive migration of MORB-type melts (Mu¨ntener and Piccardo, 2003; Piccardo et al., 2007). The re-equilibration with MORB-type melts is corroborated by the minor and trace element compositions of the olivines from these rocks. These olivines have Mn, Ni, Co and Ca compositions overlapping those (Fig. 3) of the primitive olivine phenocrysts in MORB (Sobolev et al., 2005, 2007). To our knowledge, the incompatible element compositions (Ti, Zr, Y, Y and HREE) are still unknown for the olivine phenocrysts in MORB. Note, however, that the concentrations of Ti, Zr, Y and HREE contents in the dunite olivines are markedly higher than those of the olivines from the geochemically depleted spinel-harzburgites (Fig. 3). Because the olivine-melt partition coefficients reported in the literature for Ti, Zr, Y, and HREE vary by more than one order of magnitude (cf. McDale et al., 2003; Zanetti et al., 2004; Spandler and O’Neill, 2010), we chose to not report the comparison between calculated olivine equilibrium melts and published melt compositions. The concentrations of Mn, Ni, Co, Sc and V in the olivines from the dunites give positive correlations (Fig. 2, Supplementary material), which cannot be reconciled with a process of olivine fractionation. On the basis of experimental determined olivine-melt partition coefficients (e.g., Beattie et al., 1991), segregation of olivine from a melt is expected to produce a decrease of Ni and Co with increasing Mn, Sc and V. The positive correlations between Mn, Ni, Co, Sc and V could be attributed to dissolution of plagioclase from host peridotites, given the negligible concentrations of these elements in the plagioclase from the harzburgites (Piccardo et al., 2007). The dissolution of plagioclase from the wall harzburgites is expected to produce a decrease in Mn, Ni, Co, Sc and V in the melts forming the dunite. However, the plagioclase dissolution would also produce an increase of Ca concentrations (Morgan and Liang, 2005) coupled with decreasing Mn, Ni, Co, Sc and V contents, and this relation was not observed in the dunite olivines. In addition, this interpretation argues against the idea that the dunites preserve the migrating melts from interactions with host rocks (e.g., Kelemen et al., 1995; Lundstrom et al., 2000). We conclude that the positive correlations among Ni, Mn, Co, Sc and V reflect different compositions of the melts migrating through these dunite conduits. There is a general consensus that the MORB genesis involves the contribution of a garnet-bearing, pyroxenerich (i.e., geochemically enriched) component in the mantle source (e.g., Hirschmann and Stolper, 1996; Salters and Dick, 2002; Sobolev et al., 2005, 2007; Elliott et al., 2006). The composition of partial melts of this mixed mantle source depends on the amount of the enriched compo-

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Fig. 3. Variations in Ni, Co, Ti, Zr, Cr and Ca contents (ppm) of olivine in each sample plotted relative to the Mn contents (ppm). Data are averaged per sample; the error bars represent one standard deviation of the mean value for each sample (see Table 1). The compositions of olivines from primitive MORB phenocrysts (Sobolev et al., 2005, 2007) are also plotted for comparative purposes. The compositions of olivine in equilibrium with melt obtained by fractional crystallization modeling are also reported. The chemical composition of the melts was calculated using a potential minimization program (PELE) based on the algorithms and database of Ghiorso (1985) and Ghiorso and Sack (1995), and programmed by A.E. Boudreau (http://www.nicholas.duke.edu/people/faculty/boudreau/DownLoads.html). The average composition of the dunite olivines was used to calculate the minor and trace element compositions of the hypothetical primitive melt. Mn, Ni, Co, Ti and Zr in the primary melt are 1250, 200, 50, 3200 and 26 ppm respectively. Each step consists in a decrease of 5 °C in temperature at fixed pressure of 2 kbar. For simplicity, fractionation of spinel is avoided in this modeling, as it does not affect the results. Fig. 3b also shows the composition of olivine in equilibrium with melt evolving through an assimilation fractional crystallization (AFC) process for mass assimilated/mass crystallized ratio 0.9 (see text for further details).

nent involved in the melting process (see also Herzberg, 2011). Pertermann and Hirschmann (2003) inferred that the degree of melting of this enriched component in the MORB source increases with decreasing pressure conditions. The bulk partition coefficients for Ni and Co of a garnet-bearing, pyroxene-rich assemblage is >1 (Foley et al., 2013 and reference therein). Hence, the Ni and Co contents in the partial melts of a mixed source are expected to increase at decreasing pressure, making the contribution of this enriched component gradually higher. Mn, Sc and V have high affinity with garnet (e.g., Balta et al., 2011). At

high pressure, the enriched component is melted at low degrees and garnet retains the Mn, Sc and V in the residue. High-pressure partial melts of a mixed peridotite source would be poor in these elements. At low pressures, the enriched component is largely melted, garnet is consumed and the concentrations of Mn, Sc and V in the partial melts are expected to increase. We can thereby attribute the positive correlation displayed by the concentrations of Ni, Co, Mn, Sc and V in the dunite olivines to interaction with melts from a mixed peridotite source under different pressure conditions.

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431

Fig. 4. CI normalized patterns (Anders and Ebihara, 1982) of olivines in the studied samples. Values are averaged for each sample.

Fig. 5. Ti (ppm) and Zr (ppm) versus Y (ppm) in the olivines from dunites, olivine-rich troctolites and troctolites. Each symbol represents a single spot analysis, error bars are analytical errors. Dunites are depicted in blue, whereas different colors in troctolites and olivine-rich troctolites indicate the different samples. The gray lines indicates different Ti/Y and Zr/Y ratios. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The melts migrating through the replacive dunite conduits are generally considered to be fully aggregated MORB, mainly on the basis of the compositions of spinel and clinopyroxene (e.g. Kelemen et al., 1995; Dick and Natland, 1996). The minor and trace element compositions of the olivines from the replacive dunites of the present

Fig. 6. Mn (ppm) versus Ti/Y and Zr/Y ratios in the olivines from dunites and lower crustal samples. Each symbol represents a single spot analysis. Dunites are depicted in blue, whereas different colors in troctolites and olivine-rich troctolites indicate the different samples. Fractional crystallization models are obtained from Mn, Ti, Zr and Y trends in Fig. 3. Dashed gray arrows indicate variation in the Ti/Y and Zr/Y ratios in a single sample due to interactions with interacting melts. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

study, however, reveal that these melt conduits delivered primitive melts that are not fully aggregated. This implies that the full homogenization of the different mantle-derived

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A. Sanfilippo et al. / Geochimica et Cosmochimica Acta 141 (2014) 423–439

Table 3 San Carlos olivine composition (average values, ppm). Study

This work

De Hoog et al. (2010)

LA-ICPMS

LA-ICPMS

n.

3

std

2

Li Al P Ca Sc Ti V Cr Mn Co Ni Cu Zn Y Zr Dy Er Yb

1.80 93.7 29.6 615 4.24 24.1 3.64 107 1028 144 2785 1.09 68.7 0.029 0.012 0.007 0.007 0.024

0.15 1.56 2.09 13.8 0.17 0.96 0.19 2.82 7.99 2.33 34.4 0.03 2.79 0.011 0.003 0.004 0.002 0.004

Grain Technique

Spandler and O’Neill (2010)

Grain #1

196 42 625 3.2 25 4.2 103 1057 142 2950 0.9 54 0.060 0.003

Grain #2 LA-ICPMS

Ione Probe

Ione Probe

LA-ICPMS

std

2

std

2

std

4

std

10

std

n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r

1.17 172

n.r n.r

1.08 104

0.3 1

1.66

0.13

n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r n.r

1.9 174 32 665 2.7 24 4.5 108 1085 143 2840 1.7 58 0.070 0.009

552 2.4 19 4 107 1098 186

n.r n.r n.r n.r n.r n.r n.r

563 3.04 9.7 2.7 146 1117 179

7 0.6 0.5 0.3 2 20 6

594 3.81 2.64 3.45 205 973 142 3045

16 0.07 0.87 0.11 4.50 9.70 2.10 53

0.115

n.r

0.059

0.007

n.r, value not reported.

melts did not completely occur within these dunites. Our data are consistent with recent studies of the lower oceanic crust from East Pacific Rise, which indicate that different melts extracted from the mantle crystallize within the lower crust before complete aggregation (Gillis et al., 2014). This implies that heterogeneities in the MORB sources may be preserved until the lower crust is formed, and recall the idea that chemical and isotopic heterogeneity of MORB may arise from the incomplete mixing of fractional melts at depth (e.g., Rudge et al., 2013). 4.2. Ni, Mn and Co in olivines from the lower crust: proxy for fractional crystallization versus dissolution–reprecipitation processes? The olivines from the troctolites have Ni decreasing with increasing Co and Mn (Fig. 3). As first glance, these variations are consistent with fractional crystallization of olivine + plagioclase in cotectic proportions. In particular, the correlations among Ni, Mn and Co observed for the olivines from the troctolites can be reproduced with an olivine–plagioclase assemblage with a 2:3 ratio, in agreement with the modal compositions of these rocks. Formation of the olivines from the troctolites by fractional crystallization was similarly inferred on the basis of a positive Ni versus forsterite positive correlation (Sanfilippo and Tribuzio, 2013a). The olivines from the olivine-rich troctolites have nearly constant Ni, whereas the Co contents increase with increasing Mn concentrations. These Ni, Mn and Co chemical variations are not consistent with a formation through fractional crystallization of olivine–plagioclase at 4:1 proportions, as observed in these rocks. Co and Ni are compatible in olivine (e.g., Beattie et al., 1991) and incompatible in

plagioclase. Fractionation of an olivine–plagioclase assemblage at 4:1 proportions would thereby produce a sharp decrease in Ni and Co together with increasing Mn (Fig. 3). These correlations are not observed in the olivines from the olivine-rich troctolites. The origin of the olivine-rich troctolites is commonly related to reactions between an olivine-rich matrix and migrating melts (e.g., Suhr et al., 2008; Drouin et al., 2009; Renna and Tribuzio, 2011). This process produces a change in the olivine compositions, most likely in response to olivine dissolution by the migrating melts. The olivine dissolution is associated with formation of new olivine, which re-precipitates together with plagioclase and clinopyroxene (see also Sanfilippo et al., 2013). The positive correlation between Mn and Co, and the lack of Ni variations of the olivines from the olivine-rich troctolites can derive from this dissolution– reprecipitation process. In particular, the reprecipitating olivine is progressively enriched in both Mn and Co by the synchronous fractionation of plagioclase and clinopyroxene. This process would also progressively decreases the reacting melts in MgO, thereby leading the Ni partition coefficient between olivine and liquid to increase (Hart and Davis, 1978; Matzen et al., 2013). The Ni contents of the re-precipitated olivine would thus remain relatively high, even if the reacting melt becomes progressively more fractionated. 4.3. Ti, Zr and Y in olivines from the lower crust: evidence for reactive melt migration The olivines from the troctolites and olivine-rich troctolites exhibit inter- and intra-sample heterogeneity in Ti, Zr, Y and HREE, commonly associated with markedly variable Ti/Y and Zr/Y values (Fig. 5 and Table 2). The wide

25

198

211

204 26

std 0.05 28

24

10

8 34.48 846 204 836 214 835 215 std 0.03 11

32

38

8 34.45 904 178 894 187 897 185 std 0.02 38

23

46

8 35.62 902 169 893 178 910 161 std 0.02 41

30

14

8 33.15 854 237 850 241 876 215 std 0.03 18

48

14

8 31.61 889 212 875 226 897 205 std 0.02 23

14

17

8 36.42 889 210 889 210 880 219 std 0.03 17

Olivine geothermometers (De Hoog et al., 2010) n. 8 Cr# in olivine 36.97 Temperature Al in olivine (°C) 920 Difference with Ca in Opx (°C) 216 Temperature Cr in olivine (°C) 912 Difference with Ca in Opx (°C) 225 Temperature Ca in olivine (°C) 947 Difference with Ca in Opx (°C) 190

std 0.13 118 5 1.19 1050 std 0.18 145 6 1.33 1082 std 0.08 68 6 1.28 1071 std 0.32 261 3 1.34 1091 std 0.48 366 5 1.44 1101 std 0.36 284 Ca in orthopyroxene geothermometer (Brey and Ko¨hler, 1990) n. 9 std 10 Ca in orthopyroxene (wt%) 1.60 0.10 1.40 Temperature Ca in orthopyroxene (°C) 1136 69 1099

Internal Ligurides BR4 Pl-harzburgite Serra Debbione P34b Pl-harzburgite Serra Debbione P3 Spl-harzburgite Unit Sample Rock-type

Table 4 Equilibration temperatures in harzburgites based on orthopyroxene and olivine geothermometers.

Internal Ligurides IN52 Pl-harzburgite

Lanzo LZ61 Pl-harzburgite

Lanzo LZ7 Pl-harzburgite

Lanzo LZ11 Pl-harzburgite

Avereage temperature difference (°C)

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433

ranges of Ti/Y and Zr/Y values of the olivines from the single olivine-rich troctolite and troctolites cannot be reconciled with a process of fractional crystallization from a common parental melt (see model in Fig. 6). Similar to the olivines, the spinels from the troctolites and the olivine-rich troctolites have heterogeneous TiO2 contents (0.8–2.2 wt%), which are commonly higher than those of spinels in MORB (<1 wt%, Kamenetsky et al., 2001). In addition, the spinels from these rocks frequently contain mineral inclusions of kaersutite to titanian pargasite and/or phlogopite to aspidolite (Renna and Tribuzio, 2011; Sanfilippo and Tribuzio, 2013a). These mineral inclusions are locally associated with orthopyroxene and, rarely, with Mg-ilmenite and/or Zr-loveringite. The inclusion-bearing spinels were considered to have formed by hybrid melts enriched in Cr2O3, SiO2 and in incompatible elements, such as volatiles, TiO2, Na2O, K2O and Zr (Renna and Tribuzio, 2011). Note that similar conclusions have been recently reported for the olivine-rich troctolites and melt-reacted harzburgites from Atlantis Massif (Mid Atlantic Ridge) mainly on the basis of the trace element compositions of amphibols included within the spinel (Tamura et al., in press). The involvement of these hybrid melts may also explain heterogeneous and anomalously high TiO2 contents of the spinels in these rocks. A further argument in agreement with the involvement of migrating melts in the formation of the troctolites and olivine-rich troctolites is given by the Cr2O3 contents of the clinopyroxenes oikocrysts. The melts in equilibrium with these clinopyroxenes have Cr contents too high to represent MORB-type melts (Renna and Tribuzio, 2011; Sanfilippo and Tribuzio, 2013a). Similarly, high Cr-clinopyroxenes are commonly documented for the troctolites and olivine-rich troctolites from the Mid-Atlantic Ridge (Lissenberg and Dick, 2008; Suhr et al., 2008; Drouin et al., 2009; Renna and Tribuzio, 2011) and Godzilla Megamullion (Sanfilippo et al., 2013). These clinopyroxenes are attributed to crystallization of melts reactively migrating through an olivine + plagioclase matrix and can be enriched in Cr through dissolution of spinel and/or pyroxene during previous interaction (see also Van den Bleeken et al., 2011). The variable Ti, Zr, Y and HREE compositions of the olivines from the olivine-rich troctolites and the troctolites (Fig. 5) may be therefore reconciled with interactions between former olivines and the same migrating melts that formed the spinels and the clinopyroxenes. We propose that these melts migrated through a former crystal matrix, thereby leading to modification in the incompatible elements ratios of the original olivines according to the melt migration trends depicted in Fig. 6. Note that one of the selected troctolites is spinel-free. The olivines from this troctolite show nearly constant Ti and Zr and relatively low Ti/Y and Zr/Y values, thereby indicating that this sample experienced negligible interactions with the migrating melts, which prevented the initial Ti/Y and Zr/Y values to sharply increase. We conclude that the troctolites and olivine-rich troctolites both experienced interaction with migrating melts dur-

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Fig. 7. Y (ppm) versus Ti/Y ratios in the olivine from dunites, olivine-rich troctolites and troctolites. Dunites data are averaged per sample, whereas each symbol for olivine-rich troctolites and troctolites indicates a single spot analysis. The olivines of the troctolite modeled in the assimilation fractional crystallization process (AFC) process (sample PI33) are depicted in white color. The olivines from the spinel-free troctolite (sample PI19) are also shown in yellow. The calculated chemical trends of the olivine in equilibrium with melt produced during the AFC process are depicted in gray at mass assimilated/mass crystallized ratio of 0.85, 0.90 and 0.95 (see text for details). The italic numbers indicate the proportion of melt crystallized during this process. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ing the late-stage crystallization history. The minor and trace element compositions of the olivines document that dissolution–reprecipitation processes may prevent to discriminate whether the precursor of these rocks formed by fractional crystallization or mantle-melt reactions (see discussions in Suhr et al., 2008; Drouin et al., 2010). In any cases, it seems obvious that olivine-rich troctolites and troctolites experienced reactive migrations of melts chemically similar. 4.4. An assimilation-fractional crystallization model to reproduce the reactive melt migration process To test if a reactive melt migration process may account for the formation of crustal olivines with heterogeneous Ti/ Y and Zr/Y values, we calculated an assimilation-fractional crystallization process (AFC; DePaolo, 1981), which simulates the local dissolution of a crystal matrix and the simultaneous crystallization of new phases at decreasing temperature. This model has been largely used to test reactive melt migration hypotheses in the formation of the lower oceanic crust (e.g.; Coogan et al., 2000; Gao et al., 2007; Sanfilippo et al., 2013; Lissenberg et al., 2013). In particular, we reproduced the Ti, Y and Zr variations in the olivines of the troctolite PI33, the sample showing the largest Ti/Y and Zr/Y ranges (Figs. 5 and 6). The compositions of the phases involved, the composition of melts produced during the interaction and the partition coefficients used are reported in Table EM9 of the Supplementary material. An AFC process is expected to obliterate the original composition of the phases involved, hence we assumed as assimilated and crystallized material the olivine and plagioclase from troctolite PI33. The initial melt (Melt1) is in

equilibrium with the olivine having the lowest Ti/Y and Zr/Y values (i.e. grain #4, see Table 2), which reasonably experienced the lowermost degree of interaction. We assume that Melt1 assimilates a preexisting matrix (olivine/plagioclase at cotectic proportions 1:1.5) and recrystallizes new olivine, new plagioclase and additional clinopyroxene (at proportions 3:5:2). This process produces residual Melt2 with Ti/Y and Zr/Y values progressively increased. From the composition of Melt2, we calculated the Ti/Y and Zr/Y trends of the equilibrium olivine at 10 steps of the AFC process, using variable mass assimilated/mass crystallized ratios (Fig. 7). The model shows that the Ti/Y and Zr/Y variations of the olivines from the troctolite PI33 are reproduced at mass-assimilated/ mass-crystallized ratio of 0.9. We can thereby consider the in-sample variation in Ti, Y and Zr contents as result of different stages of this reactive melt migration process. Note that the large spot diameter used in this study precluded the possibility to explore possible zonations in the selected olivines. It is thereby possible that the in-sample variability in Ti, Y and Zr contents of the olivines from the lower crustal samples can be wider than those revealed by this study. In Section 4.2 we inferred that the Ni, Mn and Co variations in the olivines from the olivine-rich troctolites are consistent with a formation by melt-rock reactions process. Differently, the Ni, Mn and Co observed for the olivines from the troctolites are virtually reproduced with a fractional crystallization process of olivine–plagioclase at cotectic proportions (Fig. 3). We hence used the same parameters used to model the Ti, Y and Zr variations in the troctolite PI33 to verify whether the same reactive melt migration event can also produce the observed Ni, Mn and

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Co variations. In particular, given the strict dependence of the olivine–liquid partition coefficient for Ni on the MgO composition of the melts (Matzen et al., 2013), we modeled exclusively the Mn and Co variations (see details in Table EM9 of the Supplementary material). Notably, the same AFC model used to mimic the Ti/Y and Zr/Y variations in the olivines from the troctolite PI33 also accounts for the Mn and Co variations in the different troctolite samples (Fig. 3b). Following this evidence, we can infer the trends depicted by the Ni, Mn and Co contents in the troctolite olivines to be at least partly constrained by melt migration processes. The lack of detectable in-sample variations for Ni, Mn and Co of the olivine in each sample is likely due to the late homogenization of these elements within a single crystal, allowed by their fast intra-crystalline diffusion in olivine (Spandler and O’Neill, 2010). 4.5. Origin of the melts reactively migrating through the lower crust In Section 4.3 we inferred that the olivine-rich troctolites and the troctolites record reactive migration of melts rich in Cr, silica and incompatible elements. These rocks form the lowermost sector of the Pineto sequence and are locally

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found in association with mantle peridotites in the Ligurian ophiolites (Renna and Tribuzio, 2011; Sanfilippo and Tribuzio, 2013a). Troctolites and olivine-rich troctolites are found in association with mantle harzburgites and dunites at Kane and Godzilla Megamullions (Dick et al., 2008; Dick et al., 2010; Sanfilippo et al., 2013). These data led us to suggest that the olivine-rich troctolites and the troctolites formed close to the mantle–crust transition. The dunites from the mantle–crust transition below an ocean ridge are considered to be products of melt– peridotite reactions mostly driven by the preferential dissolution of mantle orthopyroxene and crystallization of olivine (Kelemen et al., 1997; Koga et al. 2001). This process leads to enrichment of SiO2 and Cr2O3 in the percolating melt (Kelemen et al., 1990; Dick and Natland, 1996; Suhr, 1999). At decreasing melt mass and temperature, this melt–peridotite interaction is capable of producing enrichments in incompatible elements (Arai et al., 1997; Arai and Matsukage, 1998), as the crystallization of new olivine is associated with local crystallization of clinopyroxene (Quick, 1981; Takazawa et al., 1992; Suhr et al., 2003). A recent experimental study showed that the formation of clinopyroxene during melt–peridotite interactions is favored by cooling and depressurization (Saper and

Fig. 8. Schematic cartoon (redrawn after Grove et al., 1992; Collier and Kelemen, 2010) showing the early magmatic processes occurred in the oceanic lithosphere. The melts produced at different depths of the asthenosphere are transported through the lithospheric mantle within migration channels likely represented by replacive dunites. The melts migrating through these conduits may locally aggregate and mix within the lithospheric mantle. If these melts diffusely migrate within the peridotites, they form plagioclase-harzburgites by melt-impregnation. Most of the aggregation of the mantle-derived melts occurs at shallow mantle lithospheric levels or at the base of the igneous crust, but partly aggregated melts can be locally preserved. During period of high melt supply (A), these melts are injected to form the crystal mushes. Melts pooling at the base of the igneous crust interact with the host mantle peridotites, thereby forming an olivine-rich layer through preferred dissolution of orthopyroxene and additional crystallization of olivine. During periods of low melt supply (B), the melt mass decreases and melts residual from interaction with the mantle progressively convert this olivine-rich layer into olivine-rich troctolites. These melts can also migrate upward into the crystallizing cumulates, modifying the chemical composition of the preexisting crystal matrix forming the troctolites.

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Liang, 2014). Clinopyroxene-bearing dunites to plagioclasewehrlites may be therefore produced by interactions between melts and peridotites at decreasing temperature– pressure, conditions expected to occur at the mantle–crust transition. We thus propose that the SiO2, Cr2O3 and incompatible elements-rich melts involved in the formation of the olivine-rich troctolites and the troctolites were produced through interactions with the mantle peridotites. In a slow spreading scenario, the lower crust forms by association of multiple gabbroic intrusions, most likely derived from different magmatic episodes (Grimes et al., 2008; Lissenberg et al., 2009; Rioux et al., 2012). The injections of primitive melts are followed by periods with low or no melt supply. During periods of high melt supply, the primitive melts are injected to form crystal mushes, but may also interact with the lithospheric mantle forming a dunite layer at the mantle–crust transition (Abily and Celuneer, 2013). When the melt supply decreases, a progressive decreasing in melt mass and temperature leads to the formation of residual melts anomalously enriched in SiO2, Cr2O3 and incompatible elements. These melts may migrate upward into previously formed olivine ± plagioclase crystal mushes, thereby modifying the compositions of the original minerals and leading to the local crystallization of spinel and clinopyroxene. We propose that this process accounts for the formation of the troctolites. Melts enriched in SiO2, Cr2O3 and incompatible elements are also involved in the formation of the olivine-rich troctolites. It has been proposed that these rocks may originate from a process of reactive dissolution of a mantle dunite (e.g., Suhr et al., 2008; Drouin et al., 2009, 2010; Renna and Tribuzio, 2011; Sanfilippo et al., 2013). Following this idea, these rocks may represent portions of the olivine-rich layer formed through melt–peridotite interaction during periods of high melt supply, lately converted into olivine-rich troctolites at decreasing melt mass. 5. SYNTHESIS Minor and trace elements compositions of olivine provide information about mantle and early magmatic processes in the oceanic lithosphere. The olivines from the replacive dunites equilibrate with primitive MORB-type melts likely formed by a mixed source consisting of a depleted peridotite and a pyroxene-rich, garnet-bearing component melted under different pressure conditions. These dunites hence experienced migration of melts incompletely aggregated after their formation in the asthenosphere. This implies that that the complete aggregation of MORB-type melts did not exhaust at depth, supporting the idea that compositionally different melts can be extracted from the mantle and crystallize in the lower crust before the complete homogenization of erupted basalts. During periods of high melt supply, the melts extracted through the dunites are injected to form olivine ± plagioclase crystal mushes, but may also pool at the mantle–crust transition, producing olivine-rich layers through reactions with the mantle peridotites (Fig. 8a). When melt supply decreases, melts enriched in Cr2O3, SiO2 and incompatible elements are produced by incongruent dissolution of

orthopyroxene and crystallization of olivine plus additional clinopyroxene. We propose that these melts may locally migrate into the forming crystal mushes, thereby modifying their original mineralogy and the chemical composition of the pre-existing minerals. Troctolites and olivine-rich troctolites are the final products of these reactive migration events, likely occurred at or near the mantle–crust transition (Fig. 8b). Successive melt injections during new magmatic pulses can dissect these rocks leading to their entrapment within more evolved gabbros. ACKNOWLEDGMENTS The first author would like to thank H. Dick and H. Marschall for stimulating discussions during the cruise KN-210-5. We acknowledge the editor J. Blichert-Toft for her punctuality and dedication during the editing work. We are grateful to V. Salters and J. Lissenberg whose comments helped to increase the clarity of the data presented and their discussions. Dr. R. Tassinari (University of Ferrara) is also acknowledged for assistance during the whole rock analyses at XRF. This work was financially supported by the “Programma di Ricerca di Interesse Nazionale” of Italian “Ministero dell’Universita‘ e della Ricerca” (prot. 20099SWLYC).

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