Mineral inventory of continuously erupting basaltic andesites at Arenal volcano, Costa Rica: implications for interpreting monotonous, crystal-rich, mafic arc stratigraphies

Journal of Volcanology and Geothermal Research 140 (2005) 133 – 155 www.elsevier.com/locate/jvolgeores

Mineral inventory of continuously erupting basaltic andesites at Arenal volcano, Costa Rica: implications for interpreting monotonous, crystal-rich, mafic arc stratigraphies Martin J. Strecka,*, Michael A. Dunganb, Francois Bussyc, Eduardo Malavassid a

Portland State University, Department of Geology, Portland, OR, 97207, USA b Department of Mineralogy, University of Geneva, Geneva, Switzerland c Department of Mineralogy, University of Lausanne, Lausanne Switzerlaned d OVSICORI, Universidad Nacional, Heredia, Costa Rica

Received 28 October 2003; received in revised form 4 February 2004; accepted 15 July 2004

Abstract Except for the first 2 years since July 29, 1968, Arenal volcano has continuously erupted compositionally monotonous and phenocryst-rich (~35%) basaltic andesites composed of plagioclase (plag), orthopyroxene (opx), clinopyroxene (cpx), spinelFolivine. Detailed textural and compositional analyses of phenocrysts, mineral inclusions, and microlites reveal comparable complexities in any given sample and identify mineral components that require a minimum of four crystallization environments. We suggest three distinct crystallization environments crystallized low Mg# (b78) silicate phases from andesitic magma but at different physical conditions, such as variable pressure of crystallization and water conditions. The dominant environment, i.e., the one which accounts for the majority of minerals and overprinted all other assemblages near rims of phenocrysts, cocrystallized clinopyroxene (Mg# ~71–78), orthopyroxene (Mg# ~71–78), titanomagnetite and plagioclase (An60 to An85). The second environment cocrystallized clinopyroxene (Mg# 71–78), olivine (bFo78), titanomagnetite, and very high An (~90) plagioclase, while the third cocrystallized clinopyroxene (Mg# 71–78) with high (N7) Al/Ti and high (N4 wt.%) Al2O3, titanomagnetite with considerable Al2O3 (10–18 wt.%) and possibly olivine but appears to lack plagioclase. A fourth crystallization environment is characterized by clinopyroxene (e.g., Mg#=~78–85; Cr2O3=0.15–0.7 wt.%), Al-, Cr-rich spinel, olivine (~Fo80), and in some circumstances high-An (N80) plagioclase. This assemblage seems to record mafic inputs into the Arenal system and crystallization at high to low pressures. Single crystals cannot be completely classified as xenocrysts, antecrysts (cognate crystals), or phenocrysts, because they often contain different parts each representing a different crystallization environment and thus belong to different categories. Bulk compositions are mostly too mafic to have crystallized the bulk of ferromagnesian minerals and thus likely do not represent liquid compositions. On the other hand, they are the cumulative products of multiple mixing events assembling melts and minerals from a variety of sources. The driving force for this multistage mixing evolution to generate erupting basaltic andesites is thought to be the ascent of mafic magma from lower crustal levels to subvolcanic depths which at the same time * Corresponding author. Tel.: +1 503 725 3379; fax: +1 503 725 3025. E-mail address: [email protected] (M.J. Streck). URL: http://www.geol.pdx.edu/. 0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2004.07.018

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may also go through compositional modification by fractionation and assimilation of country rocks. Thus, mafic magmas become basaltic andesite through mixing, fractionation and assimilation by the time they arrive at subvolcanic depths. We infer new increments of basaltic andesite are supplied nearly continuously to the subvolcanic reservoir concurrently to the current eruption and that these new increments are blended into the residing, subvolcanic magma. Thus, the compositional monotony is mostly the product of repetitious production of very similar basaltic andesite. Furthermore, we propose that this quasi-constant supply of small increments of magma is the fundamental cause for small-scale, decade-long continuous volcanic activity; that is, the current eruption of Arenal is flux-controlled by inputs of mantle magmas. D 2004 Elsevier B.V. All rights reserved. Keywords: arenal; mineral inclusions; andesite; magma mixing; pyroxene; anorthitic plagioclase

1. Introduction Few volcanoes worldwide have shown continuous volcanic activity over decades. The number is even smaller if nonbasaltic volcanoes in arc settings are considered. Arenal volcano is one of the few; it is an arc volcano that has undergone multidecade, continuous eruptive activity of basaltic andesite. The current eruption started in July of 1968 and, at the time of writing this article, is in its 36th year of activity. This long and frequent sampling of magma provides a remarkable opportunity to examine critical questions about the longevity and balance of processes in subvolcanic magmatic systems. The current eruption is mainly characterized by frequent strombolian to vulcanian explosions, episodic lava flows, and rare pyroclastic flows (Malavassi et al., 1998; Melson et al., 1999; P. Cole, pers. com.). Eruption products are basaltic andesite and whole-rock compositions are monotonous (Fig. 1). Ranges of some key elements in erupted products between mid-1969 to 2000 are SiO2=53.6–54.9 wt.%, MgO=5.0–4.4 wt.%, and K2O=0.55–0.73 wt.% (Reagan et al., 1987; Streck et al., 2002, this study). Mineral modes appear mostly uniform, although mineral zoning and textures reveal a high degree of complexity in individual lava samples. In this respect, Arenal volcano resembles many arc volcanoes, especially smaller ones that erupted solely basaltic andesites. The mineralogical complexities, in light of monotonous bulk compositions, are often not explored for questions regarding processes and dynamics of magma generation beneath volcanoes such as Arenal. For this reason, this study has focused on the mineralogical complexities encountered in individual lava samples. First, we document the compositional ranges of all mineral phases, then we establish

coprecipitating mineral assemblages that are juxtaposed by the time of eruption. By piecing together wide-ranging clinopyroxene compositions with the composition and occurrence of mineral inclusions, we show that, although lavas of the current eruption are dominated by one mineral assemblage, they also contain other mineral assemblages. Hence, only part of the petrological story is recorded from the obvious and dominant mineral assemblage. On the other hand, important pieces to construct a more accurate image of the processes operating in the magmatic reservoir(s) are only preserved (a) as minor mineral components and (b) how mineral assemblages are expressed within single crystals such as the relative positions of zoning characteristics and associated mineral inclusions. The important implication is that a single crystal may contain different parts, each of which could be classified as xenocrystic, antecrystic, and phenocrystic (the name antecryst was proposed for a crystal that is out of current equilibrium but is broadly related because it is derived from an earlier evolutionary stage of the magmatic system; Hildreth, pers. com.). This study demonstrates that reliance on bulk chemical data alone to infer subvolcanic processes that generate lavas of crystal-rich, monotonous and mafic arc stratigraphies will lead to oversimplified models. A bulk chemical approach is insensitive to distinguishing among contributions of minerals derived from a variety of sources to bulk magma compositions. Furthermore, it is questionable whether analyzed crystal-rich bulk compositions ever existed as true liquid compositions from which minerals precipitated, unless careful mineralogical analysis can demonstrate this. Our model to generate basaltic andesites of the current eruption is the following. As mafic magma batches ascend from lower crustal levels to subvolcanic

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Fig. 1. Bulk compositions of Arenal lavas through time from 1968 (beginning of ongoing eruption) to 2000. Data sources for graphs are found in the graph and include Reagan et al. (1987); Streck et al. (2002), and this study. Ca#=molar Ca/(Ca+Na)100 and Mg#=molar MgO/ (MgO+FeO*)100 (see text for details).

depths, multistep mixing processes assemble mineral components and melts with diverse histories. During this ascent, mafic magma undergoes evolution to erupting basaltic andesitic compositions through fractionation, assimilation and mixing processes. We infer new increments of basaltic andesite are supplied nearly continuously to the subvolcanic reservoir concurrently to the current eruption and that these new increments are blended into the residing, subvolcanic magma. Thus, the compositional monotony is mostly the product of repetitious production of very similar basaltic andesite. Furthermore, we propose that this quasi-constant supply of small increments of magma is the fundamental cause for small-scale, decade-long continuous volcanic activity; that is, the current eruption of Arenal is flux-controlled by inputs of mantle magmas.

2. Samples and analytical methods Samples used are those of Streck et al. (2002) (Fig. 1) plus two samples obtained through William Melson

from the Smithsonian collection. Here we report on new clinopyroxene and spinel data that are added to previously obtained data and report for first time on orthopyroxene, plagioclase, and olivine data. One of the two new samples is from the active lava flow front of May 5, 1999 and the other is a sample of the pyroclastic flow erupted August 23, 2000. The latter was also analyzed by X-ray fluorescence analysis at the University of Lausanne (Philips 2400). Prior to microprobe analysis, all minerals were examined petrographically by light microscopy, Nomarski (NIDC) techniques or backscattered electron (BSE) imagery. Microprobe conditions for clinopyroxene and spinel analyses were reported by Streck et al. (2002) and were used to obtain more data mostly on inclusions and microlites of clinopyroxene (cpx) and spinel; the same conditions were used in the orthopyroxene analyses. Plagioclase analytical conditions are reported by Lunney (2002). Olivine was analyzed under the conditions used for pyroxenes with the 5-spectrometer Cameca SX-50 microprobe at the University of Lausanne. Olivine was also analyzed with the Cameca SX-50 microprobe at Oregon State University with a

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focused beam diameter of ~1 Am, acceleration voltage of 15 kV, and beam current of 50 nA. Counting times on peak and background (peak/background) were 10/5 s: for Na, Mg, Al, Si, Ca, Ti, Cr, Mn, and Fe, and 20/10 s for Ni. Groundmass glass was analyzed under two different conditions: (1) with a focused beam to increase spatial resolution (at Lausanne); and (2) with a defocused beam to minimize Na loss (at Oregon State University; cf. Lunney, 2002). The focused beam analyses clearly indicate Na loss, but otherwise yield the same compositional spread as analyses obtained with a defocused beam. This also confirms that spots analyzed with a wider beam size did not overlap with microlites. Half of 149 clinopyroxene phenocrysts, all 23 orthopyroxene (opx) phenocrysts and the majority of 120 plagioclase (plag) phenocrysts were analyzed along high-resolution core-rim traverses (3–4 Am spacing). The other half of clinopyroxene phenocrysts, one third of plagioclase phenocrysts and all olivine phenocrysts were analyzed along traverses with a point spacing every 10–40 Am. Because olivine abundance is low, we were able to analyze all, but a few olivine phenocrysts similar to clinopyroxene (cpx) (Streck et al., 2002). On the other hand, analyzed grains of orthopyroxene and plagioclase are representative of the size range and range in textures of all orthopyroxene and plagioclase. Analyses were screened for low totals (b98%) or those compromised by overlap with other minerals and were discarded. In most cases, the phenocryst host was also analyzed adjacent to the analyzed inclusion in order to correlate mineral inclusion composition with host compositions. In other cases, the location of the mineral inclusion is close (b80 Am) to the analysis traverse (see above), thus host composition was derived by projection of inclusion into the analysis traverse using optically observable zonation features. The same procedure yielded host compositions for a few inclusions that were located far (N100 Am) from the analysis traverse and where the host was not directly analyzed.

3. Occurrence and abundance of phases Mineral assemblages consist of plagioclase, clinopyroxene, orthopyroxene, and spinel. Olivine occurs sparsely and intermittently throughout the eruption

period. Hornblende is restricted to the beginning phase of the eruption (Reagan et al., 1987) and will not be discussed here because of its restricted occurrence. During the course of this study, sulfide was observed but not analyzed. Sparse sulfide inclusions were observed in orthopyroxene and clinopyroxene phenocrysts of lavas erupted in 1968, 1971, 1983, and 1991. Texturally, the largest change in lavas occurs between July 1968 (the onset of the eruption) and March 1969. Here, we consider microlites to be b~70 Am, microphenocrysts between ~70 and 300 Am, and phenocrysts N300 Am. Samples from March 1969 to the present are characterized by ~35% phenocrysts, often glomerocrystic, and microphenocrysts of plagioclase, orthopyroxene and clinopyroxene (although rarer), some oxides, and the occasional olivine. Grain size is nearly continuous between smallest microlites and largest phenocrysts. Visually, the strongest grain size gap lies between microlites and microphenocrysts. On the other hand, samples from the initial plinian phase in July of 1968 consist of only 20% phenocrysts and microphenocysts. A mixture of rapidly quenched microlites (b10 Am) and residual glass constitutes the other 80% (on a vesicle-free basis). Samples of July 1968 exhibit a more obvious porphyritic texture, mostly due to lower phenocrystic proportions combined with a lack of grain sizes between microphenocrysts and very small microlites, compared to later samples. Grain size distributions of individual phases indicate a pronounced difference going from microlites to phenocrysts. Clinopyroxene occurs mainly as large phenocrysts of greater than several hundred micrometers up to several millimeters, with a minor amount of microphenocrysts which appear most abundant early (1960s and 1970s) in the eruption. Volumetric proportion of phenocrystic clinopyroxene can reach percentages of 5% or higher (Streck et al., 2002). Clinopyroxene microlites do exist but appear to be subordinate to orthopyroxene microlites. Orthopyroxene abundances increase progressively from a few very large phenocrysts (N2000 Am) to midsize phenocrysts (~500–1000 Am), to microphenocrysts, and to abundant microlites. Phenocrystic orthopyroxene makes up ~8% while plagioclase constitutes the bulk of phenocrystic phases (F20%). Plagioclase in lavas erupted later than March 1969 exhibit a more or less continuous increase in abundance with decrease in size except the

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dip in abundance noted above between microlites and microphenocrysts. Because plagioclase is the dominant microlitic to phenocrystic phase, the overall grain size distribution is largely dominated by plagioclase. Oxide minerals (spinel) occur primarily as inclusions in mafic silicate phenocrysts or as microlites in the groundmass. However, they also occur sparsely as phenocrysts reaching sizes of up to 400 Am. Oxide phenocrysts are most abundant (F1%) and largest in samples of 1968. Olivine was found intermittently in samples of 1969, 1970, 1971, 1992, 1993, 1994, 1996 as phenocrysts (300 to ~700 Am in diameter; 1–2 grains per thin section) or as inclusions, mostly in clinopyroxene, but also once in orthopyroxene and plagioclase (see below). Because olivine proportions are very low, it is likely that olivine occurs throughout the current eruption, but wasn’t always sampled in our thin sections. Phenocrysts are always rimmed by orthopyroxene; however, olivine inclusions in clinopyroxene exist without opx selvages (see below). In summary, the mineralogy of eruptive product during the ongoing activity at Arenal is plagNNopxNcpxN spinelFolivineFsulfide (+hornblende at the onset of the eruption, cf. Reagan et al., 1987). Amounts of residual glass within individual samples vary from ~50% to ~2%. Visual estimates of volume percentages were verified with estimates based on mass balance calculations using P2O5 concentrations in the bulk sample and the groundmass glass and treating P as perfectly incompatible element. The most glass-rich sample investigated is sample Ar3/69 (erupted in March of 1969), which is supported by Fig. 2. Compilation of phenocrystic compositions (microphenocrysts and phenocrysts) of lavas from 1968 through 2000. All olivine crystals of the current eruption are rimmed by orthopyroxene, and thus analyzed crystals range from grains which preserved largely a phenocrystic character, with distinct and thin reaction rims, to grains where most of the original grain reacted to opx. N=number of phenocryts. Except for olivine, most data are based on detailed microprobe traverses (see text). Subdivisions of clinopyroxene range are defined in the text and are based on Streck et al. (2002). Inset in panel a shows observed Mg# values of cpx phenocrysts (solid bars) and of opx phenocrysts (superimposed white bars) in relation to whole-rock Mg# values, groundmass glass composition of sample Ar3/69 (3/69), and glass inclusions hosted in plagioclase phenocrysts (inc gl.). Dotted lines bracket the bulk of Mg# values of the dominant crystallization environment (see text). Also shown are curves for equilibrium relationships of Mg/Fe between melt and ferromagnesian silicates minerals for a Kd of 0.26 and 0.3 (Grove et al., 1982; Grove and Bryan, 1983; Sisson and Grove, 1993).

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the fact that elemental abundances indicate consistently to be the least evolved glass composition.

4. Composition of single phases 4.1. Clinopyroxene Clinopyroxene phenocrysts display large compositional ranges, from Mg# (=molar Mg/(Mg+Fe)100)

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higher than 85 to as low as 70 (Fig. 2). Mg# correlates positively with Cr2O3 and Al/Ti ratios, reaching maximum values of 0.7 wt.% and of 16, respectively (Fig. 3). Absolute concentrations of Al2O3 and TiO2 range from 6.5 to 1.5 wt.%, and from 0.2 to 0.8 wt.%, respectively. Lowest Al2O3 and highest TiO2 concentrations are always associated with phenocryst rims. Compositional changes are associated with fine scale oscillatory zoning (width of individual oscillations: 1– 5 Am), broader concentric growth bands (width of individual bands: 10–150 Am), and sector zoning. In addition, optically discrete cores are present in some phenocrysts. Where growth bands and optically discrete cores are evident on BSE images, they are

typically characterized by abrupt and strong chemical changes compared to neighboring areas (see discussion and Figs. 3 and 6 of Streck et al., 2002). Based on the positive correlation among Mg#, Cr, and Al/Ti and textural zones, Streck et al. (2002) distinguished among cpx as follows: (1) high-Cr cpx with Mg#N~78, Cr2O3N0.12 wt.%, and Al/TiN7; (2) low-Cr cpx with Mg#=~78–71, Cr2O3b0.12 wt.%, and Al/Ti=4–7 (see discussion below for cases where Al/TiN7); and (3) high-Fe cpx with Mg#b71 and Al/Tib4 (Fig. 2, Table 1). High-Cr cpx can be found anywhere between cores or rims of a given cpx grain. Furthermore, low-Cr cpx phenocrysts occur next to cpx phenocrysts with one or multiple discrete zones

Fig. 3. Microprobe data of all Arenal clinopyroxene analyses of this study and those (solid dots) reported by Streck et al. (2002). Open triangles represent cpx microlites and open circles represent cpx phenocrysts neighboring analyzed opx, spinel, plagioclase and olivine inclusions. Solid line is the inferred evolved terminus of the dominant crystallization environment (see text).

Table 1 Representative mineral analyses Cpx

Cpx

Cpx

Cpx

Cpx

Opx

Opx

Opx

Opx

Olivine

Olivine

Sample

5/96P3_67

8C92a.P4_3

3/70bP6_3

596P7_10

2/83a.grdx3

8A92a.P5_2

8/93aP2_39

8/96a.grdx1

9/91a.grdx2

lt.69b.ol1

7/70b.Ol1

7/70bP8a_i

Type

HCr

LCr

LCr

HFe

Mic

HiAl

LAl

Mic

Mic

Pheno

Pheno

Inc

51.1 0.42 2.83 9.75 0.29 15.31 19.65 0.38 n.d

48.5 0.54 6.36 8.75 0.21 13.91 21.05 0.29 n.d

51.1 0.57 1.81 12.94 0.34 14.00 18.22 0.31 n.d

51.6 0.42 1.36 12.30 0.50 14.93 18.00 0.28 n.d

52.2 0.27 3.78 16.5 0.43 25.5 1.47 n.d n.d

53.3 0.19 1.75 16.4 0.42 26.1 1.58 n.d n.d

54.4 0.21 0.68 16.5 0.55 25.9 1.75 n.d n.d

52.7 0.28 0.59 21.0 0.76 20.0 4.41 n.d n.d

99.8 73.7 5.3

99.7 73.9 9.1

99.3 65.8 2.5

99.4 68.4 2.5

100.1 73.3 10.9

99.8 74.0 7.2

100.0 73.7 2.6

99.9 63.0 1.6

38.6 n.d 0.06 18.3 0.24 42.2 0.11 n.d n.d. 0.13 99.7 80.4

37.1 n.d 0.03 24.4 0.45 37.3 0.16 n.d. n.d 0.09 99.5 73.2

40.4 n.d. 0.02 9.09 0.25 49.4 0.19 n.d n.d 0.18 99.6 90.6

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O Cr2O3 NiO Total Mg# Al/Ti

50.9 0.38 4.63 6.72 0.20 15.43 20.96 0.29 0.59 100.1 80.4 9.4

Spinel

Spinel

Spinel

Spinel

Plag

Plag

Plag

Sample

Spinel 2/83_1P1.i1

3/69b P5_i1

3/70bP6_i2

5/96P4_i2

8A92grd1

Sample

7/71aF5_58

771aF5_77

494aP2_i2

396aFg1.

370aFg1

Type

HCr

HAl

HMgFe

Timt

Mic

Type

Pheno

Pheno

Inc

Mic

Mic

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Cr2O3 Total Mg#

0.06 0.94 21.58 20.12 20.34 0.32 9.51 0.29 25.57 98.8 45.4

0.13 0.37 40.15 13.95 16.36 0.15 13.48 0.39 11.68 96.7 59.5

0.10 0.24 17.14 51.82 12.55 0.59 13.35 0.32 2.73 98.8 65.5

0.12 6.06 4.94 52.69 31.73 0.29 3.61 0.17 0.64 100.2 16.8

0.16 11.45 2.18 43.96 38.48 0.43 1.84 0.09 0.07 98.7 7.9

SiO2 Al2O3 FeO MgO CaO Na2O K2 O

45.61 34.61 0.55 0.05 18.09 0.98 0.01

52.80 29.08 0.80 0.11 12.71 4.19 0.11

45.08 34.10 0.82 0.05 17.95 1.38 0.02

54.88 27.71 0.88 0.05 10.84 5.03 0.18

Total An

99.9 91.1

99.8 62.4

99.4 87.7

99.6 53.9

45.30 34.68 0.79 0.06 18.40 0.89 0.03

100.2 91.9

Plag

Olivine

Plag

M.J. Streck et al. / Journal of Volcanology and Geothermal Research 140 (2005) 133–155

Phase

Mineral types are the following; for all phases: Mic=microlite, Pheno=phenocryst, Inc=mineral inclusion; for cpx: HCr=high-Cr cpx, LCr=low-Cr cpx, HFe=high-Fe cpx (see text for definitions); for opx: HiAl, LAl=phenocrystic opx compositions at upper and lower end of observed Al2O3 range, respectively; for spinel: HCr= Cr-rich spinel inclusion, HAl=Al-rich spinel inclusion, HMgFe=spinel inclusion with high Magnesioferrite component, Timt=titanomagnetite inclusion; Fe2O3 in spinel calculated with method of Droop (1987). 139

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of high-Cr cpx within the same sample. Fe-rich cpx always occurs in the outermost rims of phenocrysts and is compositionally identical to cpx microlites occurring in the groundmass (Fig. 3). Microlites are largely unzoned and the most evolved compositions correlate with the most evolved residual glass composition. Microlite ranges are Wo 37–27, En 52–48, Fs 13–26 (components after Lindsley, 1983); Mg#=75–60, Al2O3=2.8–1.2 wt.%, TiO2=0.4–0.8 wt.%, and Al/Ti=1.9–4.5. It is notable that microlite compositions continue the chemical trends exhibited by phenocrysts towards lower Mg#, lower Al/Ti, and lower Al2O3 (cf. Fig. 3). On the other hand, sample Ar3/69 (the one with highest glass proportion and least differentiated glass composition) contains microlites that overlap the field for phenocrystic low-Cr cpx (Fig. 3).

4.2. Low-Ca pyroxene Most orthopyroxene (opx) phenocrysts are compositionally restricted, ranging in Mg# from 72 to 74, occasionally reaching 77, regardless of grain size or year of occurrence (Fig. 2, Table 1). Al2O3 concentrations range from 0.8 to N4 wt.% and variations appear independent of Mg# (Fig. 4). Al/Ti ranges from 2 to 18 and some correlation with Mg# is apparent as follows. The entire range of Al/Ti is observed at the upper end of phenocrystic Mg# (72–76), while the range at Mg#V72 is narrower, from 2 to 12. As with cpx, Al/Ti is controlled by both Al and Ti concentrations, but the correlation with Al2O3 is stronger. Internal compositional variations from centers (cores) to rims of phenocrysts occur, but oscillations are restricted to the compositional ranges noted above

Fig. 4. Microprobe data of all Arenal low-Ca pyroxene analyses of this study. Solid dots are phenocrystic orthopyroxene, open triangles are microlite, and open circles are opx inclusion analyses. All inclusions shown are located within clinopyroxene phenocrysts. Solid line is the inferred evolved terminus of the dominant crystallization environment (see text). Microlites are subdivided in three groups: (a) those overlapping with phenocrystic compositions, i.e., plot right to the solid line; (b) those that overlap with sparse Fe-richest phenocrystic data, i.e., plot to the left of solid line at CaOb3 wt.%; and (c) pigeonitic microlites (CaON3.5 wt.%) which are the most evolved microlites.

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(excluding Fe-rich rims). Oscillations have various forms but the most evident zonation leads to discrete growth bands tens of micrometers in width, which are best distinguishable on BSE (backscattered eletron) images as bands of darker shades and are similar to growth bands of cpx phenocrysts (cf. Streck et al., 2002). Darker shades on BSE images are also caused by abrupt increases in Mg# of 3–4 mol%. The majority of opx inclusions in cpx phenocrysts exhibit similar compositions as opx phenocrysts. However, rare inclusions have Mg# values as high as 87, high Al2O3 (4–5.5 wt.%) and very high Al/Ti (30– 60). CaO contents of phenocrystic opx or inclusions are low, ranging from 1 to 2.2 wt.%. Also similar to cpx, phenocryst rim compositions are the most Fe-rich, overlapping with microlite compositions (Fig. 4), except that at the same Mg#, microlites tend to be richer in CaO and lower in Al2O3 than Fe-rich opx phenocryst rims. Among microlite compositions, one can distinguish three subgroups. The most magnesian microlites overlap in composition with the main compositional field of phenocrystic opx (Fig. 4 and see caption thereof). The second microlite population is compositionally more evolved and offset from the main phenocrystic field, but compositionally overlaps (as noted above) with Fe-rich rims of phenocrysts. The third population is characterized by the lowest Mg# (62F2) and significantly higher CaO (z~4 wt.%); that is, they are pigeonites. Among the nine investigated samples (Ar7/68, Ar3/69, Ar2/83, Ar8/89, Ar9/91, Ar8/92, Ar8/93, Ar8/96, Ar8/00), pigeonitic microlites were found next to opx microlites in two lava flows (e.g., Ar8/92, Ar9/91) as well as in pyroclastic flows (e.g., Ar8/93), but were not found in other lava flows nor in samples of the initial plinian blast (e.g., Ar7/68). The microlite data clearly indicate (1) pigeonite occurrence is independent of eruption style and (2) that pigeonite microlites occur next to opx microlites in the same sample. On the other hand, whether or not certain samples truly lack pigeonite would require a better sampling of low-Ca pyroxene microlites. A rare finding was a pigeonitic inclusion in An90 plagioclase. 4.3. Olivine Olivine occurs sporadically and sparsely throughout the current eruption. The number of olivine

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grains analyzed (21) include nearly all (except ~5 grains) found in existing thin sections (Fig. 2). Olivine occurs either as phenocrysts or as inclusions in primarily clinopyroxene, but one olivine inclusion was found in a plagioclase phenocryst and another was found in an orthopyroxene phenocryst. Olivine phenocrysts are always rimmed by orthopyroxene. Orthopyroxene in selvages is compositionally like those of opx phenocrysts. Opx reaction rims can be as thin as ~1/5 of the diameter of the olivine (i.e., ~100–200 Am), or can replace most of the grain, leaving only patches of unreacted olivine in the core of the former grain. In contrast, olivine inclusions in cpx phenocrysts exist without opx selvages. Phenocrystic olivine compositions are mostly evolved, ranging from Fo71 to Fo78 (Figs. 2 and 5, Table 1) and correspond well with previous data (cf. Reagan et al., 1987; Cigolini and Kudo, 1987; Beard and Borgia, 1989). The dominant composition for olivine inclusions is ~Fo75, but the total range is from Fo59 to Fo90 (Figs. 2 and 5). Fosteritic olivine inclusions (Fo90 and Fo82) were found in cpx grains with high Mg# (82) and Cr2O3 concentrations of 0.3 wt.%. Furthermore, the Fo90 olivine is located next to a high Mg# (~86) opx inclusion. One inclusion (Fo76) was found in An90 plagioclase.

Fig. 5. Microprobe data of all Arenal olivine analyses of this study. Solid dots are phenocrystic olivine and open circles are olivine inclusion analyses. Most inclusions are located within clinopyroxene phenocrysts, but sparse inclusions occur in anorthitic plagioclase.

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4.4. Plagioclase A detailed textural (employing Nomarski differential interference contrast (NDIC) imagery) and compositional analysis of plagioclase of the current eruption at Arenal volcano based on 120 representative plagioclases is forthcoming. Thus, only a brief summary is given here. Plagioclase phenocrysts in products produced throughout the current eruption are texturally complicated with a large compositional range, from ~An94 to An55 (Fig. 2, Table 1; also see Lunney, 2002; Lunney et al., 2001). The majority of phenocrysts (~80%) are cored with mottled (patchy) texture (cf. Fig. 11d) or appear as texturally flat surfaces on NDIC images. For An compositional ranges of phenocrystic plagioclase given below, the average maximum value is given first, followed by the average minimum for plagioclase segments that belong to the same textural style (Lunney, 2002). These averages are based on compositions of the same textural zone occurring in the 120 plagioclase phenocrysts analyzed. Mottled cores are irregularly shaped with An ranging from 89 to 81, whereas flat cores are mainly euhedral with a narrower range of An (90–85). Overgrowths bordering plag cores include: (1) oscillatory zoning in various textures (An77–68); (2) pitted zones, also texturally variable (An86–72); or (3) texturally homogeneous growth zones (An70–62) (Lunney, 2002). Regardless of the crystal’s interior, the majority of phenocrysts (~70%) apparently experienced a similar last crystallization episode, resulting in a homogenous growth zones (5– 20 Am) at the rims that are compositionally zoned (An69–62, occasionally down to An55), but texturally flat. Further inward from the rim growth band, phenocrysts can be grouped at best according to overall similarities in zoning style stratigraphies, including the sequence of textures, absolute compositions, and relative compositional changes. There are some notable features. Mottled cores (Fig. 11d) exhibit skeletal parts composed of high An plagioclase infilled by lower An plagioclase. The difference in An is generally ~5%; it is ~10% in a few cases, and rarely N10%. Reverse zonation is common in oscillatory-zoned areas and seldom displays abrupt increases in An. On the other hand, decreases in An content are often very abrupt in oscillatory zoned

segments and can be on the order of 10–20 mol% over distances of ~5 Am without crossing a resorption surface. The dominant occurrence of An z90 is in form of texturally flat cores or in skeletal parts of cores. However, several plagioclase phenocrysts with reverse zonation crystallized An90 (or higher) plagioclase as the most calcic portion of an oscillatory zone, which can be repeated within the same crystal 2–3 times as distinct, reversely zoned segments separated by abrupt drops in An content (Fig. 11d).

Fig. 6. Microprobe data of analyzed Arenal plagioclase inclusions (solid dots) in clinopyroxene phenocrysts and plagioclase microlites (open triangles). Single plagioclase phenocrysts often exhibit a compositional range that encompasses the entire range exhibited by all phenocrysts (An90–95 to An55).

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Plagioclase inclusions in cpx phenocrysts range from An91 to An60 and thus cover the entire range of phenocrystic plagioclase, except for the lowest An rims (Fig. 6). Microlite compositions (An89–37) also overlap with compositional ranges for phenocrysts and microphenocrysts (Fig. 6). Microlites are often compositionally zoned. High An microlites exhibit texturally flat cores with strong textural boundaries to lower An overgrowths. Maximum An contents in microlite cores appear to generally decrease through time, with values of 88–89, 82, 85, 76, and 74 in lavas of 1968–69, 1970, 1971, 1985, and 1993, respectively. The majority of analyzed microlites have rim compositions of An65 to An47.

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and some microlites of sample Ar7/68 and Ar8/00 overlap in composition with the most evolved titanomagnetites inclusions of low-Cr cpx. Microlites evolve towards Ti-rich compositions over a rather small decrease in Mg# and nearly equal Fe3+(Fig. 7). The magnetites most enriched in the magnesioferrite (cf. Haggerty, 1991) component (i.e., characterized by Mg# ~60 and Fe3+#=60–70 (Fig. 7b), low TiO2, and Al2O3 between 10 and 20 wt.%) appear to lie along a compositional trend towards the evolved titanomagnetites. Lastly, very aluminous spinel inclusions (~40 wt.% Al2O3, Mg# of 60, Cr# of 16; Table 1) have now also been discovered as inclusions in Fo80 olivine occurring in sample Ar3/69 and not only in cpx phenocrysts.

4.5. Spinel New data refine previously established compositional trends (Streck et al., 2002), which showed that spinel is compositionally variable, ranging from Cr– Al-rich spinel (occurring as inclusions mostly in highCr cpx) to inclusions of titanomagnetite found in lowCr cpx (Table 1). The most evolved titanomagnetites are found in the groundmass (Fig. 7; also cf. Fig. 3 in Streck et al., 2002). All titanomagnetite microlites in sample Ar3/69 (the sample with the most mafic glass)

5. Composition of groundmass glass and melt inclusions Groundmass glasses in selected samples, which vary in crystallinity from ~50% to ~80%, range from andesitic to rhyolitic (i.e., ~58 to ~72 wt.% SiO2), respectively. Typical lavas with crystallinities of ~70% contain dacitic glass with ~67 wt.% SiO2 (e.g., samples Ar3/82, Ar1/85, and Ar8/93; Fig. 8,

Fig. 7. Compositions of spinel inclusions in clinopyroxene phenocrysts (solid dots) and of microlites (open triangles) in the groundmass.

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Table 2), as was reported for lava erupted in 1984 and 1992 and in 1968 surge (Williams-Jones et al., 2001) and other lavas of the current eruption (Cigolini and Kudo, 1987). The following compositional variations are observed (all in wt.%) for andesitic to rhyolitic glasses (respectively): Al2O3: 15–11, Na2O: 3.5–4.5, K2O: 1–3, CaO: 7–2, MgO: 3–0.5, FeO*: 11–4, TiO2: 1.2 to b1, and P2O5: 0.3–0.7. All of these compositional variations are typical for continued crystallization during ascent and emplacement. Although FeO* decreases from 11 to 4 wt.% in andesitic to rhyolitic glass, respectively, it is rather high relative to bulk compositions and to typical Fe concentrations of calcalkaline rocks at corresponding silica values. Overlap with titanomagnetite microlites during microprobe analysis is unlikely to be the cause for high FeO* because glass analyses are consistent for single samples and microlites are sparse and easily avoided. The most evolved bulk sample (Ar7/68, Fig. 1) which is also the phenocryst-poorest sample (thus representing the greatest amount of liquid) has a Mg#=45; the least evolved groundmass glass (in sample 3/69) has a Mg#=~35. This decrease in Mg# is attributed to an increase in FeO* (from 6.2 to ~11 wt.%) from bulk sample to glass whereas MgO remains nearly constant (from 2.96 to 2.90 wt.%). This increase in FeO* but

Table 2 Selected groundmass glass and new bulk composition Sample

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2 O P2O5 Total Mg# Ca#

Groundmass glass

Bulk composition

avg3/69

avg1/85

avg8/93

58.5 1.16 14.7 10.6 0.20 2.93 6.63 4.00 0.92 0.35 100.03 33.0 47.9

66.83 1.24 12.4 7.66 0.15 1.08 3.82 4.44 1.53 0.58 99.73 20.1 32.2

67.37 1.01 12.4 6.81 0.10 1.32 3.75 4.02 2.21 0.51 99.70 25.6 34.1

Ar8/00 SiO2 TiO2 Al2O3 Fe2O3* MnO MgO CaO Na2O K2O P2O5 Total Mg# Ca#

54.9 0.63 19.2 8.05 0.15 4.43 8.74 2.97 0.73 0.19 99.53a 52.2 62.0

avg=average. a Total before normalization.

near constant MgO could result from the combined compositional effects of crystallization of abundant plagioclase, lesser ferromagnesian silicates, and minor titanomagnetite probably mostly during latest ascent and eruption. Testing this hypothesis is beyond the scope of this paper. Plagioclase-hosted melt inclusions in two samples with dacitic groundmass glass (Ar1/85 and Ar8/93) yield Mg# values between 35 and 45, thus falling in the gap mentioned above (Fig. 8). Except for highest Mg# data, data points for glass inclusions indicate an almost horizontal compositional trend towards higher SiO2 oblique to the trend defined by groundmass glass data. This trend could be due to post-emplacement crystallization of plagioclase along inclusion walls. We will further explore the relationships among compositions of groundmass glass, glass inclusions and bulk compositions below.

6. Discussion 6.1. Cocrystallizing phases and phase compositions

Fig. 8. Composition of groundmass glass (_grd), glass of plagioclase-hosted melt inclusions (_inc) compared to bulk compositions (bulk, crosses). Samples Ar8/93, Ar1/85, Ar 3/69, and AR4/ 94 contain ~33%, 31%, 51%, and 20% glass, respectively.

6.1.1. Correlation of ferromagnesian silicates Equilibrium relationships of Mg/Fe between melt and silicates minerals, coupled with the occurrence of minerals inclusions, are used to evaluate which ferromagnesian silicates could have cocyrstallized. If one assumes the same Kd (0.26 or 0.3; Grove et al.,

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1982; Grove and Bryan, 1983; Sisson and Grove, 1993) for both clinopyroxene and orthopyroxene, then cocrystallizing cpx and opx would have corresponding Mg# values. The Kd for olivine is generally slightly higher than that for pyroxene (Roeder and Emslie, 1970; Carmichael and Ghiorso, 1990); thus, an olivine cocrystallizing with pyroxene would be ~3 mol% lower in Mg# if the Kd for olivine is 0.04 higher than the one for pyroxene (cf. Insert of Fig. 2a). The effect of decreasing Fe2+/ Fe3+in the melt would be an increase in the Mg# of ferromagnesian silicates. However, this shift would equally affect pyroxene and olivine and thus would only have consequences for inferred equilibrium melt Mg# from mineral data. We base our evaluations by using total iron in Mg# calculations [=MgO/(MgO+FeO*)100]. We consider the possible effects of variable Fe2+/Fe3+in the melt. We are also aware of the possible uptake of significant amounts of ferric iron in clinopyroxene via the acmite (NaFe3+Si2O6) and/or the Fe-Tschermak (esseneite) (CaFe3+Al3+SiO6) components (e.g., Carmichael and Ghiorso, 1990). The former case is improbable to occur in calc-alkaline magmas. In the latter case, the calculated Mg# of cpx based on total Fe may be lower than the Mg# of coexisting opx and olivines. Histograms of phenocrystic clinopyroxene and orthopyroxene (including microphenocrysts) analyses show a strong contrast between the wide range of Mg# observed for clinopyroxene and the very narrow range for orthopyroxene (Fig. 2). The range in Mg# of olivine phenocrysts is between those for clinopyroxene and orthopyroxene, although existing data are more limited. Another observation is that the peaks for both orthopyroxene and clinopyroxene occur at Mg# of 73–74, as does the peak for olivine phenocrysts. Opx and olivine inclusions are surrounded by cpx hosts with similar Mg# (Fig. 9). There are some instances where inclusions are significantly (~10 mol%) higher or lower in Mg#. Interestingly, ranges of orthopyroxene and olivine compositions are extended by the inclusion data. In particular, high Mg# (~87) opx and Fo90 olivine inclusions in one cpx phenocryst (sample Ar3/70) are the first reported occurrence of such magnesian compositions in Arenal lavas (Figs. 4, 5 and 9). We favor the interpretation that these may represent relict grains rather than cocrystallized with host clinopyr-

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Fig. 9. Correlation of inclusion composition with Mg# of enclosing host cpx phenocryst composition. Superimposed white dots (or small circles) represent cpx hosts that contain high Cr2O3 (N0.12 wt.%). (a) Mg# of opx inclusions and Fo of olivine inclusions vs. cpx host (arrow indicates one Fo59 inclusion surrounded by Mg# 78 cpx host). (b) An of plag inclusions vs. cpx host. (c) MgO wt.% of spinel inclusions vs. cpx host.

oxene. This is suggested by the fact that Fo90 olivine inclusions were found only once and that, on the other hand, ~Fo80 olivines, which correspond better to the Mg# of the cpx host, are more common as inclusion

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and as phenocrysts (Fig. 5). The above suggests that orthopyroxene cocrystallized with clinopyroxene with Mg#b77, thus corresponding to a melt Mg# V47, assuming a Kd of 0.28 and excluding the one occurrence of Mg#87 opx. Olivine requires wider ranging liquid compositions (Mg# from z54 to ~42 using Kd=0.3) although most olivines appear to have crystallized from liquids as evolved as those inferred from opx compositions. There are indications, described below, that either orthopyroxene or olivine cocrystallized with cpx, but both probably rather seldom. Streck et al. (2002) identified Al/Ti as a useful parameter to distinguish among cpx populations. A comparison of Al/Ti ratios of cpx host crystals with those of opx inclusions shows that only cpx phenocrysts with low Al/Ti (b7) host opx inclusions (Fig. 10). Exceptions are the high Mg# opx inclusion discussed above and another opx inclusion with a relatively high Mg# of 77. Al/Ti of cpx hosts and opx inclusions correlate positively. Furthermore, Al/Ti of both phases reaches the lowest value (~2) in the most evolved pyroxene compositions as recorded by microlites (Figs. 3 and 4). In addition, Al/Ti of orthopyroxene reaches higher values than clinopyroxene in higher Mg# pyroxenes. Analogous to clinopyroxene with Al/Tib7, we infer crystallizing orthopyroxene with Al/Ti of b~12 is achieved by a combination of lower temperature, melt composition lower in Al2O3 and higher in TiO2, and possible lower pressure compared to elevated Al/Ti (N~15) of few opx inclusions and phenocrysts (cf. Streck et al., 2002 and discussion therein). 6.1.2. Correlation of spinel and plagioclase with pyroxene A direct way to correlate if and which plagioclase and spinel compositions cocrystallized with clinopyroxene and orthopyroxene is the correlation between the composition of the plagioclase or spinel inclusion with the composition of the surrounding cpx or opx phenocrystic host. However, this approach cannot easily distinguish between the following two scenarios which both may lead to mineral inclusions in pyroxene phenocrysts: (a) a microlite crystallized from the melt shortly before entrapment and thus its composition is representative of equilibrium conditions, (b) a microlite represents

Fig. 10. Correlation of inclusion composition with Al/Ti ratio of enclosing host cpx phenocryst composition. Superimposed white dots represent cpx hosts that contain high Cr2O3 (N0.12 wt.%). (a) Al/Ti of opx inclusions vs. host, arrow indicates one opx inclusion that plots above the observed range (cf. Fig. 4). (b) An of plag inclusions vs. cpx host. (c) Al2O3 wt.% of spinel inclusions vs. cpx host.

incompletely resorbed mineral debris originating from disequilibrium minerals. Some evidence that inclusions were truly crystallizing at the time of entrapment can be found in consistent compositional

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correlation trends between the composition of host and inclusion phases. In addition, in case that spinel inclusions were out of equilibrium with the surrounding melt before entrapment, their small size (~10–30 Am) would have allowed for either their reequilibration or resorption in andesitic to basaltic melts within 1 year (cf. Venezky and Rutherford, 1999). On this basis, it is likely that the majority of spinel inclusions in clinopyroxene reflect magmatic conditions at entrapment. Data for spinel have been previously evaluated and discussed (Streck et al., 2002). New data confirm that high-Al, high-Cr spinel inclusions occur in high-Cr cpx while titanomagnetites occur in low-Cr cpx (Figs. 9 and 10 and Fig. 7 in Streck et al., 2002). Exceptions are rare magnetites that occur in high-Cr cpx; these are characterized by high Mg#, high Fe3+, and low Ti (e.g., HMgFe spinel in Table 1) thus have high magnesioferrite components and define an end member composition for Arenal spinels (see above). Beard and Borgia (1989) also found similar magnetites as inclusions in amphibole and amphibole reaction rims but considered a non-magmatic origin for them. On the other hand, some low-T experiments (Sisson and Grove, 1993) crystallized magnesioferrite-rich magnetites, similar to the ones reported here for Arenal, suggestive of a magmatic origin. Positive correlation of MgO and Al2O3 in spinel with Mg# and Al/Ti of cpx host, respectively (Figs. 9 and 10) suggest both phases compositionally responded to some common controls mostly likely the variation of Mg/Fe and of Al/Ti in the melt. High (N80) An plag inclusions occur in clinopyroxene with low to medium (73–79) Mg# and high (F83) Mg#; but low (b75) An plag inclusions have only been found in low (b75) Mg# clinopyroxene (Fig. 9). This suggests that variably evolved liquids with Mg# ranging from ~45 to ~55 (cf. Fig. 2a inset) produced calcic (NAn80) plagioclase. In addition, change in composition from An80 to An60, while enclosing cpx hosts remain more or less constant at Mg# of 73F2 (Fig. 9b), suggests low (~43) Mg# liquids crystallized a wide range of plagioclase compositions. Both observations and inferences are compatible with plagioclase crystallization being variably delayed or suppressed according to water contents, allowing melts at different Mg/Fe ratios to acquire similarly high enough CaO/Na2O ratios or

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allowing melts with the same Mg/Fe to crystallize wide-ranging plagioclase (Sisson and Grove, 1993). This would be facilitated if crystallization of high An occurs over a range of CaO/Na2O values and thus does not only require refractory, high Ca/Na melts (Danyushesvky et al., 1997). That not only refractory melts produced high An plag is supported by the following facts: (1) the highest An plag inclusions (An88 to An91) were only found in Mg# 78 cpx associated with low Fo (59–75) olivine and titanomagnetite inclusions (Fig. 11a), and (2) microlitic plagioclase in the groundmass is as calcic as An88. We cannot rule out entrapment of disequilibrium plagioclase relicts to have had also some effect on compositional ranges of plagioclase inclusions in clinopyroxene. However, it seems unlikely that high-An plag cocrystallized already abundantly with high-Mg# (N80) clinopyroxene but were only sparsely entrapped while later crystallizing low-Mg# (b78) clinopyroxene entrapped more and higher anorthitic disequilibrium plag relicts (Fig. 9b). On the contrary, petrographic evidence rather suggests the opposite that recharging, more mafic magma entrapped plagioclase relicts that did not crystallize directly from recharging magma. Another intriguing observation is that no plag inclusion was found to occur in high (N7) Al/Ti cpx, (Figs. 3 and 10). Some of the high-Cr cpx that host plag inclusions have slightly higher Al/Ti than lowCr cpx (Fig. 10) as is expected from the previously noted correlation of Cr and Al/Ti in clinopyroxene (cf. Streck et al., 2002). The lack of high Al/Ti clinopyroxene as plag inclusion host cannot be completely due to sampling bias because selected cpx phenocrysts, particularly with portions of low Mg# (b78) clinopyroxene but high Al/Ti, were resurveyed but suspiciously lack plagioclase inclusions (Fig. 11) that otherwise frequently occur. If indeed the above indicates no (or limited) plagioclase cocrystallized with high Al/Ti cpx, regardless whether high Al/Ti is associated with high-Cr cpx or low-Cr cpx, then this is further evidence that plagioclase has a delayed occurrence in the crystallization assemblage. In other words, plagioclase appears to begin crystallizing at the moment melt conditions cause low (b7) Al/Ti in clinopyroxene (coinciding with the bulk of opx plus titanomagnetite crystallization).

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Table 3 Summary of inferred coprecipitating mineral assemblages Crystallization environment

Dominant (cpx, opx, plag, timt) Minor I (cpx, ol, CrAlsp, plag) Minor II (cpx, ol, plag, timt) Minor III (cpx, timt, ol?) Microlites (cpx, opx, timt, plag)

Clinopyroxene

Orthopyroxene

Olivine

Plagioclase

Spinel

Mg#

Al/Ti

Cr2O3 (wt.%)

Mg#

Al/Ti

Fo

An

Cr#

TiO2 (wt.%)

Al2O3 (wt.%)

71–78 ~78–85 71–78 71–78 ~60–74

~3.5–7 ~4–14 ~4–7? N7–12 2 to ~3.5

b0.12 0.12 to ~0.7 b0.12 b0.12 b0.12

71–77 none none none 60–75

3–20 none none none 2–10

none N78 b78 ? none

60–85 N75 ~90 none? 40–89

b10 10–42 b10 b10? b10

2–7 b1 2–7 2–7 6–13

2–8 10–40 b10? 10–20 b5

None means mineral is not part of assemblage. Orthopyroxene includes pigeonite among microlites.

6.2. Crystallization environments We will use the composition of phases and phase correlations, established above, to devise a general picture of the crystallization environment beneath Arenal volcano that produced the majority of erupted crystals that we call dominant crystallization environment. We also define subordinately preserved, yet clearly different environments that are revealed in the mineralogy of products erupted since July 1968. We call these minor crystallization environments. 6.2.1. Dominantly preserved by current eruption Compositional ranges of phenocrysts, mineral inclusions, and microlites combined with frequency distributions of phenocrysts, indicate that the current eruption has been mainly tapping a magma reservoir that crystallized low (b78) Mg# orthopyroxene and clinopyroxene, titanomagnetite, but possibly a wide range of plagioclase compositions (Table 3). Low-Cr cpx and orthopyroxene phenocrysts contain abundant inclusions depicting this mineral assemblage (Fig.

11a–c; but also low-Cr cpx portions in Fig. 3 in Streck et al., 2002). Although this inferred characteristic mineral assemblage stays the same, each phase has a certain compositional spread that we like to detail below. Microlite compositions, abrupt compositional changes at phenocryst rims, and compositional fields of phenocrystic compositions delimit the dominant crystallization environment towards the evolved side for orthopyroxene and clinopyroxene (see solid line in Figs. 3 and 4). At low Al/Ti (3 for cpx and opx), clinopyroxene evolved to Mg#=72–73, while orthopyroxene evolved to Mg#=73–74. At the high Al/Ti end (4 for cpx and 8 for opx), clinopyroxene and orthopyroxene evolved to slightly lower Mg# of 69– 70. It is unclear what caused this variation. The mafic end of the dominant crystallization environment for clinopyroxene and orthopyroxene is inferred to be Mg# of 78 for clinopyroxene and 77 for orthopyroxene. Both phases peak in abundance at the same Mg# of 73–74 (Fig. 2). Interestingly, the most variable chemical component for cpx and opx at any given Mg# is Al2O3. For example, at a Mg# of 73–74)

Fig. 11. Dominant and minor crystallization environments as expressed by individual phenocrysts. (a) Clinopyroxene phenocryst illustrates following crystallization environment: coprecipitating mineral assemblage of low-Cr cpx, Fo (b78) olivine, high An (N~90) plagioclase and titatnomagnetite (Minor II environment, Table 3) is represented by interior of clinopyroxene and associated inclusions while last ~100 Am towards the rim is overgrowth of dominant crystallization environment. (b) Three crystallization environments are illustrated by clinopyroxene phenocryst Ar3/70P6. The interior (inward from dark shaded band) illustrates the cocrystallized assemblage of low-Cr cpx with high (N7) Al/Ti and high (N4 wt.%) Al2O3, titanomagnetite with considerable Al2O3 (10–18 wt.%) (black arrow) and possibly olivine but lacks plagioclase (Minor III environment). This is followed by the assemblage of high-Cr cpx, Al-, Cr-rich spinel, olivine (~Fo80), Fhigh-An (N80) plagioclase (Minor I environment) and is represented here by growth band (dark shade) of high-Cr cpx with spinel inclusion (arrow). And lastly, outermost overgrowth of 50–150 Am is representing the dominant crystallization assemblage except the very rim that consists of Fe-rich cpx. Compositional profiles along black line are shown in panels f and g for this phenocryst. (c) Cocrystallizing assemblage of dominant environment are low-Cr cpx, orthopyroxene, titanomagnetite and plagioclase and is represented here throughout large opx phenocryst with inclusions of cpx (black arrows), plagioclase (white arrows), and titanomagnetite (black grains). (d) Plagioclase phenocryst with mottled core and oscillatory overgrowth that indicates reversely zoned segments recovering to An90 composition, compositional profile along black line shown in panel e.

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Al2O3 concentration ranges are N6 to b2 wt.% in clinopyroxene and ~4 to b1 wt.% for orthopyroxene. Because orthopyroxene was only found to be associated with cpx with Al/Ti of up to 7 (Fig. 10a), low Mg# clinopyroxene with Al/TiN~7 (i.e., cpx with Al2O3N~4 wt.%) appears to record a different environment (see below). Attributing that all spinel inclusions in low-Cr cpx with Al/Tib7 are derived from this dominant crystallization environment, there is still considerable compositional ranges of titanomagnetites, the most important being TiO2, ~2 to 7 wt.%; MgO, ~6–2 wt.%; and Al2O3, ~8–2 wt.% (as TiO2 increases MgO and Al2O3 decrease, Fig. 7, Table 3). Similarly, if plag inclusions near opx inclusions in low-Cr cpx (Mg# of 78–71) phenocrysts are representative of the dominant crystallization environment then this environment crystallized a wide range from FAn85 to An60 which is nearly the entire observed range (Fig. 2d). This large range can undoubtedly not only be the result of variable water concentrations but requires that Ca/Na of the melt also varied. In light of the small variation in melt Mg# to explain clinopyroxene and orthopyroxene variations, a plausible environment where this combination (small change in Mg# but large change in Ca#) could be achieved is if plagioclase grows largely in response to decompression inducing Ca/Na variation while Fe/Mg may stay relatively unchanged for some time. Crystallization driven by decompression was proposed as mechanism to generate crystal-rich magmas at Rishiri volcano, Japan (Kuritani, 1999) and Stromboli, volcano, Italy (Metrich et al., 2001). 6.2.2. Minorly preserved crystallization environments In addition to the dominant crystallization environment that the current eruption has been tapping, we argue that there are three more crystallization environments that can be distinguished. The most important crystallized high-Cr cpx, Cr + Al-rich spinel, ~Fo80 olivine, Fhigh An (z~80) plagioclase, and required liquid compositions with a Mg# of ~52–60 (Minor I environment, Table 3). Liquids of this environment are responsible for crystallizing high-Cr cpx occurring in cores and growth bands of cpx phenocrysts previously discussed (Streck et al., 2002) and most likely are basaltic magma existing over a range of crustal pressures. Similarly zoned clinopyroxenes

from Ruapehu volcano (New Zealand) were also interpreted in this way (Nakagawa et al., 2002). The composition of Cr-rich spinels implies that they are fingerprints of calc-alkaline basalt (cf. Fig. 3 in Clynne, 1999). The second environment (Minor II environment), distinct from the dominant environment, is principally delineated by the occurrence of sparsely occurring olivine phenocrysts and inclusions in the range of Fo70–76. These olivines are too evolved to have crystallized along with high-Cr cpx. In fact, the occurrence of Fo80 as inclusions in Mg# 83 cpx phenocryts confirms that a more forsteritic olivine would cocrystallize with high-Cr cpx. In addition, olivine with this evolved composition would only cocrystallize with similar Mg# orthopyroxene over a narrow temperature range (Grove and Juster, 1989). On the other hand, the stability field may completely disappear over the same temperature interval over which orthopyroxene would normally crystallize in favor of olivine under water-saturated conditions (Sisson and Grove, 1993). Disequilibrium conditions between olivine and orthopyroxene with the same Mg# in Arenal samples is indicated by the fact that no olivine grains, except inclusions in cpx phenocrysts which are protected from reacting with surrounding liquid, were observed without opx reaction rims. Petrographic analysis of cognate enclaves in Arenal lavas from the current eruption (Beard and Borgia, 1989) confirms this. In enclaves that contained both olivine and orthopyroxene, orthopyroxene always occurs as reaction rims around olivine. On the other hand, enclaves either contained unreacted olivine or opx phenocrysts, never both. Thus, it appears that there could be environments beneath Arenal volcano that crystallize olivine instead of orthopyroxene when liquids reach a generally more evolved state (Mg# between 52 and 42, but also as low as 30 to explain Fo59 olivine inclusion (Fig. 5). Support for this crystallization environment can be found in the inclusion assemblage of a clinopyroxene in sample of April 1994 (Fig. 11a). There, the interior of the cpx phenocryst contains Fo75–59 olivine, An89–91 plagioclase, and titanomagnetite inclusions. Since plagioclase An90 or above is fingerprinting environments that either contained high water concentrations or were under water-saturated conditions but low P H2O (e.g., Sisson and Grove, 1993; Danyushesvky et al.,

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1997), the association of low Fo olivine with An90 plagioclase is consistent with a water-induced crystallization of the olivine in expense of orthopyroxene. Therefore, the inner part of clinopyroxene shown in Fig. 11a exemplifies this assemblage consisting of low-Cr cpx, low Fo (~60–75) olivine, very high An (~90) plagioclase, and titanomagnetite (Table 3). The last crystallization environment (Minor III environment), that is also the most ambiguous, is characterized by low-Cr cpx similar to the dominant environment but with Al/Ti ratios above 7, titanomagnetiteFolivine (?) and likely lack plagioclase. Evidence for this environment is the following. No opx inclusion coexists with a cpx host phenocryst that has a Al/Ti ratio of greater than 7 (Fig. 10a) but there is a distinct group of cpx analyses that plot above ~Al/ Ti of 7 at a Mg# of ~74, i.e., the typical Mg# of lowCr cpx (Fig. 3). Furthermore, cpx phenocryst cores with the right combination of low Mg# and high Al/Ti are suspiciously void of any plagioclase that rather typically occurs as inclusion in other low-Cr cpx. Based on these lines of evidence, we suggest a third environment in which low-Cr cpx crystallizes but this time at higher crustal pressure and again at water-rich conditions to suppress plagioclase crystallization. The characteristic mineral assemblage for this crystallization environment is low-Cr cpx with high (N7) Al/Ti, titanomagnetite with significant Al2O3 (10–18 wt.%), MgO (5–7 wt.%) but low TiO2 (2–4 wt.%), and possibly olivine, although olivine was not found as inclusion. This environment is represented by the inner part of the clinopyroxene displayed in Fig. 11b. 6.3. Microlites, glass, and bulk compositions, and estimation of melt of dominant crystallization environment Microlites crystallizing shortly before, during, or after magma ascent are typically more compositionally evolved than phenocrystic phases in volcanic rocks because at onset of microlite crystallization residual melt percentages are low and liquid amounts are quickly reduced thus become more differentiated during continued microlite crystallization which in turn affects microlites to become more evolved (e.g., Cashman, 1992; Hammer et al., 1999). For the discussion here, microlite and residual glass compositions are important for the following

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reasons: (1) we can infer which portions of phenocryst rims are from crystallization processes coinciding with microlite crystallization by comparing phenocryst near rims with microlite chemistry (see above); (2) in turn, we can infer what the melt composition was shortly before eruption; and (3) we can search all phenocrystic mineral data for portions that may have crystallized when crystallinities were as high as observed in final eruptive products or melts as evolved as observed in residual glasses from which we in turn may infer recycled plutonic material. In regard to overlapping microlite and phenocryst data the following is suggested. In the case of clinopyroxene, conditions to crystallize most microlites were distinct from phenocryst crystallization of low-Cr cpx except for some cpx microlites of sample Ar3/69 (that also contains the most mafic, ~58 wt.% SiO2, glass) that overlap with typical low-Cr cpx. On the other hand, phenocrystic rim compositions are the most Fe-rich (high-Fe cpx) and likely cocrystallized with cpx microlites (Fig. 3) (cf. Streck et al., 2002). In case of orthopyroxene, lesser evolved opx microlites compositionally strongly overlap with opx phenocrysts compositions of the dominant crystallization environment suggesting that melt compositions and conditions to produce both were similar. Similar to clinopyroxene, the Fe-richest rims of opx phenocrysts compositionally overlap with more evolved opx microlites which likely crystallized while eruptionrelated crystallization was well underway. Yet opx phenocrysts never contain a pigeonitic rim indicating phenocrystic crystallization ceased before the most evolved low-Ca pyroxene microlites crystallized at which time the temperature was high enough and Mg# low enough to stabilize pigeonite in favor of orthopyroxene (Grove and Juster, 1989). In case of spinel, titanomagnetite microlites in the sample with the most mafic glass (Ar3/69) overlap with titanomagnetite inclusions in low-Cr cpx but others are distinctly offset to more evolved (Ti richer and Mg poorer) compositions (Fig. 7). Surprisingly, plagioclase microlites start with compositions as calcic as An88 and mostly ended at An55 but get as low as An35. This wide range and the strong overlap with phenocrystic compositions of any size of crystals could be interpreted twofold. Even shortly before eruption, storage conditions of at least some drawn-up magma was such as to allow crystallization

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of high An plagioclase (NAn85) which in turn would require Ca# [100Ca/(Ca+Na)] in the melt to be at least as high as the one observed in bulk compositions (cf. Fig. 1) combined with high water concentrations or higher Ca# under lower, but still saturated water conditions (Lunney, 2002; and cf. Sisson and Grove, 1993; Danyushesvky et al., 1997). The other interpretation of An85 microlites is that they, especially cores of them, are leftover material of relict phenocrystic grains that were not completely resorbed. Textural analysis of high-An microlites, however, does not reveal the sort of resorption textures as observed abundantly in phenocrysts. Therefore, there is no textural support for the latter interpretation. In summary, the least evolved microlites of all phases overlap with phenocryst compositions of the dominant crystallization environment suggesting that magma reservoir conditions at onset of microlite crystallization (i.e., marking likely the beginning of eruption related ascent) were such that this reservoir could crystallize most minerals of the dominant crystallization environment. All groundmass glasses experienced compositional modifications (see above) due to continued microlites crystallization, however degree of modification varied, and thus the least modified is closest to melt that could crystallize the bulk of clinopyroxene, orthopyroxene, and spinel minerals. The least modified melt is recorded by the most mafic groundmass glass of sample Ar3/69 (Table 2), which is also the sample with the greatest number of microlites that compositional overlap with phenocrystic phases of the dominant crystallization environment. On the other hand, the bulk composition that would be most compatible with being a melt composition capable of crystallizing orthopyroxene and clinopyroxene with a Mg# of 73–74 is sample Ar7/68 (cf. inset of Fig. 2). Based on the above, we infer that the actual composition to have crystallized minerals of the dominant environment lies between bulk sample Ar7/68 and glass of sample Ar3/69 and thus is likely andesitic with 56–58 wt.% SiO2. On the other hand, most bulk compositions of the current eruption are basaltic andesitic with 54 wt.% SiO2 and 5 wt.% MgO. Bulk compositions since Fall of 1969 are compatible with a Mg# of ~80 in ferromagnesian silicates and thus appear as too magnesian to have precipitated the dominantly

observed orthopyroxene and clinopyroxnene minerals (Fig. 2). From this we conclude that crystal-rich bulk compositions of the current eruption do not represent melt compositions from which most minerals of the dominant crystallization environment precipitated. Calculating Mg# with Fe2+ rather than total Fe would not help, but rather aggravate the problem because Fe2+/Fe3+ is always lower in melts compared to mafic silicates under typical fO2 conditions (Carmichael and Ghiorso, 1990). With regard to cpx and opx phenocryts and spinel inclusions, no inner mineral portion was found to have a composition as evolved as latest crystallizing microlites (excluding cpx and opx rims that partially cocrystallized with microlites). This suggest that no phenocrystic opx or cpx and no spinel inclusions have crystallized from more evolved melts than andesitic, since microlites are now embedded in dacitic to rhyolitic glass (see above). Plagioclase microlite rims are also the most evolved but cores can be as calcic as An88–89 and thus hinder a similar clear compositional division. 6.4. Mineral sources and dynamics of assembling basaltic andesites of current eruption Here, we will summarize the mineralogical characteristics of each crystallization environment before all are combined into a coherent geological model that is consistent with the mineralogical evidence and our inferences about composition and physical conditions of these crystallization environments. Minerals (entire crystals or portions of crystals) found in the products of the current eruption belong overwhelmingly to the cocrystallizing assemblage of low-Cr cpx (see definition above), orthopyroxene, titanomagnetite and plagioclase. All phases of this dominant crystallization environment indicate compositional ranges (Table 3) of which the most important are: Mg# of cpx and opx=~71 to ~78 and An of plagioclase=An60 to An85. Minerals of the dominant environment likely crystallized from andesitic (56–58 wt.% SiO2, Mg# ~42) melt and we infer low to medium pressure conditions. In addition to dominant mineral assemblage, three other assemblages appear to exist although they are much less well recorded by erupting minerals than the previous environment. The next most obvious assemblage consist of high-Cr cpx

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(e.g., Mg#=~78–85; Cr2O3=0.15–0.6 wt.%), Al-, Crrich spinel, olivine (~Fo80), and in some circumstances high-An (N80) plagioclase. This assemblage (assemblage of Minor I environment) seems to record mafic inputs into the Arenal system (cf. Streck et al., 2002). Similar compositions and assemblages have been recognized elsewhere in intermediate magmas as bfingerprintsQ of mafic parental magmas (e.g., Clynne, 1999; Nakagawa et al., 2002). The last two assemblages are the most difficult to recognize because both are associated with clinopyroxene, which is compositionally similar to that of the dominant assemblage, and occur only sparsely. One assemblage is characterized by cocrystallizing low-Cr cpx, olivine (bFo78), titanomagnetite, and very high An (~An90) plagioclase (assemblage of Minor II environment). The other (assemblage of Minor III environment) cocrystallized low-Cr cpx with high (N7) Al/Ti and high (N4 wt.%) Al2O3, titanomagnetite with considerable Al2O3 (10– 18 wt.%) and possibly olivine but appears to lack plagioclase. Because H2O destabilizes low-Ca pyroxene in favor of olivine (Sisson and Grove, 1993), we infer the difference in environment between opx- vs. olivine-bearing assemblages at low Mg# (F74) is due to the amount of water (and/or water-saturation level). Thus, the olivine-bearing assemblage is the more water-rich environment, as is also supported by the fact that this assemblage is associated with An90 plagioclase. On the other hand, it also records compositionally evolved melts with a Mg# of ~42 which likely corresponds to an andesitic melt similar to the one of the dominant crystallization environment. Although conclusive data are lacking, we argue that the apparently plagioclase-free, low Mg# environment is also recording crystallization of a mostly andesitic melt. But this time crystallization occurs at greater depth thus under higher pressure conditions (N3 kb?) which may lead to suppression of plagioclase crystallization generating melts with high Al, possibly low Ti, and low Mg# during crystallization of mafic silicates and oxides. This could explain high Al/Ti, yet low Mg# of clinopyroxene. On the other hand, microlite assemblages include the most evolved low-Ca pyroxenes (pigeonite), clinopyroxene, and titanomagnetite compositions and thus are consistent with residual glass compositions ranging from silicic andesitic to rhyolitic, with most being dacitic. This in turn indicates that phenocrytic

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phases never crystallized from more evolved melts than andesitic since no phenocryst or portion of phenocryst has a composition as evolved as latest crystallizing microlites except for rims that cocrystallized with microlites. The likelihood that the majority of minerals are derived from recycling of plutonic material where residual melts once reached dacitic to rhyolitic composition during solidification is thus unlikely (cf. Cigolini, 1998), except if one were to invoke complete obliteration of any such mineral evidence, e.g., through reequilibration following assimilation. To put the different crystallization environments into a dynamic context, it also important to consider the following. Streck et al. (2002) demonstrated that all lavas of the current eruption, even the phenocrystpoorest, most evolved lavas at the onset, contain evidence for being mixed magmas. And, growth stratigraphies of phenocrysts indicate that crystallization of the dominant environment is typically the last one to occur before eruption-induced crystallization started. Exceptions are rare rims recording crystallization from basaltic magmas (Streck et al., 2002). Our preferred model for the magma dynamics leading to erupting basaltic andesites is the following. Basaltic magma batches, supplied from the mantle nearly continuously for the last decades, ascend through an open conduit/reservoir system and mix at various crustal levels with resident, more evolved, andesitic magmas and/or fractionateFassimilate. Andesitic magmas or mingled magmas continue to ascent and further mix as they encounter other resident magmas on their way up, before arriving at subvolcanic depths well homogenized and replenish the shallowest basaltic andesitic reservoir that feeds the current eruption. In this model, mixing, evidenced so ubiquitously in the mineralogy of any given sample, is not a single event but rather a sequence of multiple mixing events occurring over a range of crustal depths and over the width of the plumbing system. Furthermore, mixing events may change in terms of proportionality and volumes of andesitic to basaltic mixing end members, and of location and frequency. Any single mixing event can unequivocally lead to different hybrids, but it is the sum of these mixing events as basalt batches move upward towards the surface and interact with resident magmas, crystal mush, or country rock that will lead to very similar end-product unless mantle input changes.

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Fingerprints of this multistage mixing-dominated evolution are preserved only in form of its mineral components, while the bulk composition is merely the cumulative product of these.

7. Conclusions By focusing on abundant mineral data for the current and relatively long-lasting (N35 years) current eruption, we have established new and important details in our understanding of the magmatic systems that feeds Arenal volcano. We disentangled the complex mineralogy and established several coprecipitating equilibrium mineral assemblages, some of which can be easily overlooked unless careful analysis is performed. Each of these assemblages appears to represent a specific combination of chemical and physical conditions of growth. On the other hand, it is reasonable to assume any transition between these environments. (2) Phenocrysts, mineral inclusions, and microlites of all phases indicate coherent trends. Overlap and compositional ranges indicate that microlites of orthopyroxene, spinel, plagioclase and, to lesser extent, clinopyroxene began crystallizing under reservoir conditions similar to those responsible for the dominant phenocryst assemblage. (3) High-An (N85) plagioclase is not necessarily associated with only high Mg#, ferromagnesian silicates and thus inferred refractory melts. Instead, new inclusion data suggest the opposite—that evolved but water-saturated (?) melt conditions were important to the crystallization of high An plagioclase. (4) Most bulk compositions of the current eruption are too mafic to have crystallized the majority of the ferromagnesian silicates. This suggests that bulk compositions of crystal-rich, mafic arc lavas, may not necessarily have existed as melt compositions from which most of the embedded minerals crystallized. (5) Although there is a variety of sources for minerals found in current Arenal lavas, essentially all minerals have crystallized last under

(6)

(1)

(7)

conditions of the dominant crystallization environment (neglecting eruption-related crystallization of outermost rims), thus overprinting any earlier history of a crystal. We conclude, at Arenal—and probably at many other comparable arc volcanoes—single larger crystals are not classifiable exclusively as xenocrysts, antecrysts (cognate crystals), or phenocrysts because certain portions of a single crystal may correspond to different categories. Probable cumulates beneath Arenal inferred from this study include gabbro, norite, wehrlite, and possibly clinopyroxenite. These lithologies correspond with those of cognate enclaves found in current lavas (Beard and Borgia, 1989). The range of lithologies is consistent with high-tolow pressure crystallization of mafic to intermediate liquids at variable water concentrations (Gaetani et al., 1993). Multitude of mineral histories and sources clearly point to the complexities of assembling the erupting basaltic andesites as final product. The mineralogical record is consistent with mixing-dominated evolution of mafic magmas during ascent acquiring a complex assembly of crystals at the time of arrival near the surface. We suggest this also occurs concurrently with the current eruption and may ultimately be responsible for the continuous volcanic activity and compositional monotony since mantle replenishments may be small and quasi-continuous providing a constant source of heat pulses. Thus, we propose that Arenal’s current activity is flux-controlled by mantle input.

Acknowledgements Support for this project came from Swiss Fonds National Grant #20-49730.96 to M. Dungan and from PSU Faculty Enhancement Grant #001521 to M. Streck. Journal reviews by Diane Smith and Suzanne Straub, which were very helpful and improved the paper significantly, are much appreciated. Bill Melson is thanked for providing the additional two samples. Fabio Capponi of University of Geneva is thanked for performing additional XRF analyses.

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