Phase equilibrium experiments at 0.5 GPa and 1100–1300 °C on a basaltic andesite from Arenal volcano, Costa Rica

Journal of Volcanology and Geothermal Research 157 (2006) 222 – 235 www.elsevier.com/locate/jvolgeores

Phase equilibrium experiments at 0.5 GPa and 1100–1300 °C on a basaltic andesite from Arenal volcano, Costa Rica Maik Pertermann 1 , Craig C. Lundstrom ⁎ Department of Geology, University of Illinois — Urbana/Champaign, 245 Natural History Bldg, 1301 W. Green St., Urbana, IL 61801, United States Received 8 July 2005; accepted 28 March 2006 Available online 6 June 2006

Abstract We present results from piston–cylinder experiments on a synthetic composition of basaltic andesite that corresponds to lavas erupted from the ongoing eruption at Arenal volcano, Costa Rica, in order to shed light on magmatic processes at upper crustal depths beneath Arenal. The starting composition represents the least evolved basaltic andesite from the initiation of stage 2 of the current eruption. Anhydrous and hydrous experiments were conducted at 0.5 GPa and 1100–1300 °C: the principal phases encountered were melt, plagioclase, orthopyroxene and clinopyroxene of variable CaO content. Glass and plagioclase compositions change in a consistent manner with decreasing temperature for both hydrous and anhydrous experiments. The phase equilibria dictate that Arenal magmas must have contained N 2 wt.% H2O in order for the erupted rocks to have once represented liquid compositions at a relatively high temperature (1200 °C) and N4 wt.% H2O if the melt was at the lower temperatures (≤1150 °C) that are more likely for the Arenal system. However, anorthite-rich plagioclase phenocrysts (N An85) commonly found in Arenal lavas cannot be accounted for by any reasonable permutation of higher temperature and water content. The close correspondence of the phase compositions (rims of plagioclase, orthopyroxene) and crystallinity observed in stage 2 lavas from Arenal and a hydrous experiment with 2 wt.% water in the melt provides evidence for Arenal magmas ponding and equilibrating at 1150 °C and ∼ 12– 14 km depth. The conclusion that Arenal lavas reflect equilibration between observed minerals and a melt with ∼ 2 wt.% H2O at 0.5 GPa, ∼1150 °C, argues that these bulk compositions are unlikely to have ever reflected fully molten liquids. © 2006 Elsevier B.V. All rights reserved. Keywords: plagioclase; anorthite; basaltic andesite; differentiation

1. Introduction High-alumina basalts (HAB) constitute a volumetrically significant portion of many subduction zone volcanoes and are generally considered to be parental to ⁎ Corresponding author. Tel.: +1 217 244 6293; fax: +1 217 244 4996. E-mail address: [email protected] (C.C. Lundstrom). 1 Now at: Washington University, Department of Earth and Planetary Sciences, St. Louis, MO 63130, United States. 0377-0273/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2006.03.043

more ubiquitous andesites. The production of andesites and more evolved rocks at subduction zones is important because of its inferred role in creating continental crust. A major debate of the past concerned whether HAB were derived from melting of an eclogite source or were differentiates from more primitive liquids (Crawford et al., 1987; Brophy, 1989). It is now widely accepted that most HAB originate from hydrous melting of a deeper peridotite source and subsequent differentiation of initially hydrous basaltic magmas at crustal depths, with mineral accumulation also possibly being important (e.g., see

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Crawford et al., 1987, for a summary of the historical perspective). Nevertheless, it remains important to understand differentiation of magmas and production of the moderately evolved erupted basalts and basaltic andesites in this setting. Arenal volcano in Costa Rica has been continuously erupting since 1968, providing a unique opportunity to study magma evolution in an arc volcano in real time. The subtle but smooth changes in lava composition through the 37+ year eruption allow examination of how a magmatic system can create and sustain production of a near steadystate whole rock composition over decadal time scales. The changes in lava composition have led to the division of the eruption into two stages with a transition period in between: stage 1 of the eruption from 1968–1971 is characterized by the effusion of relatively evolved lavas that progressively became more mafic (stage 1 of Ryder et al., 2006-this volume; Reagan et al., 1987). Stage 2 reflects the subtle change back to more evolved compositions from 1972 to present (Ryder et al., 2006-this volume). These changes in bulk composition have been modeled to reflect simple fractional crystallization or fractional crystallization plus re-charge (Ryder et al., 2006-this volume; Reagan et al., 1987) with the implication that erupted rocks once reflected fully molten magma compositions at depth. Indeed, Ryder et al. (2006-this volume) interpret the crystallinity of these highly porphyritic rocks to mostly reflect crystallization in the conduit upon eruption. Thus, there are abundant reasons to experimentally determine the phase relationships of the most mafic lava observed in the current eruption at pre-eruptive conditions of magma equilibration inferred from thermobarometry (Cigolini, 1998). Our study seeks answers to several questions: Is it possible that the erupted lavas once reflected liquids of similar composition at relevant depths? If not, how much higher in temperature is the liquidus of the erupted rock compositions as a function of their water content? Can the appearance of minerals, and their specific compositions, indicate the extent to which the erupted rocks reflect meltmineral equilibrium at the suggested pre-eruptive conditions? A notable observation from Arenal is that despite monotonous whole-rock compositions during the eruption and the relatively small changes in mineral modes of erupted lavas, individual samples from throughout the eruption exhibit a high degree of complexity in terms of textures and mineral compositional zoning (Streck et al., 2002, 2005; Szramek et al., 2006-this volume): can this complexity at least in part be explained by processes at or near 0.5 GPa in the magmatic plumbing system beneath Arenal? Here we present results from piston cylinder experiments documenting the phase equilibria of the most

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MgO-rich basaltic andesite observed during the current eruption at Arenal. This composition is given in Table 1 (AR-8, beginning of stage 2; from Reagan et al., 1987) and was chosen to better understand both the differentiation process to more evolved and Al-rich lavas in stage 1 and the evolution of lavas during stage 2. Knowledge of the phase compositions will help to understand the origin of different mineral compositions erupted (e.g., the crystallization environments of Streck et al., 2005) and to discern the conditions required for the existence of a fully molten magma. In addition, because the starting basaltic andesite is close but slightly more evolved in composition to many HAB found in other volcanic arcs (Table 1), our results should extend beyond the example of Arenal. 2. Experimental and analytical details 2.1. Starting material and experiments The synthetic starting material was prepared from a mixture of high-purity oxides, carbonates and simple silicates (SiO2, TiO2, Al2O3/Al(OH)3, Fe2O3, MgO, CaSiO3, Na2SiO3, KAlSi3O8) to obtain the desired composition, identical to AR-8 but with variable water contents. Because of the relatively MgO-poor nature of the chosen composition, it was not possible to add enough water as Mg(OH)2 to the starting material to make our 4 wt.% H2O starting material. Instead, we used Al(OH)3 to add water to form the hydrous composition. Verification of the synthetic starting material composition occurred by analysis of the glass from super liquidus experiments (given in Table 1). We also present phase Table 1 Composition of starting mineral This study

SiO2 TiO2 Al2O3 FeO MgO CaO Na2O K2O H2O Sum Mg#

AR anhydrous

AR hydrous

54.3 0.6 18.8 8.5 5.1 9.0 3.0 0.6 – 99.9 51.7

52.1 0.6 18.0 8.2 4.9 8.6 2.9 0.6 4.0 99.9 51.7

HAB

MORB

52.3 1 18.1 9.5 4.5 8.9 3.5 0.8

50.68 1.49 15.6 9.85 7.69 11.44 2.66 0.17

98.6 45.8

99.58 58.2

AR — most mafic and AL-rich stage II lava from Arenal volcano, Reagan et al. (1987). HAB — Crawford et al. (1987). MORB — typical MORB from Mid-Atlantic Ridge, Melson et al. (1976).

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composition results from one diffusion-reaction experiment from Lundstrom et al. (2005) that used a powdered natural sample of AR-8 whole rock, allowing confirmation that compositions were not dependent on the initial starting materials. Experiments were conducted in an endloaded 3/4 in. diameter piston–cylinder apparatus at the University of Illinois at Urbana-Champaign, all at 0.5 GPa pressure. The pressure cell assembly consisted of crushable MgO ceramics inside straight graphite heaters, and the heaters were surrounded by pyrex cylinders and NaCl sleeves. We employed a pressure correction of −3% (relative) to our run pressures based on the determination of the melting point of Au near 0.5 GPa, in good agreement with NaCl/pyrex based assemblies reported elsewhere (e.g., McDade et al., 2002). The hot-spot of the assembly was located by conducting five experiments with two thermocouple junctions and was found to be ∼5 mm in width and ∼2 mm below the symmetric center of the furnace length. This position differs slightly from the centered hot-spot of Hudon et al. (1994) using a similar assembly. The experiments used welded Pt–Pt90/Rh10 (type S) thermocouple wires sheathed in alumina ceramics. The sample containers were placed in the furnace hot-spot and temperatures reported take into account the temperature offset between the location of the sample and the thermocouple junction (∼10 °C). Temperatures were not corrected for the effects of pressure on the thermocouple emf. Initial experiments using D-type W/Re thermocouples resulted in unacceptable drifting at 0.5 GPa and 1100–1200 °C; the same batch of D-type thermocouple wires performed well at or above 1.0 GPa, possibly indicating oxidation of this thermocouple under the lower pressure conditions. We initially planned to run three different sample capsules of varying water content (nominally anhydrous, 2 wt.%, 4 wt.%) in each experiment. However, the graphite-lined Pt-capsules (3.2 mm outer diameter, welded shut) did not retain H2O during the experiments — while initially present and thus resulting in larger and fewer crystals, water was subsequently lost from all hydrous runs that employed Pt–C capsules. This behavior is consistent with water loss problems discussed by Truckenbrodt and Johannes (1999). Loss of water is indicated by the identical phase relationship of all three capsules in a given experiment as well as identical compositions in glass, plagioclase and pyroxenes. Furthermore, glass analyses summed to near 100%, also indicating nominally anhydrous melts. Water retention was equally poor when the Ptlined graphite capsules were placed into thick-walled (∼1.5 mm wall thickness) capsules of Ti-metal with a tight-fitting Ti-lid. We then switched to graphite-lined Au75Pd25 capsules to conduct water-bearing experiments;

when these were enclosed inside a larger Pt-capsule filled with powdered pyrophyllite (Freda et al., 2001), significant water loss still occurred. Only when enclosed in outer graphite capsules (and dependent on a particular graphite brand) did we retain water in the melt with run durations shortened from N 24 h to 3–6 h. This approach slowed or eliminated water loss to the point that meaningful results could be obtained from the hydrous experiments. In light of the difficulties to maintain water inside the capsule, we opted to characterize our results in terms of water content in the quenched glass of any given run instead of the initial bulk water content (see Results section). The validity of our anhydrous experiments was confirmed by our ability to reproduce results for a number of separate experimental runs or from multiple charges that were run in the same experiment. Due to the cumulative problems of maintaining water in the capsules, our hydrous experiment were not reproduced or confirmed by reversal experiments. The highly aluminous starting material composition also led to the production of small fine-grained clots of aluminous material in lower temperature experiments. It is possible that this reflects some of the Al(OH)3 in the starting mix failing to react completely. However, similar fine-grained material (Fig. 2c and d) was identified to be Fe-bearing Mg–Al spinel in a repeat experiment at 1150 °C (data not reported here), consistent with the typically refractory nature of spinel. The fine-grained clots occurred in both dry and wet experiments but were not present in the high melt fraction experiments (anhydrous at ≥1225 °C and hydrous at ≥1200 °C). Because they are volumetrically small, these clots have an insignificant effect on the bulk composition during the experiments. For example, based on mass balance considerations in run 29, approximately 0.5 wt.% of the total bulk Al2O3 of 18.8 wt.% is not accounted for by the major phases observed (glass, plagioclase, orthopyroxene); when neglecting the presence of clinopyroxene (which is likely to be a quench phase) in run 32, mass balance accounts for all alumina in the bulk. The overall volumetric percentage of fine-grained clots is very low, and therefore, we do not consider this problem to have significantly affected the phase equilibria results. The use of inner graphite capsules in anhydrous and hydrous experiments limits the oxygen fugacity to near or below the C–CO buffer (Ulmer and Luth, 1991). While this is a less oxidizing environment than commonly assumed for magma differentiation in volcanic arcs (∼ Ni–NiO, Parkinson and Arculus, 1989), this system condition has negligible effect on the stability of the major silicate mineral phases and does not affect the solidus and liquidus locations significantly, although it clearly affects the stability of ilmenite and magnetite. Changing the

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appearance of magnetite does affect the Mg# of residual melt and therefore also the Mg# of the mafic phases. However, the modal amount of magnetite is very small and of the total Fe in it only one third is Fe2+. The alternatives of allowing Fe bearing silicate melt to interact with the Au–Pd capsule or pre-saturating the capsules with Fe also lead to undesired experimental and laboratory problems which we chose to avoid. MELTS calculations at higher f(O2) than our experiments (see Results and Discussion sections) show that Fe-bearing oxide minerals remain late appearing phases, crystallizing well after plagioclase and pyroxenes. Thus, the difference in f(O2) between our experimental conditions and those relevant to Arenal does not affect the application of these results to Arenal. Experiments were conducted by cold pressurization to the desired run pressure, followed by slow heating at 16 °C/min to the final run temperature while maintaining run pressure. Power consumption was monitored during the runs to guard against potential drift of the thermocouple emf. Runs were quenched by shutting off the heater power, resulting in quench rates sufficiently fast to quench silicate liquids to relatively homogeneous glasses. After recovery of the capsules, the samples were mounted in epoxy and sectioned longitudinally to expose the center of the charges, followed by polishing to a finish with 0.3 μm Al2O3 slurry. 2.2. Analytical procedures After initial examination under reflected light, samples were characterized on the JEOL JSM 840 SEM housed in the Department of Geology at the University of Illinois, Urbana-Champaign. This instrument is equipped with a Kevex EDS detector and a 4Pi/Revolution processor and software package for quantitative EDS analyses with natural and synthetic standards. Secondary standards were used to assure good quality of analyses. A subset of samples (anhydrous experiments and early hydrous experiments) was also analyzed by WDS on the JEOL JXA 8900 instrument of the Department of Geology at the University of Minnesota. Careful standardization for WDS analysis of quenched glasses produced analytical totals near 100%, which showed that initially water-bearing runs in Pt–C (and also Ti–Pt–C) capsules did not maintain water, findings that were later independently verified by SIMS analysis for water in the same samples. Reported melt fractions reflect assuming complete incompatibility of potassium using the difference between run K2O content and the starting composition to calculate the degree of crystallinity. Given measurement uncertainties on K2O, melt fractions have estimated uncertainties of +/−10%.

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In order to quantify the amount of water in the quenched glasses, we used the Cameca 5f instrument housed in the Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign. 1H/30Si ratios were measured with a primary O− beam of ∼1.6⁎10− 8 A current and acceleration voltage of 12.6 kV. Secondary ions were extracted by a 4.5 kV accelerating voltage and energy filtered using an offset of 100 V and an energy window of ∼20 eV, following the approach of (Deloule et al., 1995). Standards used were two well-characterized basalt glasses (519-4-1 and D-30-1, courtesy of E. Hauri) of 0.17 and 1.54 wt.% H2O, respectively. Data were acquired for 360–600 s, long enough for the 1H and 30Si counts to stabilize after an initial drop; only 1H/30Si ratios towards the very end of the acquisition period were used for calculation of water contents. A typical calibration curve is shown in Fig. 1; because no basaltic standards with water contents beyond 1.54 wt.% H2O were available, the calibration was extrapolated linearly to the higher water contents of a few samples. Calibration curves of Hauri et al. (2002) and Deloule et al. (1995) indicate a nearly linear response between wt.% H2O and H/Si for hydrous glass compositions below 2 wt.% H2O. 3. Results Anhydrous experiments were conducted between 1100 and 1300 °C and hydrous experiments at 1150 and 1200 °C. The principal phases encountered were silicate liquid, plagioclase, clinopyroxene of variable CaO-content and orthopyroxene, with trace amounts of amphibole and ilmenite found in lower temperature experiments. For the purposes of clarity, we refer to all pyroxenes with

Fig. 1. Typical SIMS calibration for analysis of water in silicate glasses. Standards used were 519-4-1 and D-30-1 (courtesy of E. Hauri) with 0.17 and 1.54 wt.% H2O, respectively. See text for further analytical details.

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b3 wt.% CaO as orthopyroxene, with 3–14 wt.% CaO as pigeonite, and with N 14 wt.% CaO as augite. Backscattered electron images of representative samples are shown in Fig. 2 and the phase relations are summarized in Fig. 3. The compositions of the experimental glasses, plagioclase and pyroxenes are given in Table 3 and displayed in Figs. 4, 5 and 6, respectively. In the anhydrous experiments, the melt fraction decreases systematically with decreasing temperature with melt still present at 1100 °C (F = melt fraction, F ≤ 0.1). At 1300 °C, only glass and traces of plagioclase are present (F ≥ 0.99), placing the liquidus for the anhydrous composition at just above 1300 °C. The liquid compositions range from basaltic–andesitic at high melt

fraction to dacitic to rhyolitic at 1150 °C to 1100 °C, respectively (Fig. 4). Multiple experiments at the same pressure–temperature conditions (e.g., runs 22 and 28-Ti or runs 21 and 27-Ti; see Table 2) produce the same melt fraction, phase assemblage and phase composition indicating the reproducibility of these results. Phase presence and compositional trends with decreasing temperature follow general expectations for the anhydrous experiments. Plagioclase is the liquidus phase and was present in all experiments, increasing in albite content with decreasing temperature (Fig. 5). The anorthite content [reported as molecular An / (An + Ab)] of the plagioclase ranges from 0.47 at 1150 °C to 0.67 at 1250 °C. The more An-rich plagioclase composition is

Fig. 2. Backscatter electron images of run products. (a) Run #22, anhydrous, 1200 °C, scale bar is 20 μm. (b) Run #29, hydrous, 1200 °C, scale bar is 50 μm. The needle-like crystal is opx, the smaller more euhedral pyroxene crystals are zoned (opx at center, cpx on the outside). (c) Run #21, anhydrous, 1150 °C, scale bar is 20 μm. The box highlights a cluster of Al-rich unreacted starting material — some particles are similar in composition to Fe-bearing Mg–Al spinel. (d) Run #32, hydrous, 1150 °C, scale bar is 50 μm. The very small grains highlighted in the box are high-Ca clinopyroxenes; it is not clear if these grains are quench-cpx or actually represent equilibrium compositions. Note the overall larger grain sizes in the hydrous experiments; further details on experimental results are given in Table 2 and phase compositions are listed in Table 3.

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Fig. 3. Summary of experimental results as a function of wt.% water in the silicate glass. Square symbols and triangles indicate hydrous and nominally anhydrous runs, respectively. Open squares denote conditions above the liquidus for the hydrous runs. Grey triangles represent experiments with the assemblage glass +plagioclase; black triangles represent glass+ plagioclase +pyroxene(s), see Tables 2 and 3 for details on the pyroxene issue. Small subscripts adjacent to run symbols indicate trace phases present in addition to major phases mentioned above: amph=amphibole, ilm=ilmenite

generally consistent with the plagioclase rim compositions (An66–67) in a phase equilibria experiment using the natural AR-8 powder at 1200 °C with 0.5 wt.% water in the melt (see inset to : Fig. 6E in Lundstrom et al., 2005). At 1100 °C the exceedingly small grain size prevented reliable quantification of the plagioclase composition. Plagioclase remains the sole crystalline phase present at 1250 °C and 1225 °C, but is joined by pyroxene at 1200 °C. At 1200 °C, the pyroxene contains ∼5 wt.% CaO and thus can be categorized as pigeonite while at 1100 °C, the pyroxene is a CaO-rich augite. Between these temperatures at 1150 °C, two pyroxene compositions are encountered: a Ca-rich augite with ∼17 wt.% CaO and a less calcic pyroxene with just under 12 wt.% CaO. These observations are consistent with MELTS (Ghiorso and Sack, 1995) calculations which indicate that the melt is saturated with a pigeonitic pyroxene (CaO = 5– 6 wt.%) from 1220 °C down to 1080 °C, where a more augitic pyroxene (CaO = 18.2 wt.%) appears. Thus, the observed appearance of low-Ca and high-Ca pyroxene at 1200 and 1100 °C, respectively, agree well with MELTS predictions; however, the intermediate CaO content pyroxenes at 1150 °C are not predicted by MELTS. Such pyroxenes are found in other experiments: a nominally anhydrous phase equilibria experiment using a whole rock powder of AR-8 at 1150 °C produced pyroxenes with ∼10 wt.% CaO (Lundstrom, unpublished data)

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while a nominally anhydrous diffusion–reaction experiment (Lundstrom et al., 2005) also produced similar pyroxenes (Table 3). Szramek et al. (2006-this volume) also report pyroxene having ∼14 wt.% CaO in one of their experiments using a similar Arenal whole rock starting material. Bartels et al. (1991) report coexisting clinopyroxenes with 11 and 16 wt.% CaO in their basalt phase equilibria experiments at 1.2 GPa pressure. Finally, supercalcic pigeonite has been observed in igneous rocks from the Graveyard Point sill, indicating that it can also occur in nature during crystallization of basaltic liquids at moderate pressure (Markl and White, 1999). These odd CaO content pyroxenes are generally explained by the wellknown behavior of the solvus of coexisting pyroxenes narrowing with increasing temperature. Backscattered-electron images did not show any visible zoning in the pyroxene grains in the anhydrous experiments, consistent with their small grain size and the longer run times of these experiments. Due to the very fine-grained nature of the mineral grains at 1200 °C and below, positive identification of minor phases present was difficult. However, at 1150 °C we found traces (b1%) of Ca-rich amphibole and at 1100 °C, ilmenite was present. MELTS calculations indicate the arrival of ilmenite at 1060 °C. Water contents in the glasses of the anhydrous experiments were not measured directly, but based on careful WDS analysis of these and earlier, similar experiments, we consider these experiments to be nominally anhydrous: microprobe totals were consistently near 100 wt.%. Additionally, monitoring of count rates at various beam spot sizes during analyses did not indicate any loss of alkalis, supporting the absence of detectable amounts of water in the melts. In the hydrous experiments, two runs at 1200 °C having slightly different water contents in the glass (1.7 and 1.9 wt.% H2O) bracket the liquidus (Table 2, Fig. 3); phases present immediately below the liquidus (F ∼ 0.95) are plagioclase and orthopyroxene with 2.8 wt.% CaO (run 29). The dramatic lowering of the liquidus temperature is consistent with the known effect of water in destabilizing plagioclase. At 1150 °C and a water content of 1.9 wt.%, phases encountered are glass, plagioclase, ortho- and clinopyroxene (with 21 wt.% CaO) and traces (b1 wt.%) of amphibole. However, it is not clear that the clinopyroxene is a stable phase at this temperature. The few clinopyroxenes present are small and sometimes needle-like (Fig. 2), which may reflect rapid growth at quench of the hydrous melt. The dramatically lower Mg# of the one clinopyroxene analyzed in the 1150 °C experiment relative to coexisting orthopyroxene is consistent with this interpretation. Furthermore, the loss of water during the hydrous runs affected the composition of the

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Fig. 4. Composition of silicate glasses in experiments. Triangles represent anhydrous experiments, squares represent hydrous experiments. Error bars for composition are one standard deviation, as given in Table 3. The temperature uncertainty in the experiments is ±15 °C.

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Fig. 5. Plagioclase composition in experiments. Triangles represent anhydrous experiments, squares represent hydrous experiments.

pyroxenes: some grains exhibit low-Ca orthopyroxenelike compositions at the core, changing abruptly to more calcic, augitic compositions at the rims (see Fig. 2b). Presumably, the low-Ca pyroxenes grew first from melts with higher water contents, with the more calcic pyroxenes crystallizing as the water content decreased gradually during the run. Interestingly, the plagioclase crystals in the hydrous experiments do not exhibit any detectable zoning with respect to their major element components, regardless of the apparently variable water content over the course of the experiments; the An contents at 1200 °C and 1150 °C are 0.83 and 0.62, respectively. These values are higher than at comparable conditions in the anhydrous experiments, clearly demonstrating that elevated water content in the melt raises the An content of coexisting plagioclase (Fig. 5; Panjasawatwong et al., 1995). Significant loss of water did occur over the course of the hydrous experiments, indicated by deviations in water content from that expected after crystallization assuming an initial bulk water content of 4 wt.%. For instance, in run 29 the melt fraction is 0.95 and the final melt contains ∼1.6 wt.% H2O, indicating a water loss of 60% over the 24 h run. Although estimating water loss rate might help interpretation of the effect of water loss on phase equilibria, this rate appears to be highly variable. For example, two experiments run at the same temperature and pressure, having identical capsule assemblies and initial water contents, resulted in nearly identical water contents despite run durations differing from 3 to 24 h. However, despite the differences in the rate of water loss, these two experiments agree well in terms of phase equilibria as a function of melt water content; one run (with slightly lower water content) has very few crystals (F = 0.95) and abundant melt present while the other run is above the liquidus.

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Despite the water-loss problem and the associated issue of changing pyroxene compositions, our experiments appear well-equilibrated with respect to their glass and plagioclase compositions. Both phases vary smoothly and consistently in composition and abundance as a function of temperature and water content (Figs. 4 and 5). The overall small grain size observed in the charges also supports diffusive equilibration. The Ca–Na exchange coefficients for plagioclase [KdCa–Na = (plagXCa ⁎ meltXNa) / (plagXNa ⁎ meltXCa)] in our dry and wet experiments are 1.03–1.39 and 1.28–1.60, respectively, which agree well with previous experimental observations (Bartels et al., 1991). One experiment (run 27-Ti) has a relatively high KdCa–Na of 1.91. However, the errors are large on both CaO and Na2O analyses of the glass in this experiment as well as another experiment at the same temperature (run 21), reflecting the difficulty of finding and analyzing quenchfree glass in these low melt fraction runs. There is no significant difference between the KdCa–Na in these two experiments when the propagated errors are considered. Mg–Fe exchange between pyroxenes and melt is also generally consistent with expectation. For instance, KdMg–Fe [(pxXFe ⁎ meltXMg) / (pxXMg ⁎ meltXFe)] in the anhydrous runs regularly decreases with temperature from values near 0.27 at 1200 °C, down to the much lower value of 0.13 at 1100 °C. The higher values are fully consistent

Fig. 6. Pyroxene composition in experiments. Triangles represent anhydrous experiments, squares represent hydrous experiments, Xsymbols represent actual stage II pyroxene phenocrysts (both opx and cpx) from Reagan et al. (1987) and Streck et al. (2005); DR denotes coexisting pyroxenes from diffusion/reaction experiment of Lundstrom et al. (2005). Tie-lines indicate coexisting pyroxenes and small numbers adjacent to symbols give temperature (in °C) and water content (as wt.% H2O) of the respective experiments. Open squares are coexisting pyroxenes from hydrous experiment 32; the high-Ca/low Mg# composition likely represents a disequilibrium pyroxene when compared to coexisting pyroxenes in other runs (tie-lines).

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Table 2 Summary of 0.5 GPa experiments and results Run ID

Temperature

Duration

Water content

Phases present

(°C)

(h)

(wt.% H2O in glass)

Melt fraction (F)

Anhydrous experiments 26 1300 23 1250 25 1225 22 1200 28-Ti 1200 21 1150 27-Ti 1150 24 1100

1.0 30 48 48 48 48 48 72

– – – – – – – –

Glass, plag(tr) Glass, plag Glass, plag Glass, plag, pig Glass, plag, pig Glass, plag, aug, pig, amph(tr) Glass, plag, aug Glass, plag, aug, ilm(tr)

N0.99 0.83 0.77 0.63 0.69 0.36 0.33 b0.10

Hydrous experiments 29 1200 34 1200 32 1150 35 1150

24 3.0 26 3.3

1.7(4) 1.9(2) 1.9(2) 4.3(4)

Glass, plag, opx Glass Glass, plag, opx, aug, amph(tr) Glass

0.95 1 0.50 1

–

Glass, plag, aug, pig

n.a.

Diffusion/reaction experiment (Lundstrom et al., 2005) 1 1200 624

Plag = plagioclase, aug = augite (CaO N 14 wt.%), pig = pigeonite (CaO = 3–14 wt.%), opx = orthopyroxene (b3 wt.% CaO), amph = amphibole, ilm = ilmenite. Anhydrous runs were conducted in Pt–C capsules; runs 28-Ti and 27-Ti included an outer Ti-metal capsule. Hydrous runs used graphite-lined Au–Pd enclosed in an outer graphite capsule.

with previous results for KdMg–Fe between augite and moderately differentiated melt at similar temperatures (Bartels et al., 1991). The lower values are not consistent with the nominal value for KdMg–Fe between mafic phases and basaltic melt of 0.30 + /−0.03. The lower KdMg–Fe may reflect the lowering of melt Mg# with quenching in the low melt fraction experiments at lower temperature (the melt MgO content at 1100 °C is only 0.6 wt.%). However, KdMg–Fe is also known to depend on melt composition, becoming lower with increasing alkali contents and higher silica contents (Gee and Sack, 1988; Draper and Green, 1999). The hydrous experiments show the same decrease in KdMg–Fe with temperature but have a higher initial value for KdMg–Fe (0.36 at 1200 °C). A higher value of KdMg–Fe in hydrous experiments relative to dry experiments was also observed in the experiments of Gaetani and Grove (1998), with values of KdMg–Fe at similar temperature and water conditions of the same general range (0.29– 0.35). The one exception to this assessment of KdMg–Fe is the clinopyroxene observed in the 1150 °C hydrous experiment, and as argued above, this may reflect a quench crystallization effect. Comparison of our results with MELTS calculations indicates good general agreement, particularly in distinguishing the effects of water in these experiments. We ran several MELTS calculations, all at 0.5 GPa, using the AR-8 bulk composition with either 0 and 2 wt.%

H2O and constraining the f(O2) to either QFM or NNO. Under f(O2) = QFM (probably still more oxidizing than the conditions of our experiments), MELTS indicates the dry AR-8 composition should have An70 plagioclase on the liquidus at 1251 °C, with pigeonite arriving at 1220 °C. High-Ca clinopyroxene (with 18 wt.% CaO) does not appear until 1080 °C, followed by ilmenite at 1060 °C. Imposing NNO oxygen fugacity only increases the temperature of appearance of high-Ca pyroxene slightly to 1110 °C. In contrast, with 2 wt.% H2O present in the bulk, MELTS indicates that the liquidus is depressed to 1165 °C, and pigeonite (7 wt.% CaO) and high-Ca pyroxene (17 wt.% CaO) appear almost simultaneously. An80 plagioclase arrives at 1120 °C. Again, oxygen fugacity variations make little difference to magnetite stability; it only appears in the NNO run at 1005 °C. Thus, according to MELTS, the difference in oxygen fugacity between our experiments and that likely to be relevant to Arenal has no significant effect on the silicate mineral phase equilibria appropriate to interpreting Arenal compositional variations. Based on the above observations of phase homogeneity, the consistency of compositional trends within and between anhydrous and hydrous experiments, the similarity of results from repeat experiments either at the same pressure–temperature conditions or using synthetic versus natural starting materials, it appears that our anhydrous experiments represent a close approach to

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equilibrium. Water loss occurred in the hydrous experiments and this could significantly affect phase equilibria. However, the similarity in phase equilibria between two runs at the same temperature and only slightly different water contents argues that the hydrous experiment results, particularly the location of the liquidus, are robust. Although we have not performed any experiments other than those using alternative starting materials to prove equilibrium, the general consistency of the hydrous experiments results with those expected for increasing water content, indicates that these experiments define the phase equilibria of the AR-8 composition well at 0.5 GPa as a function of temperature and water content. 4. Discussion The production of the near steady-state basaltic andesite at Arenal volcano over the 37+ year eruption period provides an ideal opportunity to integrate the constraints provided by laboratory experiments into models which can explain the observed bulk and phase compositions through time. The subtle changes in bulk composition from 1968 −2003 are well matched by models of fractional crystallization and fractional crystallization plus recharge by Ryder et al. (2006-this volume). However, this interpretation assumes that the erupted porphyritic rocks were once fully molten and that most of the crystallinity in the final product reflects crystallization in the conduit during eruption. In contrast to this relatively simple explanation, detailed study of phenocrysts within lavas throughout the eruption have shown tremendous complexity with possibly four separate environments of crystallization recorded (Streck et al., 2005; see also Szramek et al., 2006-this volume). We thus focus on two key questions: 1) Do observed phase assemblages and compositions in Arenal lavas resemble the phase behavior at any particular pressure–temperature–water condition in our experiments? 2) How much water was present in magmas parental to the currently erupting lavas and are any Arenal eruptive products likely to have been fully molten liquids? A primary constraint on our interpretations of processes occurring beneath Arenal is provided by two-pyroxene geothermometry of lavas and xenoliths by Cigolini and Kudo (1987), Beard and Borgia (1989), and Cigolini (1998), all of which indicate temperatures of ∼ 1000–1120 °C at depths equivalent to 0.3–0.5 GPa pressure beneath Arenal. For comparison, the models of Ryder et al. (2006-this volume) assume temperatures of 1150 °C decreasing to 1120 °C. Our anhydrous experiments clearly indicate that Arenal magmas must contain significant amounts of water if these temperatures at depth are accurate.

231

Plagioclase, orthopyroxene and clinopyroxene of variable CaO content are the major minerals crystallizing in our experiments. Plagioclase is the liquidus phase and modally dominant mineral in both dry and wet experiments but orthopyroxene is also a near liquidus phase in the wet experiments. Augitic clinopyroxene does not appear until significantly lower temperatures in both wet and dry experiments. These observations are consistent with observations from Arenal lavas in which plagioclase dominates the phenocryst mode (mode of ∼ 40% in samples from stage 2) with orthopyroxene the second most abundant phase (mode of ∼ 8% in stage 2: Reagan et al., 1987). Clinopyroxene in Arenal lavas has lower modal abundance (1–7%), CaO contents of at least 18 wt.%. and highly variable Cr contents (Streck et al., 2002). Although clinopyroxene is identified as a primary phase crystallizing in all four of the environments identified by Streck et al. (2005), its importance relative to orthopyroxene ranges from equal to subordinate, depending on opinion. There are both similarities and differences in the plagioclase compositions seen in our experiments and those observed in Arenal lavas. Plagioclase compositions in Arenal samples, representing both mineral inclusions enclosed within clinopyroxenes and mm to cm sized phenocrysts, range in composition from An55 to An96 with single phenocrysts sometimes encompassing this entire range (Streck et al., 2005). However, the majority of plagioclase phenocryst rims fall within a limited compositional range from An62–69 (Streck et al., 2005), providing comparison with our experiments. This rim composition is consistent with equilibrium with an anhydrous melt of AR-8 composition at 1250 °C (An67) or a hydrous melt with ∼2 wt.% water at 1150 °C (An62). Based on the known temperature constraints, observed plagioclase rims imply crystallization at 1100–1150 °C with ∼2 wt.% water in the melt. Because the An-content of plagioclase regularly increases with temperature and the water content of the melt, it may be possible that similar plagioclase rim compositions could exist with a more water-rich melt at lower temperature although this is not constrained by our experiments. However, our experimental results fall short of producing plagioclase of An85 or above, similar to the compositions like that commonly found in Arenal lavas. For instance, our experiment at relatively high temperature (1200 °C) and water content (1.7 wt.% H2O) produced only a trace of An82 plagioclase, still significantly less anorthitic than the most Ca-rich plagioclase observed in Arenal lavas. To explain these extreme compositions would require a combination of higher temperatures and/ or higher water contents in the magmas. However, run 35

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Table 3 Phase compositions from experiments Run ID Phase SiO2

Al2O3

FeO MgO

Phase equilibrium experiments 26 Glass 53.5(9) 0.67(7)

19.2(5)

23

18.5(2)

8.02 (31) 7.73 (34) 0.61 (13) 8.80 (46) 0.45 (5) 9.30 (15) 0.61 (11) 13.7 (6) 9.87 (25) 0.54 (6) 13.8 (9) 7.01 (31) 0.78 (29) 11.6 (5) 15.2 (1.2) 5.97 (49) 0.70 (10) 11.4 (1.5) 3.87 (7) 11.8 (9) 9.6 (9) n.a. 16.1 (4) 8.8 (2) n.a. 13.9 (3) 12.3

Glass 55.1(7) Plag

25

22

28-Ti

21

27-Ti

24

29

30.8(9) 0.77(4)

51.8(3)

Glass 56.4(5) Plag

53.1(4)

Pig

55.1(4)

Glass 57.4(4) Plag

53.6(5)

Pig

55.6(4)

Glass 63.8(1.1)

16.9(2) 30.6(2)

0.89(6)

15.3(2) 28.7(5)

0.12(5)

1.17(28)

1.04(12) 15.3(3) 29.6(4) 0.19(6)

2.13(52)

1.08(13) 13.4(6)

Plag

57.0(6)

Aug

54.1(4)

0.40(6)

2.29(55)

Pig

53.9(5)

0.41(4)

1.76(9)

Glass 68.2(1.3)

27.1(8)

1.21(20) 13.0(2)

Plag

56.4(8)

Aug

54.2(6)

0.31(8)

Glass 72.2(3)

0.70(7)

Aug

0.22(8)

54.7(6)

Glass 54.0(7)

28.0(7) 1.98(46) 12.9(3) 0.91(17)

0.7(3)

17.4(4)

50.6(7) 54.1(3)

29.4(3) 0.2(1)

0.5(2) 3.5(8)

Glass 57.4(2)

0.9(1)

16.8(2)

29.8(2) 0.2(1)

0.5(2) 3.4(4)

Plag Opx 32

0.64(7)

50.7(5)

Glass 55.4(6) Plag

TiO2

Plag Opx

53.2(4) 54.4(1)

Aug

50.8

0.1

2.6

Na2O

K2 O 0.55 (6) 0.66 (6) 0.12 (2) 0.71 (4) 0.14 (2) 0.87 (3) 0.19 (3) n.d.

5.56(19)

9.16(23)

2.74(7)

5.26(27)

8.42(17)

2.75(10)

0.36(20) 13.8(2)

3.24(8)

6.0(31)

2.66(14)

0.18(4) 4.66(22)

7.65(17) 13.3(2) 7.83(17)

0.26(10) 12.0(2) 25.5(8) 4.60(26) 0.19(3)

3.56(14) 2.88(8) 4.30(16)

5.18(1.12) 0.09(3) 7.93(12) 12.3(3)

26.3(1.5)

4.23(1.1)

1.95(22)

4.97(49)

2.88(9)

0.87 (6) 4.24(15) 0.17 (2) 0.07(2)

15.9(4)

17.2(1.3)

3.57(30) 1.54 (13) 5.18(19) 0.27 (6) 0.33(6) n.d.

18.3(1.3)

11.8(1.7)

0.20(2)

1.22(17)

3.80(45)

0.19(12) 10.0(3)

0.13(4)

10.4(5)

16.6(1.0)

16.9(2.0)

0.58(6)

1.73(9)

Total

Mg# An

Kd

Method n

99.3(4)

55.3

WDS

12

99.1(4)

54.8

WDS

15

0.67 1.39 WDS

10

WDS

13

0.64 1.30 WDS

15

WDS

10

0.58 1.03 WDS

14

99.6(6) 98.9(4)

54.8

100.1(4) 98.1(4)

47.2

99.2(5) 100.9(4)

76.9

0.27 WDS

17

99.9(5)

45.4

WDS

15

0.59 1.05 WDS

14

77.3

0.24 WDS

19

97.4(2.1) 33.1

WDS

11

0.47 1.39 WDS

12

100.6(6) 102.3(5)

100.5(8) 101.8(7)

70.9

0.20 WDS

5

n.d.

101.6(6)

68.3

0.23 WDS

4

3.66(15) 1.83 (12) 5.26(24) 0.23 (4) 0.33(8)

98.9(9)

26.7

WDS

10

0.49 1.91 WDS

14

101.7(5)

72.2

0.14 WDS

14

99.0(2)

21.2

WDS

3

101.7(2)

67.6

0.13 WDS

5

13.7(2)

19.6(5)

5.0(3)

10.2(1)

2.1(2)

1.0(1) 100.0

48.4

EDS

4

17.1(6) 22.7(3)

2.2(1) 2.8(3)

0.2(1) 0.5(1)

n.d.

100.0 100.0

71.5

0.83 1.60 EDS 0.37 EDS

4 6

4.2(1)

8.0(3)

2.8(1)

1.1(1) 100.0

46.2

EDS

4

12.8(3) 25.1(3)

3.5(2) 2.3(2)

0.2(1) 0.7(1)

n.d.

100.0 100.0

76.4

0.62 1.28 EDS 0.26 EDS

5 5

0.8

n.d.

100.0

62.9

0.50 EDS

1

0.7(1)

100.0

69.7

EDS

2

0.7(1)

100.0

71.9

EDS

3

11.7

21.7

8.6(2.1) 14.7(5)

4.6(19)

101.2(5)

2.44 (5) 0.80(10) n.d.

Diffusion–reaction experiment (Lundstrom et al., 2005) 1 Pig 53.0(6) 0.6(2) 3.2(3) 14.8 19.1(1.2) (1) Aug 52.3(4) 0.7(1) 3.2(2) 11.6 16.7(2) (3) Notes to Table 3:

CaO

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indicates that with 4 wt.% water, the AR-8 composition is above the liquidus and hence unable to produce any plagioclase. Thus, our experiments dictate that the NAn85 plagioclase observed in Arenal lavas cannot be explained by any simple process of crystallization at 0.5 GPa. It is possible that the high-An plagioclase phenocrysts could originate by crystallization at lower pressures and temperatures in the conduit during eruption (see the 0.2 GPa experiments of Szramek et al., 2006-this volume). However, melt water contents in these water saturated experiments are likely to be N 5 wt.%. Thus, the erupted Arenal lavas would have to be fully molten at greater pressures (like those here), raising the issue of what previous thermobarometry results mean (Cigolini, 1998). Furthermore, the large size of many anorthitic plagioclase crystals and their generally homogeneous cores are unlikely to result solely from crystallization in the conduit (at b 0.2 GPa) during eruption. The pressure–temperature–water content conditions inferred from the plagioclase rim compositions appear generally consistent with those inferred from pyroxene equilibria in our experiments. At 1150 °C and 2 wt.% water in the melt, orthopyroxene compositions (Mg# 76, 2.3 wt.% CaO) closely match the relatively homogeneous orthopyroxenes found in Arenal lavas (Mg# 74– 78, 1.3–1.8 wt.% CaO; Reagan et al., 1987). Relating the presence and composition of clinopyroxene in the experiments to Arenal observations is more difficult for two reasons. First, the importance and relative stability of clinopyroxene in Arenal lavas remains a question: the clinopyroxene mode during stage 2 is less than 1% according to Reagan et al. (1987), while Streck et al. (2005) indicate modes of up to 7%. Second, our experiments clearly show that high Ca clinopyroxene is not stable at 1200 °C under either hydrous or anhydrous conditions but that it does appear at lower temperatures. Unfortunately, while the clinopyroxene found in hydrous experiment at 1150 °C (run 32) did have high CaO contents like those observed in Arenal lavas, its texture and Mg# are interpreted to reflect it being a quench product and not yet stable at 1150 °C. A conservative interpretation of our hydrous experiment results is that at 0.5 GPa clinopyroxene appears at temperatures ≤ 1150 °C. Thus, the similarity in orthopyroxene and plagioclase crystal rim composition between experiments and Arenal lavas indicate that a magma with ~ 2 wt.% H2O could have equilibrated with these minerals at ∼ 1150 °C

233

and 0.5 GPa pressure prior to eruption. The crystallinity of the AR-8 composition at these conditions would have approached 50%, also similar to the observed phenocryst abundance during stage 2 of the eruption (Reagan et al., 1987). Nevertheless, several observations, including plagioclase core and clinopyroxene compositions, remain inconsistent with a simple crystallization process at any temperature–pressure–water content condition. The origin of highly anorthitic plagioclase in igneous rocks from diverse tectonic settings remains perplexing. Given the AR-8 composition and the textures observed, the observed N An85 plagioclase crystals observed in Arenal lavas are not explained by any possible permutation of the pressure–temperature–melt water content conditions. These crystals could reflect crystallization from a calcium and/or aluminum-rich magma but no melt of such composition has yet to be observed, even as melt inclusions. Furthermore, plagioclase in Arenal lavas form a continuum of compositions with N An85 plagioclase occurring as phenocrysts, megacrysts, mineral inclusions, and even microlites (Streck et al., 2005). This observation argues that these crystals reflect a common process occurring in the crust rather than reflecting exotic crystals (xenocrysts) or exotic melts. The anorthitic plagioclase in Arenal lavas could originate from a diffusion–reaction process occurring during interaction between wall rock gabbros and the basaltic andesite magma (Lundstrom et al., 2005). Briefly, in piston cylinder experiments that juxtaposed AR-8 with a leucocratic gabbro lying within a temperature gradient, diffusive exchange between these two partially molten materials produced a plagioclase-rich boundary zone between them. Surprisingly, the cores of the plagioclase in the boundary zone became overly enriched in anorthite component during this process while the rims remained in Ca–Na exchange equilibrium. Isotopic tracers indicated full chemical communication between plagioclase cores and melt during diffusion–reaction; the end result of this process was that the changing plagioclase core compositions served to buffer the changes in melt composition due to the diffusive exchange between the materials (Lundstrom et al., 2005). Diffusion–reaction thus provides a viable process that could explain the occurrence of anorthitic plagioclase in Arenal lavas. The wide variations in compositions of clinopyroxenes in Arenal lavas may also provide evidence for diffusion–reaction processes occurring in a crustal mush zone beneath Arenal. For instance, clinopyroxene Cr2O3 contents range from below

Notes to Table 3: Blank entry indicates element was not analyzed; n.d. indicates element not detected. Mg# = molecular Mg / (Mg + Fe) ⁎ 100; An = plagioclase anorthite content. Kd = (plagCa/meltNa) / (plagNa/meltCa) for plagioclase and Kd = (pxMg/meltFe) / (meltMg/pxFe) for pyroxenes. n indicates the number of spots analyzed per phase. Mg/Fe Kd for diffusion/ reaction experiment not determined because of the bulk composition gradient across the basaltic andesite/ gabbro interface. See Table 2 for mineral abbreviations and definitions.

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detection limits to 0.7 wt.% (Streck et al., 2005). Because bulk rock compositions are ∼50 ppm Cr (Ryder et al., 2006-this volume), crystallization from the corresponding melt cannot explain the high Cr2O3 contents observed in clinopyroxene. The diffusion–reaction experiments show that Cr diffuses from the colder gabbro into the hotter basaltic andesite and thus this process could provide an explanation for the erratic and sometimes high Cr2O3 contents of clinopyroxenes (Lundstrom et al., 2005). We have suggested that the observed phase compositions in Arenal lavas are consistent with a melt composition close to that in our 1150 °C hydrous experiment equilibrating with ∼ An65 plagioclase rims and Mg# ∼75 orthopyroxenes as it ponded in the crust below Arenal. The surrounding gabbroic wall rock became partially molten and interacted with the magma in a diffusion– reaction process, creating the anorthitic-cored plagioclase and possibly variable composition clinopyroxenes. Evidence for input of crustal materials into Arenal anorthitic plagioclase comes from large variations in 87Sr/86Sr observed within single plagioclase crystals from throughout the eruption (Lundstrom and Tepley, 2006-this volume). The question remains as to whether the erupted rock bulk compositions ever represented a fully molten magma. Based on the phase equilibria results at 0.5 GPa pressure, erupted Arenal rocks could have represented a liquid only if the magma contained N 2 wt.% water at temperatures N 1200 °C or if it contained N4 wt.% water for temperatures ≥ 1150 °C. Given petrologic constraints that dictate temperaturesb 1150 °C, melt water contents would have to be N4 wt.%. Olivine-hosted melt inclusions found in mafic tephras erupted from Arenal 950 and 1250 years before present typically have water contents of ∼2–3 wt.% (Wade et al., 2006-this volume). The most primitive melt inclusions reach values as high as 4 wt.% H2O with estimated minimum pressures of entrapment of ∼0.2 GPa (Wade et al., 2006-this volume). These results give a general idea of water contents in Arenal magmas although it is not clear how currently erupting lavas directly relate to such older lavas. Nevertheless, these results document that magma water contents high enough to result in current eruption lavas being nearly fully molten depending on the exact temperature and pressure conditions have existed in the Arenal system in the recent past. There remains a disconnect between the idea of an initially fully molten magma and these later equilibration conditions, if our inference that erupted lavas closely approximate our hydrous 1150 °C experiment is correct. This is because the experimental result indicates equilibrium between the 2 wt.% water melt and 50% crystals for this bulk composition. If this reflects a derivation from a

fully molten magma, initial water contents would be only 1 wt.%. Only under extreme thermal conditions would the AR-8 bulk composition then be fully molten. Degassing of water at this depth is unlikely given that the melt remains water undersaturated at 0.5 GPa. In summary, combining the water contents observed in olivine-hosted melt inclusions (Wade et al., 2006-this volume) with our phase relationships as a function of water content indicates that erupted bulk compositions could have represented a liquid originally. However, the good match for phase compositions and crystallinity between our 1150 °C hydrous experiment and observed lavas argues that erupted magmas record lower temperature, lower water pressure conditions that are inconsistent with the premise that the composition was ever fully molten near 0.5 GPa. Instead, we suggest that as magmas pond and equilibrate at these crustal depths, they are modified by diffusion–reaction with surrounding wall rocks leading to magmas with signatures of plagioclase accumulation. Diffusion–reaction could then be an important process that explains the observation that many porphyritic HAB have accumulated plagioclase (Crawford et al., 1987). 5. Conclusions Phase equilibria experiments at 0.5 GPa pressure on the most mafic basaltic andesite from the current eruption at Arenal volcano show consistent relationships of changing melt composition, plagioclase composition and crystallinity as a function of temperature and melt water content. The close correspondence of the phase compositions (rims of plagioclase, orthopyroxene) and crystallinity observed in stage 2 lavas from Arenal and a hydrous experiment with 2 wt.% water in the melt provides evidence for magmas ponding and equilibrating at 1150 °C and ∼12– 14 km depth. However, the experiments cannot account for the origin of unusually An-rich plagioclase phenocrysts in Arenal lavas, unless magmas at depth were unrealistically hot or water-rich. Instead we suggest that the anorthitic plagioclase reflect diffusion–reaction processes (Lundstrom et al., 2005) of interaction with surrounding crustal gabbros. Combined with maximum water contents observed in olivine-hosted melt inclusions from other earlier Arenal eruptions (Wade et al., 2006-this volume), it is possible that this bulk composition could have been fully molten at plausible temperatures at depth beneath Arenal. However, such a scenario is inconsistent with the stronger observation of our experiments, namely the indication of melt–crystal equilibration at lower water contents. We therefore conclude that Arenal bulk rocks are unlikely to have ever reflected fully molten magmas.

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