Degassing at the SoufrieÁ re Hills Volcano, Montserrat, Recorded in Matrix Glass Compositions

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1 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 8 PAGES 1503± Degassing at the SoufrieÁ re Hills Volcano, Montserrat, Recorded in Matrix Glass Compositions C. L. HARFORD 1, R. S. J. SPARKS 1 * AND A. E. FALLICK 2 1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, BRISTOL BS8 1RJ, UK 2 SUERC, SCOTTISH ENTERPRISE TECHNOLOGY PARK, RANKINE AVENUE, EAST KILBRIDE G75 0QF, UK RECEIVED JANUARY 11, 2002; ACCEPTED MARCH 19, 2003 Compositions of matrix glasses from the current eruption of Soufriere Hills Volcano indicate that decompression-driven crystallization results in 20±70 wt % groundmass crystallization during eruption and variable degassing. Variations in crystallinity and volatile contents (water and chlorine) of matrix glasses are attributed to variations in extrusion rates and residence times in the lava dome. Residual water contents in pumice clasts (02±06 wt %) indicate minimum pressures of 11± 37 MPa in 1997 Vulcanian explosions. Residual water contents of 16 wt % in a ballistic block ejected in sub-plinian explosive activity on 17 September 1996 imply larger pressure drops (20 MPa). Variable residual water contents in dome samples are consistent with pressure variations of up to 9 MPa in the lava dome interior. Large variation in chlorine contents between lava blocks compared with explosion pumice clasts indicates that shallow-level processes dominate degassing. Low melt chlorine contents of dome samples cannot be explained by open- or closed-system degassing, especially when crystallization is taken into account. Instead, heterogeneous chlorine leaching by circulation of groundwater vapour in the dome is proposed. Variable and elevated matrix glass dd values can also be attributed to interaction with isotopically heavy surface waters. HCl emission during the current eruption can be accounted for by Cl loss from the melt, consistent with the melt being undersaturated in chlorine. KEY WORDS: andesite; degassing; chlorine; lava dome INTRODUCTION The exsolution and escape of volcanic gas from magma are major controls on volcanic activity. Understanding degassing is an important goal in volcanology, because the fluxes and inventories of volcanic gases can be monitored by remote-sensing techniques and analytical methods. These data provide information to assist forecasting activity and mitigating the effects of volcanism. The current eruption of Soufriere Hills Volcano, Montserrat, has exhibited a wide variety of eruptive styles, ranging from sub-plinian and Vulcanian explosive eruptions to extrusion of a viscous lava dome at rates from 505m 3 /s to 9m 3 /s (Robertson et al., 2000). Transitions in eruptive style have been attributed principally to ascent-driven degassing and crystallization, because of their enormous effect on magma rheology (Melnik & Sparks, 1999; Voight et al., 1999; Sparks et al., 2000). During magma ascent, decompression-driven volatile exsolution is accompanied by crystallization, because volatile loss causes a rise in the liquidus temperature of the magma (Blundy & Cashman, 2001). This paper presents a study of the volatiles (water and chlorine) and hydrogen isotope compositions of glasses from a suite of samples from the current eruption of Soufriere Hills Volcano to investigate degassing processes. The study complements and develops the study of degassing at the Soufriere Hills Volcano by Edmonds et al. (2001). The Soufriere Hills Volcano is well suited to such a study because of the availability of both a well-documented sample suite, which covers a variety of eruptive activities, and a wealth of complementary information on the volcanic system. Experiments simulating chlorine behaviour in magma of the Soufriere Hills Volcano (Signorelli & Carroll, 2001) allow comparisons to be made with samples from the current *Corresponding author. Steve.Sparks@bristol.ac.uk Journal of Petrology 44(8) # Oxford University Press 2003; all rights reserved

2 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 8 AUGUST 2003 eruption. Degassing is assessed through analyses of the major elements and volatiles (chlorine and water) in matrix glasses. Comparing post-eruptive volatile concentrations in matrix glasses with pre-eruptive concentrations inferred from melt inclusions allows assessment of degassing during magma ascent. Modelling of water degassing is facilitated by analysis of the hydrogen isotope composition of matrix glass water. Hydrogen isotope compositional variations can be influenced by whether a degassing system is open or closed, and can be influenced by kinetic effects and interactions with external waters. Analysis of major elements and volatiles in the same matrix glass samples allows investigation of the relationship between crystallization and degassing processes. This information can help interpret investigations of gas fluxes using remote-sensing techniques. In particular, changes of Cl/S measured by Fourier transform IR spectroscopy (FTIR) (Edmonds et al., 2001) can be interpreted in the context of Cl degassing processes. SAMPLING AND ANALYTICAL METHODS Samples were selected to cover a broad range of eruptive conditions, including explosion-derived pumice and ballistic clasts, and lava blocks from periods of varying dome extrusion rates (Table 1). For safety reasons it was impossible to sample the lava dome directly. Hence lava blocks were collected from blockand-ash-flow deposits formed by episodic dome collapse. Samples were selected from block-and-ash flows thought likely to have involved collapse of the active flow lobe of the lava dome and not to have remobilized large proportions of material from older parts of the dome. Matrix glasses were analysed for major elements and chlorine at Bristol University on the JEOL JXA-8600 electron microprobe with four wavelength-dispersive spectrometers. Analytical techniques were optimized to minimize loss of sodium whilst at the same time ensuring good precision (Appendix A). Glass areas were selected so as to allow as large an analysis area as possible, often 5 mm 4 mm, and always greater than 4 mm 3 mm. Each glass area was analysed twice with a rastered beam and an accelerating voltage of 15 kv. First the glass was analysed for major elements at the low beam current of 2 na. The background was measured only on the initial analysis, which was not included in the mean. Sodium stability tests indicate that sodium loss is thus minimized to an estimated 5±10% relative (Appendix A). Subsequent analysis of the same glass area for chlorine, present only in small quantities, was carried out at 10 na with a peak count time of 120 s. Five to 10 glass analyses were made at each beam current to assess sample heterogeneity. For homogeneous samples multiple analyses also reduce uncertainties for the mean glass composition. Separate standardization routines were carried out at each beam current, using a beam diameter of 5 mm and the following standards: albite (Na), olivine (Mg), wollastonite (Si, Ca), synthetic Al 2 O 3 (Al), adularia (K), synthetic SrTiO 3 (Ti), hematite (Fe), synthetic MnO (Mn) and NaCl salt (Cl). To check the standardization, secondary standards KK1 albite and KN18 glass were analysed at 2 and 10 na, respectively, with a 15 mm beam diameter. The predicted precision for a single microprobe analysis of chlorine, based on counting statistics, ranges from 002 to 004 wt % for glasses containing 033 and 005 wt % Cl, respectively. The measured standard deviation for chlorine in KN18 containing 032 wt % is 001 wt % (for 26 analyses during the course of this work; predicted standard error of the mean is 0008). For major elements the single analysis precision based on counting statistics is better than 7% relative, except for Fe (20%) and minor elements (Ti, Mg, Mn). It is difficult to obtain a direct measure of water in matrix glasses. The glass areas are too small for FTIR analysis, and assessment of water by electron microprobe difference methods is not reliable for these tiny glass patches as a result of sodium loss. Instead, a selection of samples was analysed for water and hydrogen isotopes by bulk extraction at SUERC (Scottish Universities Environmental Research Centre). Anhydrous samples of the bulk matrix were prepared by coarse crushing and hand-picking to remove amphibole crystals. In some cases amphiboles were removed using heavy liquid (tetrabromoethane) separation and then hand-picking (Table 2). Coarse grain size fractions, generally 1±14 mm and occasionally as fine as 250±500 mm, were used to minimize surface water contribution (Newman et al., 1988), facilitate removal of amphiboles and minimize grain size fractionation of glass. Step-heating experiments were performed to assess water release as a function of temperature, to determine a suitable heating schedule for water extraction (Appendix B). The resultant extraction schedule involved overnight degassing at 200 C to remove surface water, before heating to over 1400 C for a period of around 2±6 h, until no further gas was evolved. Released water was converted to H 2 by reaction with hot uranium and the yield measured manometrically (Fallick et al., 1993). Extractions were generally performed at least in duplicate. Hydrogen isotope results are expressed as dd values in parts per thousand (%) relative to V-SMOW (Vienna Standard Mean Ocean Water). The water content of the matrix glass was 1504

3 1505 Table 1: Summary of new EMPA results for matrix glasses (sorted by increasing K 2 O), compared with results for microprobe results for mineral glass inclusions, average groundmass and experimental samples, and average whole-rock composition by XRF Sample no. Composition (wt %) Eruption date WR K 2 O Sample Estimated magma Estimated glass (wt %) 2 Cl degassed 3 Na 2 O MgO K 2 O CaO TiO 2 FeO MnO SiO 2 Al 2 O 3 Cl Unnorm. total (wt %) 1 description extrusion rate (m 3 DRE/s) content 1s error Matrix samples MVO58Na /09/96 ÐÐ dome lava 1.5 MVO1205b /09/96 ÐÐ pumice fall MVO1205c /09/96 ÐÐ pumice fall MVO1205a /09/96 ÐÐ pumice fall MVO /09/ pumiceous ballistic MVO /05/97 ÐÐ dome lava MVO305a Aug.97 ÐÐ pumice fall MVO /12/ pumiceous block MVO1109d /06/99 ÐÐ dome lava MVO Sept/Oct banded block MVO /09/ glassy ballistic MVO /08/ dome lava MVO /01/97 ÐÐ dome lava MVO /08/96 ÐÐ dome lava MVO44 5, /05/96 ÐÐ dome lava MVO58K 5, /09/96 ÐÐ dome lava MVO /02/ dome lava MVO /02/ dome lava MVO /01/97 ÐÐ dome lava MVO /07/96 ÐÐ dome lava MVO /08/ dome lava mean 98.4 (kg/m 3 WR) HARFORD et al. MATRIX GLASS COMPOSITIONS, SOUFRIEÁ RE HILLS

4 1506 Table 1: continued Sample no. Composition (wt %) Eruption date WR K 2 O Sample Estimated magma Estimated glass (wt %) 2 Cl degassed 3 Na 2 O MgO K 2 O CaO TiO 2 FeO MnO SiO 2 Al 2 O 3 Cl Unnorm. total (wt %) 1 description extrusion rate (m 3 DRE/s) content 1s error Non-matrix samples MI in quartz MI in plag Groundmass av WR av Experimental samples Mon6ar ÐÐ n.a. 180 MPa; 900 C 53 M14b ÐÐ n.a. 130 MPa; 830 C 34 (kg/m 3 WR) All compositions are normalized to 100% anhydrous for comparison. n.a., not analysed. 1 Whole-rock (WR) XRF values where available. 2 Assuming K 2 O slightly compatible. Assumptions used (with estimated 1s error): plagioclase 0.2 (0.03) wt % K 2 O, 70 (3) wt % of anhydrous crystals; amphibole 0.18 (0.01) wt % K 2 O, 7 (0.5) wt % of whole rock. WR K 2 O contents from XRF values where available (3% relative; wt %), otherwise WR K 2 O average of 53 analyses of (0.08) wt %. Errors in glass contents calculated by error propagation. 3 Calculated using difference between plagioclase melt inclusion and matrix glass chlorine contents, and estimated corresponding glass contents. 4 Not true glassððfinely crystalline intergrowth of plagioclase and quartz. 5 Water content by bulk extraction available (Table 2). 6 Denotes contains groundmass silica phase. 7 Devine et al. (1998): Plag melt inclusion (MI) n ˆ 26, Quartz MI n ˆ 3. 8 Average groundmass composition from 200 rastered EMPA of MVO34, Barclay et al. (1998). 9 Average of WR XRF analyses for current eruption, n ˆ 53 (G. Zellmer, personal communication, 2000). 10 By definition. JOURNAL OF PETROLOGY VOLUME 44 NUMBER 8 AUGUST 2003

5 HARFORD et al. MATRIX GLASS COMPOSITIONS, SOUFRIEÁ RE HILLS Table 2: Data on the water content and hydrogen isotope composition of samples from the Soufriere Hills eruption MVO no. Split Date Sample type Best WR glass Bulk H 2 O Approx. glass H 2 O dd SMOW (%) erupted estimate (%) (wt %) (wt %)y mean SD mean SD MVO121 az 9/16/96 explosion pumice fall MVO58 bz 9/16/96 dome lava MVO57 az 9/16/96 explosion ballistic MVO57 bz 9/16/96 explosion ballistic MVO57 cz 9/16/96 explosion ballistic MVO57 dz 9/16/96 explosion ballistic MVO57 e 9/16/96 explosion ballistic MVO57 f 9/16/96 explosion ballistic MVO174 az 12/18/96 pumice MVO174 bz 12/18/96 pumice MVO53 iz 9/16/96 dense pumice MVO53 jz 9/16/96 dense pumice MVO53 kz 9/16/96 dense pumice MVO53 lz 9/16/96 dense pumice MVO53 mz 9/16/96 dense pumice MVO53 nz 9/16/96 dense pumice MVO1191 d 9/16/96 dense ballistic MVO1191 c 9/16/96 dense ballistic MVO1191 e 9/16/96 dense ballistic MVO1191 f 9/16/96 dense ballistic MVO1191 g 9/16/96 dense ballistic MVO1191 h 9/16/96 dense ballistic ÐÐ MVO244 c Jul.97 explosion pumice flow MVO244 b Jul.97 explosion pumice flow MVO244 c Jul.97 explosion pumice flow MVO658 b 26/12/97 dense ballistic MVO658 c 26/12/97 dense ballistic MVO /09/96 dome lava MVO /09/96 dome lava MVO /09/96 dome lava MVO /03/97 dome lava MVO /03/97 dome lava MVO1205a 1 17/09/97 explosion pumice fall MVO1205a 2 17/09/97 explosion pumice fall MVO /05/97 dome lava MVO /05/97 dome lava MVO /01/97 dome lava MVO /01/97 dome lava MVO1205b 1 16/09/96 explosion pumice fall MVO1205b 2 16/09/96 explosion pumice fall MVO /01/97 dome lava MVO /01/97 dome lava MVO305a 1 Jul.97 explosion pumice fall Samples are crushed whole rock with amphibole phenocrysts removed generally by hand-picking. MVO, Montserrat Volcano Observatory. Values in bold type represent estimate of whole-rock glass content from glass composition; others represent guesstimates. yapproximate glass water content estimated using whole-rock glass estimate (see text). ztetrabromoethane used in sample preparation. 1507

6 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 8 AUGUST 2003 estimated from the measured water content (relative error 10%), and applying a correction to account for the anhydrous crystals in the samples analysed. Estimates of matrix glass content based on electron microprobe analysis (EMPA; see below) were used where available. Where glass could not be analysed by electron microprobe, glass contents were estimated by comparison with other samples (12 wt % for dome samples, 32 wt % for pumice clasts), and these estimates were then used to calculate approximate glass water contents. RESULTS Matrix glass compositions The electron microprobe major-element analyses show that the matrix glasses are rhyolitic and span the compositional range from 765 to 793 SiO 2 (normalized to 100% anhydrous; Table 1). Repeat analyses on each sample gave major-element 1s variation comparable with counting statistics precision, indicating major element homogeneity within the samples studied. Here K 2 O is used as an indicator of the evolution of the melt phase in the magma as a result of crystallization, as K is nearly incompatible in the crystallizing mineral assemblage in the Soufriere Hills magma. K 2 O constitutes only around 01±04 wt % of plagioclase and 018 wt % of amphibole crystals (Murphy et al., 2000). Amphibole does not crystallize during magma ascent, as it is unstable at low pressures (Barclay et al., 1998). The matrix glass compositions vary from 20 to 59 wt % K 2 O, indicating a wide range of sample crystallinities. The matrix glasses are richer in K 2 O than most melt inclusions in phenocrysts (Barclay et al., 1998; Devine et al., 1998a; Edmonds et al., 2001), a groundmass composition (determined by rastered EMPA; Barclay et al., 1998), and the average whole-rock composition [determined by X-ray fluorescence (XRF); Murphy et al., 2000, G. Zellmer, personal communication, 2002], indicating that the matrix glasses have experienced a greater degree of crystallization than these materials (Table 1). Matrix glass K 2 O contents were used to estimate the glass contents of samples by mass balance of K 2 O. The model involves a mass balance of K 2 O using observed proportions and K contents of the phenocrysts (principally plagioclase and amphibole) and bulk-rock K 2 O contents (Murphy et al., 2000). Estimated glass proportions vary from 34 to 10 wt % (Table 1). These glass estimates have small uncertainties (1s of 05±46 wt % absolute) associated with errors in XRF and electron microprobe K 2 O measurements, and assumptions involved in the mass balance calculation (Table 1). The glass contents estimated are consistent with estimates by image analysis and results reported in Edmonds et al. (2001). The estimated glass content of MVO47 is 25 wt % by mass balance and 22 vol. % by image analysis (assuming an average density of 2400 kg/m 3 for glass and 2700 kg/m 3 for whole rock, this corresponds to 20 wt % glass). Many samples proved unsuitable for reliable EMPA of matrix glass because of the lack of sufficiently large matrix glass areas and are excluded from the dataset. Glass estimates for some of these samples by image analysis indicated glass proportions as low as 5 vol. %. There is much greater variation in glass composition and groundmass crystallization in lava dome samples than in pumice clasts. The variation between two lava blocks from the same block-and-ash-flow deposit is, in fact, as great as the variation between lava blocks from different block-and-ash-flow deposits. This variability can be attributed to two factors: first, dome collapses sample regions in the dome with different degassing and cooling histories, and hence different ultimate crystallinities; second, block-and-ash flows can erode blocks deposited from previous flows (Cole et al., 2002). An exception to major-element glass homogeneity occurs in samples where there appear to be two `glasses', as illustrated by analyses MVO58K and MVO58Na (Table 1). Similar observations have been made for Mount St. Helens samples (Cashman, 1992). Blundy & Cashman (2001) recently examined samples with apparently heterogeneous glasses by transmission electron microscopy (TEM). They concluded that the heterogeneities constituted an intimate mixture of a K-rich true glass, and very finely crystalline intergrowths of feldspar and quartz. The Montserrat results provide additional evidence to support this interpretation. The K-rich zones contain chlorine, whereas the Na-rich zones contain no chlorine, consistent with the former zones comprising glass, and the latter zones comprising volatile-poor microcrystalline regions. Hence, where this texture was observed, only the K-rich, true glass was analysed. Matrix glass chlorine contents Matrix glass chlorine contents are highly variable, ranging from 031 to below the detection limit (Table 1, Fig. 1). Chlorine values are high in pumice clasts (031±027 wt %), and variable in ballistic and lava dome samples (028±003 wt %). The standard deviation of repeat chlorine analyses is generally comparable with counting statistics precision, indicating chlorine homogeneity within these samples. However, the 1s variation for MVO47 and MVO52 (008 and 010 wt %, respectively) significantly exceeds the counting statistics precision (004 wt %), indicating chlorine heterogeneity. Edmonds et al. (2001) also 1508

7 HARFORD et al. MATRIX GLASS COMPOSITIONS, SOUFRIEÁ RE HILLS Fig. 1. Matrix glass K 2 O and chlorine contents. Data on melt inclusions in phenocrysts after Devine et al. (1998a) are also shown. reported variable matrix glass chlorine contents in a similar range of 032 wt % to below the detection limit of 004 wt %. Matrix glass chlorine concentrations are mostly lower than values reported for plagioclase melt inclusions of up to 044 wt % (Devine et al., 1998a; Edmonds et al., 2001). Devine et al. (1998a) also measured chlorine contents of 026 wt %, in melt inclusions in quartz. Both matrix glass values and melt inclusion chlorine contents are below the solubility limit of chlorine as determined experimentally (Signorelli & Carroll, 2001) for rhyolitic melt compositions similar to those found at Soufriere Hills Volcano in the presence of vapour and brine. Signorelli & Carroll (2001) used average groundmass composition as the starting material. Those workers found solubilities of 048±068 wt % Cl at pressures in the range 250±25 MPa, respectively. At pressures between 100 and 200 MPa, where the melt composition is relatively constant, chlorine solubility is inversely correlated with pressure (Signorelli & Carroll, 2001). Metrich & Rutherford (1992) also found a negative correlation between pressure and Cl solubility in experiments on hydrous peralkaline rhyolites. In contrast, Webster et al. (1999) found that Cl solubility decreases with decreasing pressure from 2 kbar to 1 atm with water-poor felsic, intemediate and mafic melts. These experiments involved coexistence of silicate melts with a brine but without a vapour phase, and are thus less relevant to the Montserrat situation. Changing melt composition, in particular increasing silica activity or increasing [(Na K)/Al] molar (from peraluminous to metaluminous compositions), can result in a moderate decrease in chlorine solubility (Metrich & Rutherford, 1992; Carroll & Webster, 1994; Webster & De Vivo, 2002). Experimental glasses of Signorelli & Carroll (2001) range in composition from 730 to 777 wt % SiO 2 with [(Na K)/Al] molar of 067±081, whereas natural samples include more evolved compositions, ranging from 765 to 793 wt % SiO 2 (normalized to 100% anhydrous) with [(Na K)/Al] molar of 067±099. Some of the dome sample matrix glasses therefore have slightly higher SiO 2 and [(Na K)/Al)] molar than the 25 MPa experimental glasses of Signorelli & Carroll (2001), which might result in slightly lower Cl solubility. Thus, for pressures between 25 and 100 MPa for the Soufriere Hills andesite, Cl solubility decreases with increasing pressure. The increase in SiO 2 and [(Na K)/Al)] molar as a result of crystallization will have a small counteracting effect. These effects are, however, minor, such that Cl solubility remains approximately constant during magma ascent with concurrent crystallization. Matrix glass water and hydrogen isotopes Bulk extraction from amphibole-free separates gave low water contents of 002±04 wt % and dd values ranging from 40 to 100% (Table 2). Estimated 1509

8 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 8 AUGUST 2003 Fig. 2. Groundmass dd and water contents from bulk extraction. Symbols represent single analyses; detailed results are given in Table 2. Error bar represents 1s error for initial melt dd value, derived from hornblende phenocryst analyses. Details of models are given in text. glass proportions were used to calculate approximate glass water contents from bulk water values, as described above. Removal of amphibole and other heavy mineral phases, and fractionation of glass out of the coarse-grained separate analysed can cause the proportion of glass in the crushed sample analysed to be unequal to that in the whole-rock. However, K 2 O contents of glass residues after fusion (for hand-picked samples) were similar to whole-rock K 2 O by XRF (averages of 076 wt % and 077 wt %, respectively), indicating that effects caused by removal are minor. The uncertainties in K 2 O analyses by electron microprobe and XRF are larger than the error in glass content estimates introduced by these procedures. The results confirm that matrix glass water contents are low: 51 wt % (Table 2, Fig. 2). One exception is MVO57, a glassy ballistic from the 17 September 1996 sub-plinian explosive eruption (Robertson et al., 1998) for which six individual analyses gave an average matrix glass H 2 O content of wt %. dd variation of up to 20% for splits of the same sample is significantly greater than expected from analytical precision (3% for mineral separates) (Table 2). Variation of dd is unlikely to represent incomplete water extraction for two reasons. First, there is no systematic relationship between dd values and water content for repeat analyses of the same sample, which could have been related to processes of water release. Second, after water extraction, fused glasses had no detectable retained water by FTIR, i.e wt % H 2 O, which corresponds generally to less than a few percent of the total water extracted from the sample. The very low bulk water content of these samples may increase analytical uncertainties, but the magnitude of the variation suggests that there is dd heterogeneity within the samples. Ion microprobe studies (Harford & Sparks, 2001) have shown marked dd heterogeneity within and between amphibole crystals in early erupted samples (before April 1996). In contrast, ion microprobe analysis showed variations within the analytical precision of this technique (8%) in amphiboles erupted after spring Repeat analyses by bulk extraction of fresh amphibole separates (MVO121, 17 September 1996 explosion pumice) gave a mean dd of 52% 10% (ranging between 36 and 64%), comparable with values obtained for other volcanic amphiboles in subduction zone settings (Hildreth & Drake, 1510

9 HARFORD et al. MATRIX GLASS COMPOSITIONS, SOUFRIEÁ RE HILLS 1992; Taran et al., 1997; Kusakabe et al., 1999). However, dd variation of 10% (1s) is significantly greater than the analytical precision of 3% (Harford & Sparks, 2001). DISCUSSION Eruptive conditions The difference in composition and crystallinity observed between samples is interpreted as due to differences in eruptive conditions and associated time spent at shallow levels. The most K 2 O-poor, least crystalline samples are pumice clasts (with estimated glass contents of 34±31 wt %), erupted during explosions. During Vulcanian explosions, material from the uppermost several hundred metres of the conduit was evacuated on a timescale of less than a few minutes (Druitt et al., 2002), giving insufficient time for crystallization. During the periods of Vulcanian explosions, magma extrusion rate was estimated at 9 m 3 /s, comparable with the highest lava extrusion rates (Sparks et al., 1998; Druitt et al., 2002). For the pumice clasts, ascent from the magma reservoir at an estimated depth of 45 km (Barclay et al., 1998) to the surface, assuming a conduit area of 700 m 2, as discussed by Melnik & Sparks (1999) and Robertson et al. (1998), would have taken only 4 days. These estimates are consistent with ascent speeds estimated by studies of amphibole breakdown reactions (Devine et al., 1998b). During the sub- Plinian explosion of 17 September 1996, when it is inferred that the majority of the conduit was evacuated (Robertson et al., 1998), pumice clasts may have spent as little as an hour in transit to the surface from depths of up to 4 km. In contrast, samples of the lava dome experienced a relatively slow transit to the surface. These samples are more crystalline, with estimated glass contents of 31±5 wt %. Magma ascent from the chamber to the surface would have taken between 5 and 80 days for typical extrusion rates of 8±05 m 3 /s (Sparks et al., 1998), assuming a conduit area of 700 m 2. In addition, these samples would have spent an unknown period in the lava dome, probably ranging from a few days to several months. The amount of crystallization thus reflects both ascent time and residence time in the dome before collapse, explaining the lack of correlation between glass K 2 O content (a proxy for groundmass crystallinity) and magma extrusion rate (Fig. 3). A further modifying control on crystallization is each sample's individual thermal and degassing history, which may vary significantly dependent on position within the shear lobes of the lava dome (Watts et al., 2002). These factors account for the large variation in matrix glass composition and groundmass crystallization in lava dome samples compared with pumice clasts. Chlorine degassing In the following discussion glass K 2 O content is used as a proxy for crystallinity. The chlorine contents of matrix glasses do not show a simple relationship with either magma extrusion rate or K 2 O content. Pumice clasts erupted during periods of rapid magma extrusion have low K 2 O glass contents and crystallinities, and high chlorine contents, similar to those of most plagioclase melt inclusions. Data on melt inclusions in plagioclase (Devine et al., 1998a; Edmonds et al., 2001) extend the data to higher chlorine contents and lower glass K 2 O contents. These data are consistent with the interpretation that most of the melt inclusions were trapped before magma ascent. There is an approximate inverse correlation between K 2 O and Cl for these matrix glass samples (Fig. 1), indicating that Cl degassing accompanies crystallization. However, at higher crystallinities (high K 2 O), ballistics and lava dome samples have very variable chlorine contents, which do not relate to K 2 O. Both high and very low Cl contents are observed in highly crystalline samples (Fig. 1), indicating that the relationship between late-stage crystallization and Cl degassing is complex. Lack of correlation between matrix glass Cl content and magma extrusion rate for lava dome samples (Fig. 3) may in part be attributed to variable extrusion rates and residence times in the dome, resulting in variable degassing histories as well as variable crystallization histories as discussed above. The chlorine contents of both matrix glasses and melt inclusions are all well below the solubility limit of chlorine in Soufriere Hills melts of 05±07 wt % in the pressure range 25±250 MPa (Signorelli & Carroll, 2001). Melt inclusions in plagioclase (Devine et al., 1998a; Edmonds et al., 2001) are assumed to represent melt compositions in the magma reservoir, and hence reflect pre-eruption volatile contents. In the absence of degassing, increasing crystallization of chlorineincompatible groundmass phases during magma ascent should be accompanied by increasing chlorine in the melt. However, the chlorine contents of the matrix glasses are lower than those of the plagioclase melt inclusions. These observations, together with the tendency for Cl to decrease with higher crystallinity (higher glass K 2 O), indicate chlorine degassing on ascent. Edmonds et al. (2001) observed similar evidence for chlorine degassing during magma ascent for their studies of matrix and melt inclusion glass compositions. Chlorine degassing can occur when the combined solubility limit of the volatile species present is 1511

10 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 8 AUGUST 2003 Fig. 3. Estimates of approximate magma extrusion rates (DRE m 3 /s) at the time of sample collection (after Sparks et al., 1998) plotted against matrix glass K 2 O (wt %) and Cl (wt %) of samples. reached. The lack of a simple relationship between Cl and H 2 O in matrix glasses (Fig. 4) suggests that a single process does not control chlorine degassing at Soufriere Hills Volcano. Cl±H 2 O degassing can be modelled in terms of equilibrium partitioning of chlorine (as a trace element) between the melt phase and an accompanying aqueous fluid phase or vapour plus brine (Villemant & Boudon, 1999) (Fig. 4). It is assumed that no significant fluid phase exists in the magma chamber and that a water-dominated 1512

11 HARFORD et al. MATRIX GLASS COMPOSITIONS, SOUFRIEÁ RE HILLS Fig. 4. fluid phase forms during ascent by decompressioninduced water saturation. Starting melt volatile contents of 43 wt % H 2 O and 034 wt % Cl are assumed based on melt inclusions in plagioclase measured by FTIR (Barclay et al., 1998) and EMPA (Devine et al., 1998a). The chlorine partition coefficient between the aqueous phase and the melt, D Cl, is defined as D Cl ˆ CCl v =Cm Cl where C v Cl and C m Cl denote chlorine concentration in the aqueous fluid phase (vapour in the 1513

12 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 8 AUGUST 2003 Fig. 4. Water and chlorine contents of Soufriere Hills glasses, and models of degassing. Water contents measured by bulk extraction indicated by filled symbols; estimated water contents denoted by open symbols. Melt inclusion water contents measured by FTIR (Barclay et al. 1998). Matrix glass chlorine contents measured by electron microprobe [melt inclusion data from Devine et al. (1998a)]. (a) Models with constant D Cl showing open- vs closed-system degassing without crystallization, effects of concurrent crystallization as prescribed by the parameter k (see text), and effect of D Cl (D Cl ˆ 10 for all models, except for curve indicated with D Cl ˆ 50). (b) Models using variable D Cl estimated from the experiments of Gardner et al. (1998) and J. E. Gardner (personal communication, 2000), using a linear positive dependence of D Cl on pressure (D Cl ˆ 6 at 110 MPa and D Cl ˆ 2 at 10 MPa). Low melt Cl results can be fitted only with late elevation of D Cl as shown by lower two continuous lines. (c) Models to account for Soufriere Hills melt compositions [using variable D Cl as for (b)], involving late-stage fluxing by external water as indicated by arrows. chlorine-undersaturated case) and melt, respectively. There are no direct measures of D Cl for the Soufriere Hills melt compositions. The 25±150 MPa experiments of Signorelli & Carroll (2001) were conducted in the subcritical region of the system NaCl±H 2 O, where melts are chlorine saturated and two immiscible fluids (vapour and brine) are present (Shinohara et al., 1989). Therefore only apparent D Cl values can be determined. These values (21 at 25 MPa to 34 at 150 MPa) can be considered as a maximum for true D Cl for the chlorine-undersaturated case, if there is only a vapour phase present. Villemant & Boudon (1999) modelled data for Mont Pelee, Martinique, largely using a D Cl value of 10. D Cl exhibits a strong positive pressure dependence (Shinohara et al., 1989). This is supported by experiments on rhyolitic glass at 850 C, which indicate that D Cl decreases from 8 at 150 MPa to 2 at 20 MPa (Gardner et al., 1998; J. E. Gardner, personal communication, 2000). These results were used to estimate D Cl at varying pressure for the models presented (Fig. 4b). Degassing models are also presented with constant D Cl to illustrate key points (Fig. 4a). Closed-system degassing is derived from a simple mass balance derived directly from the definition of D Cl : CCl m ˆ CCl o = 1 D Cl f D Cl Š 1 where f is the fraction of melt remaining ( f ˆ 1 at start of degassing): f ˆ 1 CH o 2 O Cm H 2 O ; CCl o ˆ fccl m 1 f CCl v : C Cl and C H2 O refer to concentration in the melt of chlorine and water, respectively, and the superscripts o and m refer to initial and final melt conditions, respectively. Open-system degassing (Rayleigh distillation) can be described as follows: CCl m ˆ CCl o f d 2 with d ˆ D Cl ± 1 and f as above. The fractionation effects of both open- and closedsystem processes become more marked as f decreases. In the early stages, when f is large (5075), opensystem trends resemble closed-system equilibrium 1514

13 HARFORD et al. MATRIX GLASS COMPOSITIONS, SOUFRIEÁ RE HILLS trends (e.g. Wilson, 1989). Because the melt contains only 43 wt % water, the minimum f attained through water degassing is 096, and evolution never extends beyond the early stages. Therefore chlorine fractionation during water loss is not markedly greater in an open system than in a closed system (Fig. 4). The samples with high observed chlorine melt contents can be reasonably well accounted for using realistic values for D Cl of Gardner et al. (1998) and J. E. Gardner (personal communication, 2000) and an open- or closed-system degassing model (Fig. 4b). However, these models cannot generate the low to very low melt chlorine contents of some dome samples, without requiring very high D Cl values of 50 (Fig. 4a). Villemant & Boudon (1999) proposed a model of simultaneous crystallization and open-system degassing, with D Cl of 10±30, to account for low chlorine contents of dense samples from Mt Pelee, Martinique. The degassing models presented above require modification to account for concurrent crystallization because chlorine is incompatible in the crystallizing phases. Two important resultant effects must be taken in account. First, the effective bulk partition coefficient for the crystalline assemblage removed from the melt, D B Cl, is less than the partition coefficient for the aqueous phase alone, D v Cl (referred to as D Cl above), as follows: D B Cl ˆ D v Cl V= S V Š 3a where V and S are the masses of aqueous fluid and crystalline phases removed, respectively. We make the simplifying assumption that crystallization and degassing occur simultaneously, and that crystallization is proportional to degassing. We thus introduce a factor k ˆ S/V and thus D B Cl ˆ D v Cl 1 k : 3b Second, the fraction of melt remaining for a given degree of water degassing decreases as follows: f ˆ 1 1 k C o H 2 O Cm H 2 O : 4 Substituting both the modified expressions for f and D B Cl for D Cl into equations (1) and (2) gives expressions for crystallization accompanying closedand open-system degassing, respectively. Assuming an initial groundmass glass content of 43% (from mass balance using average groundmass composition, Table 1), shallow-level groundmass crystallization varies between 21 and 77 wt % (for end glass contents of 34 and 10 wt %, respectively). Around 35±4 wt % of water is degassed from the melt on ascent, resulting in bulk k values varying from about 5 to 20 (Fig. 4b). The two effects of crystallization counteract one another. Higher f for a given degree of H 2 O degassing means that higher degrees of fractionation are reached. However, low D B Cl means that less Cl is partitioned out of the melt. For high k, D B Cl becomes 51 and Cl partitions preferentially into the melt, resulting in Cl enrichment with water degassing and crystallization (Fig. 4c). In all cases, melt Cl is elevated over that in the open-system model with no degassing (i.e. k ˆ 0) (Fig. 4b and c). Therefore simultaneous degassing and crystallization cannot account for the low melt chlorine concentrations at Soufriere Hills Volcano, unless the value of D Cl exceeds 50, as also recognized by Edmonds et al. (2001). Kinetics cannot account for enhanced chlorine loss either, because diffusion of chlorine is several times slower than that of water in granitic melts (Bai & van Groos, 1994). In explosive degassing, where diffusion controls may become dominant (Hort & Gardner, 2000) chlorine may be left behind in the melt relative to water. Thus, if water escaped rapidly in an open system, volatile differentiation would involve melt chlorine enrichment rather than depletion. There are five possible explanations for the low melt chlorine concentrations observed. First, a marked reduction in chlorine solubility at low pressures could result in low melt chlorine. Changing melt composition, in particular increasing silica activity or increasing [(Na K)/Al] molar (from peraluminous to metaluminous compositions) or decreasing Mg, Ca and Fe, can result in a decrease in chlorine solubility (Metrich & Rutherford, 1992; Carroll & Webster, 1994). However, these effects could only lower chlorine solubility moderately, and in addition the inverse dependence of chlorine solubility on pressure deduced from experiments in the presence of vapour would counteract this. Second, at low pressures, where no experimental constraints are available, an elevated D Cl (of order 50 or more) could result in late-stage chlorine depletion (Fig. 4b). However, D Cl depends positively on pressure (Shinohara et al., 1989; Gardner et al., 1998) so very high values of D seem unlikely. Further, if this were the correct interpretation then a problem would emerge of how to explain the high chlorine contents of other glasses with high K 2 O. Third, there is the possibility that a Cl-rich brine can separate at low pressures, resulting in very high distribution coefficients between the bulk fluid and silicate melt. J. E. Gardner (personal communication, 2000) has found values of 100±300 for D Cl for such systems. Unfortunately, there are no experiments under pertinent conditions to test this hypothesis. Fourth, latestage formation of a chlorine-rich mineral phase, for example apatite, could account for elevated bulk partitioning out of the melt. However, considering apatite comprises only 1 wt % of the lava, it would require 1515

14 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 8 AUGUST 2003 both an unfeasibly high D Cl for apatite of 4150 and an unachievable 47 wt % chlorine concentration in the apatite. A final possibility is that enhanced water circulation results in additional chlorine degassing from the melt without requiring significantly elevated D Cl. Assuming D Cl ˆ 2, a mass of water equivalent to 90% of the melt mass would be sufficient to reduce the melt Cl from 03 to 005 wt %. In fact, many samples with low melt Cl are highly microcrystalline with effective bulk k [equation (3)] of up to 20 as discussed above. Degassing with k of 20 would result in 07 wt % Cl at 05 wt % H 2 O with open-system degassing (Fig. 4c). We have assessed the effects of flushing of meteoric water by a finite-difference calculation in which a small amount of water is equilibrated with a unit mass of melt and then outgassed and replaced by another small amount of water. This is repeated until the chlorine content has reduced to the observed values of the most chlorinepoor glasses. Assuming that k ˆ 20 and D Cl ˆ 2, reduction to 005 wt % Cl requires flushing by 130% of the melt mass of water. This is equivalent to 13% of the lava mass for a sample with 10% final melt, or 300 kg water/m 3 magma. Water rising from magma at a lower level in the conduit is unlikely to provide this flux, as such water should already be chlorine charged. It is also unlikely that such a large water flux could be accounted for by an excess vapour phase sourced from the magma chamber. However, groundwater and hydrothermal circulation could account for such volumes. Assuming an average rainfall of 2 m/year and that magma extrusion over the eruption has averaged m 3 /year, an area of 10 km 2 would receive sufficient rainwater to leach chlorine to this extent from all the magma erupted. Samples have variable melt Cl, therefore we propose that leaching of chlorine by escaping steam is heterogeneous. Vapour passes preferentially through permeable areas of the dome, such as cracks, as evidenced by localized areas of steam emission from the dome. The continuation of steam venting, at times very vigorous, during the period of no dome extrusion (Norton et al., 2002) is consistent with a model of circulation of surface waters through the dome. Another possibility is that interaction of water with the glass and removal of Cl occur in the pyroclastic flow deposits by infiltration of rain and circulation of surface waters. However, our samples were collected within a few days of emplacement, limiting the time available for interaction. This model has implications for the cooling and crystallization history of the lava dome. Hydrothermal circulation would lead to localized cooling, although this has to be limited to avoid hydration of glass. Some cooling-driven crystallization may thus be important in some parts of the dome. Below, our discussion of the hydrogen isotope data provides independent support for involvement of external water. Water degassing Matrix glass water contents are low (51 wt %) compared with water contents of plagioclase melt inclusions (43 05 wt % by FTIR, Barclay et al., 1998), which are inferred to represent pre-eruption melt concentrations. This indicates that water degasses efficiently during eruption at Soufriere Hills Volcano. Matrix glass water contents can be interpreted as minimum pressures at which the melt is water saturated. The solubility model of Moore et al. (1998) is used for the Soufriere Hills melt compositions at an estimated magma temperature of 860 C (Murphy et al., 2000). The most water-rich matrix glass is that of the dense glassy ballistic sample MVO57, erupted during the 17 September 1996 sub-plinian explosive eruption, and interpreted as part of the conduit wall broken off from a relatively deep level (Robertson et al., 1998). The glass water content of 16 wt % H 2 O corresponds to a minimum pressure of 20 MPa. Assuming conduit pressure equals magmastatic pressure, and an average magma density of 2400 kg/m 3, this pressure equates to a depth of 830 m. However, several lines of evidence suggest that overpressures of several MPa develop in the conduit at Soufriere Hills Volcano, and that this therefore represents a maximum depth estimate. Ground deformation during 1996 has been modelled in terms of a pressure source at 700 m depth with an overpressure of 10 MPa and total pressure (overpressure plus magmastatic) of 25 MPa (Shepherd et al., 1998). Tilt-meter cycles have been interpreted in terms of a pressure source at 400 m depth (Voight et al., 1999). Melnik & Sparks (1999) developed dynamic flow models of lava dome extrusion, which suggest that overpressures of up to 4±8 MPa develop at depths of a few hundred metres in the conduit. A pressure of 20 MPa from the water content of a small ballistic can be compared with the peak explosion pressure of 275 MPa estimated for this eruption from the largest ballistics (Robertson et al., 1998). Minimum pressures of 11±37 MPa are estimated for pumice clasts based on matrix glass water contents of 03±06 wt % (Table 2). If the pumices were quenched on fragmentation, this represents lower fragmentation pressures compared with the rapid decompression experiments of Aldibirov & Dingwell (1996), in which a pressure drop of 12 MPa resulted in spontaneous fragmentation of Mount St. Helens dacite. If pumice expansion continues during ejection then 11± 37 MPa represents minimum fragmentation pressures (Gardner et al., 1996). Water contents of glasses from lava dome samples of 014±10 wt % can be related to 1516

15 HARFORD et al. MATRIX GLASS COMPOSITIONS, SOUFRIEÁ RE HILLS equilibrium minimum pressures of 03±9 MPa. This pressure range is consistent with that which might be expected within a thick lava dome with deep regions of overpressure, which is suddenly sampled by a large dome collapse. The Soufriere Hills dome has reached heights of up to 300 m during its growth (Watts et al., 2002). Overburden pressures of up to 7 MPa can be expected at the base of the dome. Overpressures above magmastatic pressure can be generated during extrusion (Melnik & Sparks, 1999), so that pressure estimates using residual water contents of glasses cannot be simply used to infer depths. Hydrogen isotope compositions of matrix glasses are controlled by the original isotopic composition, whether degassing occurs as an open or a closed system, whether kinetic effects are important, and whether isotopic exchange occurs with external waters. Taylor et al. (1983) first related hydrogen isotope compositions of lava to degassing processes, modelling degassing in terms of open- and closed-system end-members. Standard expressions (e.g. Holloway & Blank, 1994) equivalent to those used to model chlorine degassing above are used to calculate the effect of degassing on the end isotopic composition of melt. Using mass balance considerations, closed-system degassing is described using the approximation dd f dd i 1 F 10 3 ln a v m 5 where dd f and dd i are the final and initial hydrogen isotopic compositions in the melt, F is the mole fraction (normalized) of the volatile species remaining dissolved in the melt (unity at the beginning, zero at the end), and a v±m is the bulk vapour±melt fractionation factor. Open-system degassing (Rayleigh fractionation), is modelled by an expression derived from equation (2) above: dd f dd i 10 3 F a v m 1 1 : 6 Water dissolves in silicate melts as both water molecules and hydroxyl groups. The hydrogen isotope fractionation between these two species and water vapour is different, and means that the bulk fractionation factor for the melt, a v±m, varies with water content (Newman et al., 1988; Dobson et al., 1989). The bulk fractionation factor between water vapour and melt can be expressed as (Taylor, 1986) a v mˆx H2 O m a v H2 Om 1 X OH m a v OHm where X H2 O m and 1 X OHm represent the mole fractions of H 2 O as molecular H 2 O and OH in the melt. Recent work shows that water speciation is also strongly dependent on melt temperature (e.g. Kohn, 2000). a v OHm was taken as 1040, from the experiments of Dobson et al. (1989) at 750±850 C. a v H2 O m was calculated using this value for a v OHm, and a value of bulk a v±m of [Taylor (1986) at 950 C with 31 wt % H 2 O]. Speciation of water in the melt for this bulk a v±m value was estimated from Nowak & Behrens (1995) for 414 wt % H 2 O at 850 C, adjusted to 31 wt % H 2 O based on the speciation model in glass of Silver et al. (1990). a v H2 O m was thus estimated as For degassing models, a v±m was estimated for each water content, using the same procedure, and varies from 1020 to 1037 at water content from 4 to 01 wt %, respectively. The parent magma chamber melt before degassing is assumed to have a water content of 43 wt % based on the water content of melt inclusions in plagioclase (Barclay et al., 1998). Using a hbl±v (hornblende±vapour) of 0983 at 750 C from Suzuoki & Epstein (1976), and a v±m at melt water content of 4 wt % estimated above as 1020, a hbl±m is estimated to be Assuming hydrogen isotope equilibrium between the melt and hornblende phenocrysts, the starting melt is estimated to have a dd composition of 55 10%. The Soufriere Hills matrix glasses do not show a systematic H 2 O±dD relationship (Fig. 2). There is a broad similarity between these results and those for similar eruptions of crystal-rich andesites and dacites. Matrix glass dd results from the 1991 eruption of Unzen Volcano, Japan, also show a non-systematic H 2 O±dD relationship, interpreted as a result of latestage kinetic degassing (Kusakabe et al., 1999). Varying combinations of closed- and open-system and kinetic degassing, linked to repose period before eruption, were invoked to account for complex H 2 O±dD relationships of Mount St. Helens matrix glasses (Anderson & Fink, 1989). The Soufriere Hills data are also consistent with degassing by varying combinations of open-system, closed-system and kinetic degassing. This result is not surprising, as a very wide range of isotopic values can be generated by these processes; open- and closed-system degassing result in D depletion, whereas kinetic degassing results in D enrichment through faster diffusion of hydrogen- than deuteriumbearing water. However, most of the samples lie above the model dd±h 2 O curves, even considering the uncertainty in initial dd, suggesting that a D-enrichment process occurs. Kinetic degassing is one possible explanation. However, interaction with surface waters could also result in D enrichment. Meteoric waters in the region average at around 10% (International Atomic Energy Agency, 1992) and seawater has a dd value close to 0%. Chiodini et al. (1996) measured dd of fumarolic condensates and spring waters on Montserrat, and found values varying between 13 and 22%. Circulation of ground water through the dome and upper conduit has already been proposed to 1517