Journal of Volcanologyand Geothermal Research 108 2001) 85±106 www.elsevier.com/locate/jvolgeores

Volcanic SO2 emissions and the abundance and distribution of exsolved gas in magma bodies

P.J. Wallace*

Ocean Drilling Program, Texas A & M University, College Station, TX 77845, USA

Abstract

The mass of sulfur dioxide SO2) released bymost explosive volcanic eruptions greatlyexceeds the amount originally dissolved in the erupted volume of silicate melt. Several lines of evidence suggest that this discrepancyis due to the presence of

SO2-bearing gas bubbles in the magma before the time of eruption. Comparison of remote sensing data for SO2 emissions with conventional petrologic estimates suggests that the discrepancycan be resolved if andesitic and dacitic magmas contain about 1±6 wt% exsolved gas, equivalent to 5±30 vol%, prior to eruption. Such large mass fractions of exsolved gas in pre-eruptive magma are consistent with previously published physical models in which crystallization-induced gas exsolution gradually increases overpressuring of a magma reservoir, eventuallytriggering an eruption. Simple mass balance models for SO 2 emissions from magma bodies in which there is an upwards increasing gradient in exsolved gas mass fraction i.e. gas-rich at the top) yield SO2 vs. eruptive volume trends that are similar to those observed for eruptions ranging in size from 0.01 to 10 km3 of magma. Despite the manyuncertainties involved, these patterns are consistent with the hypothesisthat subvolcanic magma reservoirs are separated from volcanic vents bycupolas of gas-rich magma that supplymuch of the gas released in explosive eruptions. The volume fraction of exsolved gas inferred from SO2 data for the upper regions of manypre-eruptive magma bodies ,30 vol%) is similar to the percolation threshold at which gas bubbles become suf®cientlyinterconnected to allow permeable gas ¯ow through a bubble network. Thus this value mayre¯ect a physicallimitation on the maximum exsolved gas volume fraction that can occur at the roof zone of a magma reservoir because anyadditional gas would be lost byadvective ¯ow through the permeable bubble network. Andesitic and dacitic magma in crustal reservoirs are probablygas saturated due to recharge and underplating bybasaltic magma that is saturated with H 2O±CO2±S gas. Comparison of repose times, eruptive volumes, and basaltic magma supplyrates for a spectrum of volcanic systemssuggests a relativelysteadystate ¯ux of S from Earth's mantle to atmosphere through manyandesitic and dacitic magma systems. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Sulfur dioxide emission; Exsolved gas; Magma bodies

1. Introduction H2SO4 aerosols that can affect both climate and atmo- spheric chemistryLamb, 1970; Hammer et al., 1980;

The volatile constituents H2O, CO2, and S playan Self et al., 1981; McCormick et al., 1995). Numerous important role in the evolution and eruption of measurements of SO2 ¯uxes from active volcanoes magmas. In addition, SO2 injected into the strato- have been made using remote sensing techniques sphere byexplosive eruptions is converted into see review bySymondset al., 1994), including both ground and air-based use of the ultraviolet * Tel.: 11-979-845-0879; fax: 11-979-845-0876. correlation spectrometer COSPEC) and the satellite- E-mail address: [email protected] P.J. Wallace). based Total Ozone Mapping Spectrometer TOMS).

0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0377-027300)00279-1 86 P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106

Comparison of such SO2 emission data with petrolo- eruptions on Earth's climate and atmosphere. In addi- gic studies of dissolved S in magmas based on tion, exsolved gas in crystallizing magma bodies can analyses of quenched melt inclusions trapped inside trigger volcanic eruptions Blake, 1984; Tait et al., phenocrysts has led to a conundrum, known as the 1989; Woods and Pyle, 1997) and contributes to `excess' S problem, concerning S mass balance during some magmatic-hydrothermal ore deposits Burnham, volcanic eruptions Stoiber and Jepson, 1973; Rose et 1979a; Hedenquist and Lowenstern, 1994). The al., 1982; Andres et al., 1991). Concentrations of purpose of this paper is to quantifythe abundance dissolved S in magmas before eruption are commonly and distribution of pre-eruptive exsolved gas in a far too low by1 to 2 orders of magnitude) to account number of magmatic systems from which there have for the total mass of SO2 released during the eruption been recent, relativelywell-studied eruptions. In the as measured byremote sensing techniques Fig. 1). following sections, I begin byreviewing petrologic Excess eruptive S is observed in most eruptions for evidence for pre-eruptive gas saturation in magmas which remote sensing and melt inclusion data are and methods for constraining the concentration of S available, and in particular, is characteristic of explo- in the exsolved gas phase. sive eruptions of intermediate and silicic magma in subduction zone settings Andres et al., 1991). In contrast, basaltic eruptions from divergent plate 2. Pre-eruptive gas saturation and volatile boundaries and hot spots, such as Hawaiian and gradients in magma bodies Icelandic volcanoes, often do not show evidence of excess S emissions Fig. 1). Water, CO2, S-species, and Cl are the most abun- Most SO2 emission estimates that are based on melt dant volatile constituents present in magmas. These inclusions in volcanic phenocrysts assume that the volatiles maybe present both as dissolved species in onlyS released during an eruption is S that was origin- silicate melt and exsolved in a separate gas phase. If allydissolved in the silicate liquid melt) portion of the sum of partial pressures of the dissolved volatiles the magma. However, there is a growing bodyof in a silicate melt is equal to the local con®ning pres- evidence, based on petrologic, remote sensing, and sure, then a separate multicomponent gas phase will volcanic gas data, that a large proportion of the SO2 be in equilibrium with the melt Verhoogen, 1949), released during explosive eruptions mayactuallybe presumablyin the form of bubbles. If a magma is gas- contained in a gas1 phase before eruption Anderson, undersaturated, then volatiles will be present in solu- 1975; Luhr et al., 1984; Sigurdsson et al., 1990; tion only, and their concentrations will be controlled Andres et al., 1991; Gerlach et al., 1994, 1996; bycrystal±liquid differentiation processes. Although Wallace and Gerlach, 1994; Gerlach and McGee, gas can be exsolved byisobaric crystallization 1994; Giggenbach, 1996). As a result, eruptions of second or resurgent boiling), gas mayalso be redis- exsolved-gas-rich silicic magmas can release large tributed within magma bodies due to its greater buoy- amounts of SO2 derived from pre-eruptive exsolved ancyrelative to surrounding silicate melt and crystals gas, despite the fact that such magmas generallyhave e.g. Candela, 1991), or to the dynamics of magma verylow concentrations of dissolved S. Techniques transport e.g. Wilson, 1998). Thus for a gas saturated for quantifying the mass fraction of exsolved gas in magma, the mass fraction of exsolved gas is not magma before eruption are thus important for under- strictlylimited bypressure or solubilitysuch that a standing SO2 ¯ux data used for forecasting eruptions gas-rich magma could have a bulk volatile content and for assessing the potential impacts of volcanic that is greatlyin excess of what could be dissolved in the melt at upper crustal pressures #3 kbar).

Dissolved H2O concentrations for manysilicic 1 In this paper, I use the word `gas' as de®ned byVerhoogen magma bodies have been estimated using thermo- 1949), to describe the lowest densityphase that is in equilibrium dynamic calculations for biotite±sanidine±magnetite with silicate melt 1 crystals. This serves to distinguish the gas equilibrium e.g. Hildreth, 1977; Whitneyand Stor- phase from an immiscible hydrous saline melt ¯uid) that may coexist with silicate melt 1 crystals 1 gas under certain conditions mer, 1983; Luhr et al., 1984). In the case of the Bishop of temperature, pressure, and bulk composition Roedder, 1992). Tuff, this method has also been used to infer a gradient P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 87

12 Fig. 1. Volcanic SO2 emissions, in megatons 1Mtˆ 10 g†; vs. total volume of erupted magma. Data are taken from Gerlach and Graeber, 1985), Andres et al. 1991) and references therein), Bluth et al. 1993), Allard et al. 1994), Gerlach et al. 1994), Gerlach and McGee 1994),

Self and King 1996), Roggensack et al. 1996), and Thordarson et al. 1996). SO2 emissions were measured using remote sensing methods except for the eruption of the Bishop Tuff BT; see text). Most of the remote sensing data are from the TOMS, except for the data for Fuego determined byCOSPEC), Agung estimated from stratospheric optical depth measurements), and Laki estimated from information on

atmospheric turbidity). Uncertainties of ^50% for both SO2 emission and eruptive volume are less than to slightlygreater than the size of the symbols. Uncertainties in SO2 emission data are generallyconsidered to be about ^30% for the TOMS data and ^20±50% for COSPEC see discussion in Symonds et al., 1994). Data are shown for the following eruptions: A) Agung, 1963; EC) El Chichon, 1982; Et) Etna, 1975; F) Fuego, 1974; L) LaÂscar, 1989; Lk) Laki, 1783±1784; Lq) Lonquimay, 1989; ML) Mauna Loa, 1984; M) Mount St. Helens, 1980; Pc) Pacaya, 1972; P) Pinatubo, 1991; Rb) Rabaul, 1994; R) Redoubt, 1989±90; Rz) Ruiz, 1985; S) Stromboli, annual average. Data for silicic bulk compositions dacitic to rhyolitic)are shown bysquares, intermediate-compositions andesitic) by®lled circles, and basaltic composition s

bytriangles. Shown for comparison are predicted relationships between SO 2 emission and eruptive volume for rhyolitic dashed line), andesitic solid line), and basaltic short dashed line) melts calculated byassuming that the onlySO 2 released during the eruption is from S that is originallydissolved in silicate melt. Note that the SO 2 emissions for all eruptions, with the exception of Mauna Loa and Laki, are at least one order of magnitude greater than predicted for the appropriate bulk composition by syneruptive degassing of dissolved S only. This clear discrepancyover manyorders of magnitude in eruptive volume indicates that a major additional source of S must be involved in all of these eruptions.

in dissolved H2O, with concentrations increasing Newman et al., 1988; Westrich et al., 1988; Martel upwards in the magma bodyHildreth, 1977). Gradi- et al., 1998). That such magmas are erupted non- ents in dissolved H2O in silicic magma bodies have explosivelyrequires that water be lost prior to extru- more recentlybeen con®rmed using ion probe and sion, which mayoccur if magma decompression infrared spectroscopic techniques on melt inclusions induces suf®cient vesicularityto form a magmatic in phenocrysts Anderson et al., 1989; Skirius, 1990; foam that is highlypermeable to gas Tayloret al., Westrich et al., 1991; Lowenstern, 1993; Wallace et 1983; Eichelberger et al., 1986, 1989, 1995; Westrich al., 1995). However, changes in eruptive style from et al., 1988; Jaupart and Allegre, 1991; Westrich and Plinian column to pyroclastic ¯ow, sometimes Eichelberger, 1994; Woods and Koyaguchi, 1994; inferred to result from decreases in volatiles, do not Sparks, 1997). appear to be related to differences in pre-eruptive It has commonlybeen assumed that magmas of inter- dissolved water Westrich et al., 1988; Skirius, mediate to silicic composition onlybecome gas satu- 1990; Lowenstern, 1993; Wallace et al., 1995). Even rated during shallow ascent and emplacement, during effusive non-explosive) eruptions of silicic magma eruptive decompression, or during advanced pegmati- appear to involve magma with initiallyhigh dissolved tic) stages of plutonic consolidation. Kennedy1955),

H2O concentrations Eichelberger et al., 1986; drawing on the earlier thermodynamic analysis of 88 P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 Verhoogen 1949), argued that saturation with respect to Luhr et al., 1984; Luhr, 1990; Wallace and Gerlach, water was onlylikelyto occur at the top of magma 1994; Gerlach et al., 1996). chambers due to the variations in P Oimposedby Thus it appears that volcanic eruptions release H2 hydrostatic equilibrium in a magma body with signi®- abundant `excess' SO2 and that subvolcanic magma cant vertical extent. However, Anderson 1975) bodies contain signi®cant exsolved gas. Two impor- reviewed evidence that some andesitic magmas are tant possibilities therefore arise: 1) to forecast the gas saturated during their formation from more ma®c amount of SO2 release for certain volcanic eruptions; magma and suggested, based on available CO2 and SO2 and 2) to con®rm and estimate the amount of emission data, that magmas in subvolcanic reservoirs exsolved gas in subvolcanic magmas. Critical to are generallysaturated with a multicomponent gas doing this, as discussed below, is an assessment of phase. Holloway1976) recognized that the small but the total mass fraction of S-species that is present in

®nite solubilityof CO 2 in silicic magmas at crustal pres- the pre-eruptive gas phase. sures could result in saturation with an H2O±CO2 gas phase during all stages in the magmatic evolution of granites, including partial melting, ascent and emplace- 3. S abundance and speciation in pre-eruptive gas ment, and crystallization.

Signi®cant CO2 in rhyolitic melts has been detected The solubilitybehavior and activity-composition using infrared spectroscopic techniques on obsidians relations of S in silicate melts are complex due to

Newman et al., 1988) and on melt inclusions in multiple valence states H2S, S2,SO2,SO3) and the phenocrysts Anderson et al., 1989; Skirius, 1990; occurrence of non-volatile S-rich phases e.g. immis- Lowenstern, 1994; Wallace and Gerlach, 1994; cible Fe±S±O liquid, pyrrhotite, anhydrite). Two Wallace et al., 1995). Negative correlations between basic approaches can be used to constrain the mass

CO2 and incompatible trace elements in the Bishop fraction of S that is present in a pre-eruptive exsolved Tuff suggest that the magma was gas saturated during gas phase. Despite uncertainties inherent in each crystallization, with CO2 partitioning into a coexisting approach, the two yield a fair degree of consistency, gas phase Wallace et al., 1995). Pre-eruptive gas making it possible to place limits on the S content of saturation of the dacitic magma erupted at Mount pre-eruptive magmatic gas. Pinatubo in 1991 has also been demonstrated based The ®rst approach involves phase equilibria and on melt inclusion and experimental data Wallace and thermodynamic modeling to estimate fugacities of

Gerlach, 1994; Gerlach et al., 1996; Rutherford and SO2,SO3,H2S, and S2. Bycombining analysesof Devine, 1996). The occurrence of high-temperature pyrrhotite, the relationship between pyrrhotite compo-

¯uid inclusions in phenocrysts from some silicic sition and fS2 Toulmin and Barton, 1964), and inde- volcanic rocks provides further evidence of gas pendent estimates of T±f ±f from mineral O2 H2O saturation during pre-eruptive crystallization Roed- equilibria, it is possible to calculate the fugacities of der, 1992; Lowenstern, 1995). Quantitative modeling all major S-species Whitneyand Stormer, 1983; for the magma bodyof the Bishop Tuff using H 2O± Whitney, 1984; Luhr et al., 1984; Luhr, 1990). If a CO2 solubilityrelations reveals a pre-eruptive gradi- gas phase is present, then the calculated fugacities can ent in exsolved gas, with gas contents varying from be converted into mole fractions of the various ,1 wt% in the deeper regions of the magma bodyto S-species using fugacitycoef®cients calculated with nearly6 wt% near the top Wallace et al., 1995, 1999). the Redlich±Kwong equation of state Holloway, Several wt% of exsolved gas has also been inferred for 1987). Application of this method to the 1980 silicic magma at Mono Craters, California, based on Mount St. Helens and 1982 El ChichoÂn eruptions

H2O and CO2 data for rhyolitic obsidians Newman et yields estimates of 1.9 and 1.2±6.0 mol% Stotal al., 1988). Similar amounts of exsolved gas are esti- H2S 1 SO2), respectively, in the pre-eruptive gas mated for the magma bodies of the 1982 El ChichoÂn phase Whitney, 1984; Luhr, 1990). The main disad- and 1991 Mount Pinatubo eruptions byassuming that vantage with the thermodynamic approach is that all SO2 released measured byremote sensing techni- combined uncertainties in the various parameters ques) was stored in the erupted volume of magma can easilyyielda one order of magnitude uncertainty P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 89 in the mass fraction of total S in the coexisting equili- possible to estimate the original S content of the brium gas. In addition, manysilicic rocks contain magmatic gas e.g. Giggenbach, 1975, 1996; Gerlach, pyrrhotite only as inclusions in phenocrysts Whitney 1979). High-temperature gas discharges from conver- and Stormer, 1983), which raises a question as to gent margin andesitic to dacitic volcanoes have mol% whether it was a stable phase at the time of eruption. Stotal [ ˆ 100 SO2 1 H2S)/H2O 1 CO2 1 SO2 1 A variant on the thermodynamic approach is to H2S)] that varies over two orders of magnitude from analyze trapped melt glass) inclusions in phenocrysts 0.04 to 7.3 Symonds et al., 1994; Giggenbach, 1996). for concentrations of dissolved H2O, CO2, and S, and The lowest values of Stotal tend to be from samples use experimental solubilitystudies and derived ther- collected from degassed lava domes Symonds et modynamic models e.g. Burnham, 1979b; Silver et al., 1994). The remaining values are mostlybetween al., 1990; Blank et al., 1993) to estimate fugacities for 0.3 and 7 mol% Stotal, corresponding to CO2/Stotal of each component. Although this method works well for 0.2±10. These values are in good agreement with the calculating H2O and CO2 contents of exsolved estimates from Mount St. Helens and El ChichoÂn magmatic gas Newman et al., 1988; Skirius, 1990; based on thermodynamic methods and the estimates Wallace and Gerlach, 1994; Gerlach et al., 1996), it is for Mount Pinatubo based on melt inclusion volatile dif®cult to applyto S because of lack of information concentrations and experimental partitioning studies. concerning activity-composition relations of dissolved The range of values from 0.3 to 7 mol% for Stotal are 22 SO4 in silicate melts. Estimates of S concentration in also in good agreement with recent experimental pre-eruptive gas in the 1991 Mount Pinatubo dacitic determinations of S partitioning between melt and magma, based on this method and the experimental coexisting vapor at relevant oxygen fugacities data of Baker and Rutherford 1992), suggest some- Keppler, 1999). Because of the general consistency where between 0.1 and 4.0 mol% Stotal see discussions of Stotal estimates from thermodynamic calculations, in Gerlach et al., 1996, and Baker and Rutherford, solubilitymeasurements, experimental partitioning 1996a). The high end of this range of values agrees studies, and volcanic gas discharges, the range well with recent experimental measurements of S parti- from 0.3 to 7 mol% Stotal will be used as upper tioning between rhyolitic melt and vapor at oxygen and lower limits in calculations throughout the fugacities comparable to that of the Pinatubo dacitic rest of this paper. magma Scaillet et al., 1998). It is important to note that the ratio of SO2 to H2Sin The second approach for estimating S concentra- exsolved magmatic gas is dependent on temperature, tions in pre-eruptive exsolved gas in magmas is pressure, and the fugacities of oxygen and H2O. Over through analysis of volcanic gases. Two major disad- the range of T, P, f , and f O conditions of most O2 H2 vantages of this approach should be kept in mind in intermediate to silicic magmas Carmichael, 1991), the following discussion. First, volcanic gases gener- S in pre-eruptive gas will either be predominantly allyare sampled from fumarolic non-eruptive) states H2S or will occur as subequal amounts of H2S and of activity. As such, they probably represent degas- SO2 Carroll and Rutherford, 1985; Luhr, 1990). sing at much lower pressure than the deeper, pre- The reduced S in the gas phase probablybecomes eruptive stage relevant to large explosive eruptions. oxidized to SO2 during eruptive decompression and Furthermore, sampled volcanic gases maycome from subsequent atmospheric transport Varekamp et al., shallowlystored magma that has alreadypartially 1984; Gerlach and Casadevall, 1986; Bluth et al., degassed during earlier eruptive activity. Neverthe- 1995). In more oxidized magmas, such as the 1991 less, volcanic gases represent our onlyreal sample Mount Pinatubo dacite, most of the S in the pre- of gases evolved from decompressing and crystalliz- eruptive gas phase occurs as SO2 Westrich and ing magma at depth. A second general problem is that Gerlach, 1992; Rutherford and Devine, 1996). much of the variation in H2O, CO2 and S concentra- Throughout the rest of this paper, for reasons of tions in volcanic gases can be caused bysecondary, simplicity, the concentration of Stotal in pre-eruptive nonmagmatic processes Symonds et al., 1994; exsolved gas will be recalculated to equivalent wt%

Giggenbach, 1996). However, techniques exist for amounts of SO2. Thus the range of values discussed evaluating these secondaryprocesses, making it above for pre-eruptive exsolved gas in andesitic and 90 P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 dacitic magmas 0.3±7 mol% Stotal) is equivalent to explain remote sensing data of SO2 emissions is that approximately1±20 wt% SO 2 in the gas phase, unerupted magma at depth degasses during eruption, because most of the gas is low molecular weight H2O. therebycontributing SO 2. These two contrasting hypotheses represent endmembers in a continuum of 3.1. Estimation of SO2 released by the eruption of the possibilities involving variable proportions of pre- Bishop Tuff eruptive exsolved gas and syneruptive loss of gas from unerupted magma. Other potential sources of Using the techniques based on thermodynamic S, such as decomposition of phenocrystic anhydrite analysis and phase equilibria, it is possible to estimate in an eruptive plume Devine et al., 1984) or vapor- the total amount of S that would be present in pre- ization of S-rich hydrothermal brines, are unlikely to eruptive exsolved gas in a magma bodygiven an esti- be signi®cant Gerlach et al., 1996). That gas is indeed mate of the exsolved gas mass fraction. For the large lost from unerupted magma during shallow emplace- rhyolitic magma body that erupted at 0.76 Ma to form ment and crystallization associated with activity the Bishop Tuff, the exsolved gas content vs. depth before and after large explosive eruptions, and during pro®le from Wallace et al. 1995) can be used to dome extrusion i.e. open-vent systems), is unques- predict the total amount of SO that would have 2 tionable. At issue are the relative contributions of been released during the eruption. This can be done pre-eruptive exsolved gas and syneruptive loss of byusing the exsolved gas gradient together with erup- gas from unerupted magma during sustained Plinian tive volume estimates Hildreth, 1977; Hildreth and and pyroclastic-¯ow-generating eruptions. Given the Mahood, 1986; Wilson and Hildreth, 1997), S-species strong case that has been made for a minimal contri- fugacities calculated from magnetite±pyrrhotite equi- bution from unerupted magma in several recent, well- librium Whitney, 1984), and fugacity coef®cients studied eruptions Mount St. Helens, Gerlach and calculated using the Redlich±Kwong equation of McGee, 1994; Redoubt, Gerlach et al., 1994; Mount state. The S concentration in the exsolved gas phase Pinatubo, Gerlach et al., 1996), the hypothesis that all calculated using this method is ,1.5 mol% S total SO is derived from erupted magma needs to be quan- ,5 wt% SO ), in good agreement with values calcu- 2 2 titativelyinvestigated. Additional detailed considera- lated bythermodynamic modeling of multicomponent tion of unerupted magma degassing will be discussed gas composition constrained bypyrrhotite saturation in a later section. and melt inclusion volatile contents for the Bishop If all of the SO released during an eruption is Tuff B. Scaillet, pers. commun., 1998). This value 2 originallycontained, as both dissolved and exsolved for mol% S is signi®cantlyhigher than previously total species, within the volume of erupted magma, then the estimated byScaillet et al. 1998). The Bishop difference between remote sensing and petrologic eruption is thus estimated to have released estimates can be used to estimate the amount of SO ,1300 ^ 600 Mt of SO , of which ,30% probably 2 2 that is contributed bythe gas phase. The total mass of would have been removed from the plume byadsorp- SO released during an eruption can be expressed as tion onto ash particles Rose, 1977; Varekamp et al., 2 the sum of SO that is contained in pre-eruptive gas 1984; Gerlach and McGee, 1994), and the remaining 2 and SO released during decompression from exsolu- 900 ^ 600 Mt injected into the atmosphere. 2 tion of dissolved S-species in the silicate melt: 64 4. Exsolved gas contents of magmas inferred from SOTotal ˆ SOin gas 1 Sdissolved in melt 2 2 32 SO2 emissions If it is assumed that the remote sensing estimate

One hypothesis to account for the discrepancies represents the total amount of SO2 released during between remote sensing and petrologic eruptive an eruption, then the amount of SO2 that is contained exsolution) estimates of SO2 emissions Fig. 1) is in pre-eruptive gas can be estimated bycombining Total that magmas in crustal storage reservoirs contain the remote sensing data SO2 ) with estimates, signi®cant exsolved gas before eruption. An alterna- based on melt inclusions, of the amount of tive hypothesis that has frequently been invoked to dissolved S that is degassed from the melt during P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 91

Fig. 2. Mass of SO2 estimated to be in pre-eruptive exsolved gas vs. eruptive magma volume in recent eruptions for which both remote sensing and petrological data are available see text for description of methods). Symbols are the same as in Fig. 1. Representative error bar shown at upper right corresponds to an uncertaintyof ^70% relative) based on assumed uncertainties of ^50% for both erupted magma volume and total SO2 emission.

Fig. 3. Mass of SO2 estimated to be in pre-eruptive exsolved gas vs. eruptive magma volume calculated as in Fig. 2. Dashed and solid lines show the amounts of SO2 that would be released per unit volume of erupted magma for magmas with pre-eruptive exsolved gas contents of 1 and 5 wt% concentration of SO2 in gas ˆ 20 wt%) and 5 wt% concentration of SO2 in gas ˆ 1 wt%). These values for total S-species concentra- tions expressed as equivalent SO2) in the exsolved gas phase are chosen to be representative of intermediate to silicic magmas saturated with magnetite 1 pyrrhotite ^ anhydrite Whitney and Stormer, 1983; Whitney, 1984; Luhr, 1990) and are consistent with analyses of high- temperature gas discharges from convergent margin volcanoes Symonds et al., 1994; Giggenbach, 1996). The data for basaltic eruptions

Etna, Stromboli, Pacaya) are not shown because basaltic magmas contain gases that are richer in SO2 than the values adopted here for intermediate to silicic magmas Gerlach, 1979, 1980; Symonds et al., 1994; Giggenbach, 1996), and hence the exsolved gas contents represented bythe horizontal lines are not appropriate. For the two smallest intermediate-composition eruptions LaÂscar and Lonquimay), the estimated mass of gas is roughlyequivalent to the mass of erupted magma if the exsolved gas has an SO 2 concentration of about 10 wt% SO2 short dashed line). 92 P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106

dissolved in melt eruption S ). Normalizing the mass of SO2 contained about 5±20 vol% exsolved gas, equivalent in pre-eruptive gas to the volume of erupted magma to about 1±5 wt% gas Wallace and Gerlach, 1994; 3 Mt SO2 in gas/km magma) provides a useful indica- Gerlach et al., 1996; Rutherford and Devine, 1996). tor of the mass of SO2 contributed from pre-eruptive The estimates for El ChichoÂn and Mount Pinatubo gas per unit volume of erupted magma Fig. 2). agree well with the values depicted in Fig. 3. It is Several important features are suggested bythe interesting to note that although the 1982 El ChichoÂn data shown in Fig. 2. First, smaller eruptions appear and 1991 Mount Pinatubo eruptions have received to have a greater mass of SO2 per unit volume of considerable attention as being S-rich, the systematics magma contributed from a pre-eruptive gas phase displayed in Fig. 2 show that these eruptions are not than larger eruptions. Second, when eruptions of a anomalous compared with other andesitic and daci- given volume are compared, andesitic magmas have tic±rhyolitic eruptions, respectively. a greater amount of SO2 contributed from pre-eruptive The data and analysis shown in Fig. 3 suggest that gas than dacitic and rhyolitic magmas. Third, persis- intermediate to silicic magmas in eruptions of tentlyactive open-vent basaltic volcanoes Etna and 0.01±10 km3 of magma appear to contain about Stromboli) appear to have extremelylarge masses of 1±6 wt% exsolved gas. Within this range of eruptive

SO2 contributed from pre-eruptive gas. Such large volume, smaller volume eruptions contain either SO2 emissions with little or no erupted material systematically higher gas contents, higher proportions from open-vent systems of relatively low viscosity of total S in the gas phase, or both. Differences in basaltic magma have led to so-called `endogeneous magma composition and temperature, both of which growth' models. In these systems, shallow-stored can stronglyaffect the solubilityof dissolved S magma probablydegasses without eruption, sinks to Buchanan et al., 1983; Carroll and Rutherford, deeper levels of the magmatic system, and eventually 1987; Luhr, 1990; Wallace and Carmichael, 1992; solidi®es to form a high densityroot beneath the Baker and Rutherford, 1996a), are explicitly volcano Francis et al., 1993; Kazahaya et al., 1994; accounted for in this calculation, so the patterns Allard, 1997). Finally, it should be noted that data for shown in Fig. 3 cannot be due to correlations between Mauna Loa and Laki are not shown in Fig. 2. For these erupted volume and magma composition or tempera- volcanoes, relations between measured SO2 emissions ture. The correlations shown in Fig. 3 are statistically and total erupted magma indicate that all SO2 could be signi®cant. For the four silicic eruptions, the correla- produced from exsolution of dissolved S Fig. 1; see tion coef®cient r) for the log transformed data is 0.98; also Andres et al., 1991, and Thordarson et al., 1996). for the seven intermediate-composition eruptions If the total concentration of S-species in a pre- .0.01 km3, r is 0.87. Estimated exsolved gas contents eruptive gas phase is known or can reasonablybe for eruptions in the range from 0.01 to 0.1 km3 are as estimated, as discussed in the previous section, then high as 5±6 wt%. For the two smallest intermediate- the values shown on the vertical axis in Fig. 2 can be composition eruptions LaÂscar and Lonquimay), the converted into estimated mass fractions of exsolved estimated mass fraction of pre-eruptive gas is much gas in a magma before eruption. The amounts of SO2 greater, such that the mass of gas is roughlyequivalent that would be released per unit volume of erupted to the mass of erupted magma Fig. 3). magma for various total exsolved gas contents 1 and 5 wt% gas) and exsolved gas S concentrations

1 and 20 wt% SO2) are shown in Fig. 3. Two of the 5. Contributions of SO2 from syneruptive eruptions shown in Fig. 3 El ChichoÂn, 1982, and degassing of unerupted magma Mount Pinatubo, 1991) have been the focus of detailed analytical and experimental studies in An alternative hypothesis to the large mass frac- which pre-eruptive exsolved gas mass fractions have tions of pre-eruptive exsolved gas inferred above is been estimated. The El ChichoÂn magma body that during explosive eruptions, unerupted magma at evidentlycontained ,20 vol% exsolved gas before depth degasses and contributes SO2. Unfortunately, in the eruption, equivalent to ,5 wt% gas Luhr et al., most cases where syneruptive degassing of unerupted 1984; Luhr, 1990). The Mount Pinatubo magma body magma has been invoked, there is little detailed P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 93 consideration of mechanisms, mass balance, or the are too rapid for excess gas accumulation to occur by time and length scales over which such a process bubble rise relative to magma in the conduit Wilson, could occur. Recently, however, the idea has been 1980). Convective processes maybe important during quantitativelytested using data for several well- repose period degassing in open conduits e.g. Kaza- studied eruptions. For the 1989±1990 Redoubt and haya et al., 1994), but they are unlikely to be impor- 1991 Pinatubo eruptions, seismic data have been tant on the timescale of an explosive eruption, nor is it used to interpret the volume of the underlying clear how such a process would operate during the magma reservoir Power et al., 1994; Mori et al., high magma conduit ¯uxes characteristic of such 1996). In both cases, the total volumes of unerupted eruptions Wilson et al., 1980). magma that are likelyto be present at depth are The most obvious mechanism bywhich large quan- grosslyinadequate to account for the measured SO 2 tities of S could escape rapidlyfrom a deep 5±10 km) emissions from major eruptive phases solelybyexso- bodyof unerupted magma would be for decompres- lution of dissolved S in unerupted melt Gerlach et al., sion to cause suf®cient exsolution of volatiles so that 1994, 1996). Similar relationships have been noted for gas bubbles form an interconnected network, resulting the 1980 eruption of Mount St. Helens Gerlach and in a magmatic froth de®ned as #90 vol% gas McGee, 1994). It thus appears that for some explosive bubbles; Candela, 1991) with substantial gas perme- eruptions, there is an insuf®cient volume of magma in ability. The critical porosity at which vesicles become the underlying reservoir to provide all of the released suf®cientlyinterconnected to allow loss of gas prob-

SO2 purelybysyneruptive degassing of dissolved S ablylies in the range from 30 to 60 vol% Eichelber- from unerupted magma. ger et al., 1986; Candela, 1991; Klug and Cashman, An additional problem with the hypothesis that 1996). For a magma with an initial dissolved water syneruptive degassing of unerupted magma is a signif- content of 6 wt% H2O,2 kb), a highlypermeable icant source for SO2 is the physical mechanism by froth would be formed at pressures #300±700 bars, which such gas loss could occur. Because gas exsolu- equivalent to 60 and 30 vol% porosity, respectively. tion is driven bydecompression during magma ascent Similarly, a magma with an initial dissolved water and eruption, it is dif®cult to envision a process by content of 4 wt% H2O and 800 ppm CO2 ,2 kb) which gas could separate from deeply-stored but uner- would reach the critical porosityat pressures less upted magma on eruptive timescales. In many than 200±400 bars. Furthermore, to remove suf®cient regards, the problem of rapid gas loss from a large dissolved S from the melt would probablyrequire bodyof unerupted magma is similar to the problem pressures much lower than 200±700 bars, given the of how large quantities of H2O can be lost from relativelysmall pressure dependence of S in inter- ascending silicic magmas in effusive non-explosive) mediate and silicic melts Luhr, 1990). Although silicic eruptions Taylor et al., 1983; Eichelberger et wall rocks probablyhave suf®cient strength for a al., 1986; Jaupart and Allegre, 1991; Woods and conduit to remain open with magma pressures less Koyaguchi, 1994; Eichelberger, 1995). Liquid state than lithostatic Carrigan et al., 1992; Sparks et al., diffusion of volatiles dissolved in silicate melt is 1994; Eichelberger, 1995), it is questionable whether manyorders of magnitude too slow for anyappreci- this degree of underpressure could be achieved in a able quantityof either H 2O or S to be transferred from deep magma body, initially at ,2 kb pressure, while unerupted magma at depth on the timescale of an still mantaining an open conduit. In order to maintain explosive eruption Shaw, 1974; Baker and Ruther- an open magma conduit during an eruption, pressures ford, 1996b). Furthermore, for relativelyviscous must remain relativelyhigh in the deep reservoir and intermediate to silicic composition magmas, time- lower parts of the system Blake, 1981; Druitt and scales for buoyant rise of bubbles will be much too Sparks, 1984; Tait et al., 1989). This probably slow compared to eruptive timescales Taylor et al., precludes substantial decompression-driven degas- 1983) to deliver substantial amounts of gas from deep, sing of deeply-stored, unerupted magma on the time- unerupted magma, into the roof zone of the magma scale of an explosive eruption. chamber where magma is entering the conduit. Like- Notwithstanding the problems discussed above, if wise, upward transport rates of magma in the conduit pressure in a deep 5±10 km) magma reservoir can 94 P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106

Fig. 4. Intrusive:extrusive ratio for individual eruptions calculated byassuming that all excess SO 2 released during the eruption is derived from syneruptive degassing of unerupted magma. This diagram represents an endmember model for SO2 emissions that can be compared with the alternative hypothesis, shown in Fig. 2, in which all SO2 is derived from pre-eruptive exsolved gas dispersed throughout the erupted volume of magma. drop low enough for rapid formation of permeable of volatiles are lost from a magmatic system during a magmatic froth on a scale of 10±100 km3 of given eruption e.g. Tait et al., 1989). magma, then the framework presented in Fig. 1 A novel variant of the unerupted magma hypothesis imposes strong constraints on the proportions of uner- was recentlyproposed byKress 1997) for the 1991 upted magma that would have to degas in order to Mount Pinatubo eruption. When S solubilityin sili- account for observed SO2 emissions. For each erup- cate melts is examined as a function of oxygen fuga- tion considered in Fig. 1, an `apparent' intrusive to cityat constant temperature, there is a solubility extrusive magma volume ratio can be estimated by minimum near the nickel±nickel oxide buffer assuming that all SO2 comes from syneruptive degas- NNO) that corresponds to the change in dissolved S sing of dissolved S onlyFig. 4). For the majorityof speciation from dominantlyreduced S to dominantly eruptions considered, the apparent intrusive to extru- oxidized S Katsura and Nagashima, 1974). It is well sive ratios predicted in this wayvaryfrom 60:1 to accepted that intrusion of a new batch of ma®c magma 200:1, whereas for four eruptions Pacaya, Redoubt, into the Mount Pinatubo magma reservoir triggered Fuego, and Agung), the apparent intrusive to extrusive the explosive eruption in 1991 Pallister et al., 1996). ratios are much lower. Thus the unerupted magma Kress 1997) proposed that mixing between reduced hypothesis requires both a very large volume of basaltic magma saturated with immiscible Fe±S±O magma at depth and suf®cient underpressures in the liquid and oxidized dacitic magma saturated with reservoir during the eruption so that this large magma anhydrite would have resulted in a hybrid magma bodyis depleted in most of its H 2O and S and with intermediate oxygen fugacity. The hybrid certainlyall of its CO 2). Depletion of volatiles from magma is postulated to have been at an oxygen fuga- a large bodyof magma during a single eruption bythis citynear the S solubilityminimum, causing most process would result in rapid solidi®cation due to the dissolved S to exsolve into the vapor phase, and strong effect of water loss on the liquidus and solidus resulting in anhydrite breakdown, which would temperatures Eichelberger, 1995). Such total deple- contribute additional S to the vapor phase. However, tion in volatiles is not consistent with the observed the dacitic pumice and ash erupted on 15 June 1991, at repose periods for manyvolcanic systems, which Pinatubo contain no basalt, no basalt commingled can be interpreted to indicate that onlya small amount with dacite, and no intermediate hybrid magma P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 95

Fig. 5. A) Model gradient light solid line) in pre-eruptive exsolved gas content used for calculations shown in Figs. 6 and 7. This gradient was determined bybest ®t to the data in Fig. 2 for eruptions of 0.01±10 km 3 of magma; that is, the gradient was derived so as to yield straight lines in Fig. 7 that would have the same general trend as the data. Note that the absolute values for wt% exsolved gas are dependent on assumed chamber diameter and gas composition, but the relative shape of the model gradient is independent of anyassumed parameters and is determined onlybythe slope of the data in Fig. 2. The heavysolid line shows the gradient deduced for the pre-eruptive Bishop Tuff magma bodyusing melt inclusion data Wallace et al., 1995). B) Schematic showing a magma bodywith an exsolved gas gradient approximatelycorresponding to that shown in A).

Pallister et al., 1996), so the postulated hybrid mum in low temperature, hydrous silicic melts is magma of Kress 1997) was not erupted. Onlythe greatlydiminished relative to high-temperature, anhy- minor volume ,0.1 km3) of hybrid andesitic drous basaltic magmas cf. Fig. 4 of Carroll and magma that erupted in the days prior to the large Rutherford, 1987, and Fig. 3 of Katsura and Naga- explosive eruption contained anyevidence of shima, 1974). Indeed, the relations shown byCarroll magma mixing, and rapidlyquenched scoria of this and Rutherford 1987) suggest that at T , 9008C; mixed material contained euhedral unresorbed) there is little or no minimum in S solubilityacross a anhydrite Pallister et al., 1996). wide range of oxygen fugacities.

The redox hypothesis for the origin of excess SO2 is In summary, the hypothesis that syneruptive problematic because it requires that the intial degassing of unerupted silicic magma supplies the endmembers are on opposite sides of the S solubility `excess' SO2 observed in sustained explosive erup- minimum, which occurs near the NNO buffer. tions appears to be problematic for two main reasons. However, basaltic magmas from convergent plate First, because of the verylarge volumes of unerupted margins are typically more oxidized than the value magma that would have to degas, and second, adopted byKress 1997), and are commonlyin the because of the verylow pressures that would have range from NNO to NNO 1 1 Brandon and Draper, to be sustained in the deep reservoir feeding the erup- 1996; Lange and Carmichael, 1996; Metrich and tion. Furthermore, new intrusion of ma®c magma Clocchiatti, 1996; Luhr, 1997; Wallace and Carmi- into a silicic magma reservoir shortlybefore eruption chael, 1999). Such an oxygen fugacity for basaltic is also problematic as a major source of `excess' SO2 magma involved in mixing would place it on the because the oxygen fugacities typical of ma®c arc same side of the S solubilityminimum as silicic magmas are probablyclose to those of more evolved magma, so mixing would not cause S exsolution. magmas. Despite these reservations, however, the

Furthermore, the magnitude of the S solubilitymini- extent of SO2 contributions from unerupted magma 96 P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106

6. Models of SO2 emissions from magma bodies with exsolved gas gradients

Most volcanic eruptions are characterized byemis-

sions of SO2 that are 10 times or more greater than the amounts that could have been derived from exsolution of dissolved S from the erupted volume of magma. If

minimal SO2 is derived from syneruptive degassing of unerupted magma, then the remote sensing data imply that intermediate to silicic magmas contain about 1±6 wt% exsolved gas Fig. 3). One hypothesis for the trends observed in Fig. 3 is that magmas in crustal storage reservoirs not onlycontain exsolved gas before eruption but that there is a vertical gradient in the mass fraction of gas such that the gas mass fraction is greatest in the roof zone of the reservoir Fig. 5). Evidence for the existence of such a gradient Fig. 6. Fraction of the maximum possible SO emission i.e. if all 2 in a magma reservoir is based on H2O, CO2, and trace magma was erupted) is shown as a function of the fraction of the element concentrations of melt inclusions in quartz total magma volume that is erupted for a range of model eruption volumes created bydifferent depths of drawdown in a cylindrical phenocrysts from the Bishop Tuff Wallace et al., magma body. The shape of the curve for the model gradient is 1995, 1999) interpreted in the context of the revised independent of model parameters and is determined solelybythe eruption chronologyWilson and Hildreth, 1991, shape of the gradient in Fig. 5A). 1997). For a magma reservoir with such an exsolved

gas gradient, the amount of SO2 released per unit still remains the greatest source of uncertaintyin volume of erupted magma varies depending on the using remote sensing SO2 data to estimate the maximum depth of drawdown in the magma reservoir amounts of exsolved gas that are in magma bodies i.e. the depth to which the reservoir is tapped during prior to explosive eruptions. the eruption; Fig. 6). In the case of the Bishop magma

Fig. 7. Mass of SO2 estimated to be in pre-eruptive exsolved gas vs. eruptive magma volume, as in Fig. 2. Diagonal lines show predicted trends for variable drawdown depths in cylindrical magma bodies with radius of 0.1±4 km based on the gradient shown in Fig. 5A). P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 97 body, the gradient in exsolved gas content is such that Vulcanian and Strombolian eruptions, which suggest the gas-rich cap makes up only ,30% of the total that segregation of gas at shallow depth leads to episo- 800 km3 of erupted magma but contributes ,70% of dic explosions Allard et al., 1991; Blackburn et al., the total SO2 emission Fig. 6). The systematics 1976; Chouet et al., 1974; Self et al., 1979; Wilson, displayed in Figs. 5 and 6 suggest that if small erup- 1980). In contrast, sustained Plinian and pyroclastic tions preferentiallytap the gas-rich top of a larger ¯ow-forming eruptions typically involve larger magma reservoir, then a larger proportion of their volumes of magma, and are characterized byhigh total erupted SO2 should be from pre-eruptive magma ¯ux and rapid ascent from crustal reservoirs exsolved gas Wallace, 1997). ,5±10 km deep), which precludes segregation of This hypothesis can be tested by considering gas from magma before fragmentation Wilson, geometricallysimple magma bodies of a range of 1980). sizes, all of which are characterized bya gradient in Based on comparison with model trends Fig. 7), exsolved gas content Figs. 5 and 6). For a cylindrical the SO2 emission vs. eruptive volume data are consis- magma bodyof a given radius and depth, a wide range tent with the hypothesis that smaller eruptions prefer- of eruptive volumes can result depending on the depth entiallytap the gas-rich upper portions of larger of drawdown during a particular eruption, notwith- magma bodies. Most pyroclastic eruptions greater standing the complex withdrawal dynamics that may than ,1km3 are compositionallyheterogeneous occur in natural magmatic systems. Rather than speci- with respect to major and/or trace elements e.g. fying a particular exsolved gas gradient, it is possible Hildreth, 1981), and manythat are one to two orders to solve, byinverse methods, for an appropriate gradi- of magnitude smaller show distinct zonation. For ent based on the observed SO2 emission data Fig. 2). example, two of the smaller eruptions shown in Thus the exsolved gas gradient used in the calcula- Fig. 7 are zoned from dacite to andesite Ruiz, tions Figs. 5 and 6) was determined bybest ®t to the 0.03 km3, and Redoubt, 0.015 km3). Upward data for eruptions of 0.01±10 km3 of magma; that is, increases in exsolved gas content result in stable the gradient was derived so as to yield straight lines in densitystrati®cation and maythus help preserve Fig. 7 that would have the same general trend as the major and trace element compositional gradients in data. Considering that this gradient was derived solely magma bodies Wallace et al., 1995). from trends in the data shown in Fig. 2, it is remark- It is important to note that for an eruption of a given ablysimilar in form to the gradient deduced for the size there maybe as much as one order of magnitude

Bishop Tuff magma bodybyan entirelyindependent difference in the amount of SO2 in gas per unit volume method based on melt inclusion volatile contents of erupted magma e.g. Mount St. Helens and El Wallace et al., 1995). ChichoÂn in Figs. 3 and 7). Although this can be The results of simple models based on variable explained in part bydifferences in magma bodyradius drawdown depths demonstrate that for a plausible and drawdown depth, variations in other parameters range of magma reservoir sizes, variable depths of are probablymore important: 1) maximum gas magma withdrawal can yield SO2 emission patterns content at the top of the magma body; 2) concentra- similar to those that are observed based on remote tion of S in the pre-eruptive gas phase; 3) extent of sensing data Fig. 7). However, for the smallest pre-eruptive gas loss during transport of magma from volume eruptions shown in Fig. 7 LaÂscar and deep to shallow reservoirs; and 4) in¯uence of reser- Lonquimary), it is dif®cult to explain the very high voir, vent, and conduit geometryon withdrawal 3 apparent values of SO2 in gas per km of magma dynamics. Amounts of pre-eruptive magmatic gas erupted unless the uppermost part of the magma are likelyto varyfrom one eruption to another, but bodycontains a headspace of gas e.g. Giggenbach, a main cause of observed variation probablyis an

1996). A more plausible alternative is that SO2 emis- upward increase in exsolved gas content. The calcu- sion data for verysmall volume eruptions re¯ect lated trends in Fig. 7 are not meant to implythat all separation and concentration of gas relative to magma bodies have similar amounts of gas but merely magma during ascent and shallow storage. This inter- that gradients in exsolved gas contents can lead to the pretation is supported bydata and physicalmodels for observed patterns in data for SO2 emissions. Many 98 P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 magma bodies are probablycharacterized byless on melt inclusion H2O, CO2, and trace element data to steep gradients. As the magnitude of the gradient studyan eruption for which remote sensing data are diminishes, the diagonal trends shown in Fig. 7 will available. This method works well for the Bishop Tuff ¯atten out until theybecome horizontal for a magma magmatic system, in the upper parts of which frac- with homogeneous gas content Fig. 3). Ultimately, tional crystallization was the dominant process there are manyuncertainties in assessing exsolved gas responsible for differentiation Michael, 1983; Lu, contents in magma bodies, and the unique character- 1991), but the method will be dif®cult to applyin istics of each of the magmatic systems portrayed in magmatic systems where magma mixing plays an Fig. 3 underscore the need for further detailed studies important role. Nevertheless, application of the melt of individual eruptions. Nevertheless, the models inclusion method to the 1991 Mount Pinatubo yields shown in Fig. 7 serve to illustrate how pre-eruptive an estimated exsolved gas content of 3.6 ^ 1.2 wt% gradients in exsolved gas contents would, under a Wallace, unpublished data, cited in Wallace et al., simpli®ed set of circumstances, be manifested in 1995), consistent with the range of estimates based volcanic SO2 emissions. on other methods e.g. Fig. 3). Another wayin The exsolved gas gradient used in the modeling was which the exsolved gas gradient hypothesis could be derived so that trends for eruptive volume vs. SO2 tested is byusing time series data from remote sensing released would yield a straight line with negative techniques. For example, the 1982 eruption of El slope on a log±log diagram Fig. 7). Anysuch solu- ChichoÂn occurred in three closelyspaced major pulses tion will yield an exsolved gas mass fraction that that occurred over eight days. Combination of TOMS- decreases logarithmicallywith depth in the magma derived SO2 emissions with eruptive volumes for each chamber, assuming constant SO2 mass fraction in of the three pulses might make it possible to detect a 3 the gas phase and constant dv/dz, where v is volume change in SO2/km of magma over time S.J. Schae- withdrawn from the chamber and z is depth. Although fer, pers. commun.). such a logarithmic gradient is a continuous function, it results in a gas distribution with two distinct regions, a gas-rich top with a relativelysteep gradient, and a gas- 7. Gas accumulation and eruption triggering poor lower region with relativelyconstant gas content Fig. 5). With such a gradient, the upper, gas-rich Physical models of eruption mechanics suggest a region occupies a small fraction ,10%) of the total basis for expecting exsolved gas contents at the tops chamber volume. The fraction of the reservoir that of magma bodies that are comparable to those inferred consists of gas-rich magma in such a model is in from SO2 emission data. Volcanic eruptions are accord with the fraction of a magma bodythat is likely believed to occur when pressure in a magma reservoir to be evacuated during a given eruption Smith and exceeds lithostatic pressure byan amount greater than Shaw, 1979; Crisp, 1984; Shaw, 1985). The exsolved the tensile strength of the surrounding wall rocks gas gradient portrayed in Fig. 5 is somewhat analo- Blake, 1981). Such overpressuring can be caused gous to that used byBlake 1984) and Druitt and byan in¯ux of new magma from below or bycrystal- Sparks 1984), in which an exsolved gas-rich cap lization-induced gas exsolution Blake, 1981, 1984; representing a small fraction of the chamber overlies Tait et al., 1989). In the latter process, physical a gas-undersaturated dominant volume. Their model- models suggest that the critical overpressure is ing suggests that onlyin large caldera-forming erup- achieved when the exsolved gas content of the tions does less gas-rich magma get expelled from the magma reaches about 1 vol%, or if gas saturated deeper regions of a magma reservoir as the cauldron magma onlyoccupies a small fraction of the total block collapses and subsides until lithostatic pressure reservoir volume i.e. a gas-rich cap), as much as is reestablished. 10 vol% Blake, 1984). It is important to note that There are several methods bywhich the hypothesis the deduced maximum volume fraction of exsolved that exsolved gas gradients generallyexist in magma gas in these calculations is stronglydependent on the reservoirs could be tested. One test would be to use maximum degree of in¯ation of the reservoir that can the method developed byWallace et al. 1995) based occur bycompression of the enclosing wall rocks P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 99

Fig. 8. Schematic of an explosive eruption cycle for a magma reservoir with a gradient in the mass fraction of exsolved gas. See text for details of model.

Blake, 1981, 1984). The critical volume fractions of of repeated eruptions from a magma body. If after an exsolved gas cited above are based on a maximum eruption, the pressure in the reservoir relaxes to its volumetric expansion DV=V† of the reservoir of initial pre-overpressured) value, then the volume of 0.001. As an example of the effects of uncertainties erupted magma will be replaced bynewlyexsolved in this value, if the maximum volumetric expansion gas Fig. 8). In the closed system case Tait et al., DV=V† of the reservoir is 0.002, then the maximum 1989), no new magma is injected into the reservoir gas volume fraction in a small gas saturated layer at during the subsequent repose period, and pressure the top of the reservoir would be 20 vol% instead of graduallyincreases due to crystallization-induced 10 vol%. Given the factor of 2±3 uncertaintyinherent gas exsolution. Alternatively, in an open system, pres- in estimating the maximum possible extent of volu- sure increases in the chamber through both recharge metric expansion Blake, 1981, 1984; Tait et al., 1989; byma®c magma and crystallization-induced gas exso- Woods and Pyle, 1997), the maximum gas volume lution in the overlying differentiated magma Fig. 8). fraction in the upper part of a magma reservoir Because crystallization is caused by heat loss, the could be as high as 20±30 vol% exsolved gas. extent of crystallization-induced gas exsolution in The model of Blake 1984) for overpressuring due the differentiated magma will be controlled bythe to crystallization-induced gas exsolution was relative proportions of hot, ma®c recharging magma extended byTait et al. 1989) to consider the effects and differentiated magma. Eventuallythe critical 100 P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 overpressure in the chamber will again be reached and magnitude in eruptive volume, it has been estimated another explosive eruption will occur. that a basaltic magma ¯ux of approximately It is intriguing that the volume fraction of exsolved 0.01 km3/year is required to sustain an individual gas at the tops of manymagma bodies maybe about long-lived crustal magmatic system Smith, 1979;

30 vol% ,6 wt%), based on both the SO2 emission Shaw, 1985; Trial and Spera, 1990). If an average of data Fig. 3) and melt inclusion methods Wallace et 1000 ppm of S is degassed from mantle-derived al., 1995). As ®rst pointed out byCandela 1991) in basalt, then this basaltic magma ¯ux for an individual regard to degassing of plutons, mathematical models magmatic system yields a ¯ux of 0.052 Mt SO2 per predict that 30 vol% porosityrepresents the percola- year. This value provides an estimate of the amount of tion threshold at which spherical gas bubbles become mantle-derived S that is delivered into the base of an suf®cientlyinterconnected to allow permeable ¯ow intermediate to silicic composition magmatic system through a network of bubbles e.g. Sur et al., 1976; that is periodicallyrecharged with ma®c magma. Such Garboczi et al., 1995; Rintoul and Torquato, 1997). an estimate can be combined with the general rela- Thus 30 vol% mayrepresent a physicallimitation on tionship between repose time and eruptive magma the maximum amount of exsolved gas that can occur volume Smith, 1979; Trial and Spera, 1990) in at the top of a magma reservoir. Once this threshold is order to compare the ¯ux of SO2 released to Earth's reached, gas will be lost out of the top of the system by atmosphere from explosive eruptions with the ¯ux of advection through a permeable bubble network, and S derived from basaltic magmas that sustain crustal will probably be transferred into an overlying hydro- magma reservoirs. thermal system or leaked out to the surface. Smith 1979) suggested an approximate correlation between repose time and eruptive magma volume for intermediate and silicic magmatic systems that can be 8. Sources and ¯uxes of S in magmatic systems expressed as:

3 Isotopic studies of S in intermediate and silicic log repose time year†ˆlog magma volume km † 1 3 magmas have demonstrated that the S is magmatic in origin and is not dominantlyderived byassimila- Using a similar approach, Trial and Spera 1990) tion of crustal material Rye et al., 1984; Williams et provided data see their Fig. 1) that suggests repose periods somewhat shorter than those of Smith 1979), al., 1990; McKibben et al., 1996). The ultimate source 3 of S in these systems must therefore be from ma®c especiallyfor eruptions involving 1±100 km of magma, because it is well established that silicic magma. Given the uncertainties involved, and the magma reservoirs are created and sustained through natural variabilityin repose times, a general relation- long-term replenishment bymantle-derived basaltic ship consistent with Trial and Spera 1990) and Smith magma during inter-eruption repose periods Smith, 1979) can be expressed as: 1979; Hildreth, 1981; Shaw, 1985). Andesitic and log repose time year†ˆlog magma volume km3† dacitic magma in crustal reservoirs are probablygas saturated due to recharge and underplating bybasaltic 12:5 ^0:5† magma that is saturated with multicomponent H2O± CO2±S gas. Vapor transfer from underplated basaltic Based on this volume±repose time relationship, it magma into an overlying silicic magma body is prob- appears that for intermediate to silicic systems in ablya slow process that occurs during the long repose which individual eruptions are on the order of 3 periods that separate eruptive episodes. If the S in 0.01±1 km , the ¯ux of SO2 from explosive eruptions crustal magmatic systems is ultimately derived from is approximatelyin steadystate with the mantle- the mantle via basaltic magma, then the ¯ux of S derived S supplyrate Fig. 9). For example, an ande- should be directlyrelated to the basaltic magma ¯ux. sitic eruption of 1 km3 will be preceeded byan ,300 Based on the relationship between eruptive year repose period, during which time 3 km3 of basal- volumes and repose periods for numerous intermedi- tic magma is added to the system from below, contri- ate to silicic magma systems spanning many orders of buting 16 Mt of SO2. When the system erupts, ending P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 101

Fig. 9. Volcanic SO2 emissions vs. total volume of erupted magma for the intermediate and silicic eruptions shown in Fig. 1. The relationship between characteristic repose period shown on upper horizontal axis) and volume of differentiated magma erupted shown on lower horizontal axis) is based on Trial and Spera 1990). Solid line indicates steadystate balance between the amount of S introduced byprolonged degassing of 3 basaltic magma at the base of the system during the repose period and the amount of SO2/km of differentiated magma released byexplosive eruptions. The solid line is calculated assuming that an individual magmatic system has a basaltic magma ¯ux of 0.01 km3/year and 1000 ppm 3 S degassed from basalt, yielding a S ¯ux from the mantle of 5.2 Mt SO2/km of basalt. Dashed lines correspond to an uncertaintyof ^ one-half order of magnitude in the relationship between repose period and eruption size. the repose period, the systematics of Fig. 9 indicate each of the ,20 subaerial eruptions that occur that associated with the 1 km3 andesitic eruption is annually, the volume of magma erupted which is about 10±20 Mt of SO2. Thus the mass of SO2 related to VEI) can be used to estimate the length of released bythe eruption approximatelybalances the the repose period preceeding the eruption based on the mass of SO2 added via mantle-derived basaltic relationship given above. The repose period time can magma during the repose period. The balance then be used to estimate the total mass of basaltic between eruptive SO2 ¯uxes in systems characterized magma that has entered each system, based on a by0.01±1 km 3 eruptions probablyre¯ects the ef®- ¯ux of 0.01 km3/year, which in turn allows an esti- ciencywith which S is transferred from basaltic mate of the total mass of mantle-derived SO2 intro- magma at the base of the system to the overlying duced into the system. If the hypothesis of a steady differentiated magma. state balance is correct, then the mantle-derived SO2 The hypothesis that long-lived crustal magmatic that was introduced into each magmatic system during systems have an approximate steady state balance the repose period should be released during the explo- between input of S from basaltic magmas and loss sive eruption that ends the repose period. The global of S from explosive eruptions can be tested byusing annual ¯ux of SO2 from explosive eruptions can be the mantle-derived S ¯ux to estimate a global annual calculated byassuming: 1) a steadystate balance

SO2 ¯ux to the atmosphere. The number of subaerial between mantle supplyand explosive loss of SO 2; eruptions that have occurred in the last 200 years in 2) the historical record of explosive volcanism for different size categories based on the Volcanic Explo- the last 200 years; 3) the relationship between erup- sivityIndex VEI) have been compiled byMcClelland tive volume and repose period given above; and 4) a et al. 1989). These subaerial eruptions occur domi- basaltic magma ¯ux of 0.01 km3/year. The resulting nantly $94%; McClelland et al., 1989) from volca- estimate for the global ¯ux from explosive eruptions noes associated with convergent plate margins. For is 2.7 Mt SO2/year, a value that agrees well with an 102 P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 estimate of 0.3±3 Mt SO2/year calculated using magmas before eruption. Intermediate to silicic 3 remote sensing data for SO2 emissions Pyle et al., magmas in eruptions of 0.01±10 km appear to have 1996). Such agreement substantiates the hypothesis about 1±6 wt% exsolved gas. Within this range of that the mantle supply¯ux of SO 2 is approximately eruptive volume, smaller volume eruptions contain in steadystate with the SO 2 ¯ux from explosive either systematically higher gas contents, higher eruptions. proportions of total S in the gas phase, or both. For The two eruptions in Fig. 9 that involve less than verysmall volume eruptions  ,0.01 km3 of magma), 0.01 km3 of magma LaÂscar and Lonquimay) have apparent exsolved gas contents are highlyvariable, 3 ratios of SO2/km magma that are signi®cantlyhigher and probablylargelyre¯ect shallow segregation of than would be expected for a steadystate systemwith gas relative to magma, such as occurs in Strombolian a ¯ux of 0.01 km3/year of basaltic magma. Such a and Vulcanian eruptions. For eruptions .0.01 km3, pattern could be caused byshallow gas segregation, the SO2 emission vs. eruptive volume data are consis- as previouslydiscussed for Vulcanian and Strombo- tent with the hypothesis that smaller eruptions prefer- lian eruptions. In contrast, eruptions much larger than entiallytap gas-rich upper portions of larger magma 1km3 Pinatubo, Bishop Tuff) fall somewhat below bodies. The volume fraction of exsolved gas deduced the steadystate S ¯ux value. These lower values could for the upper regions of manymagma bodies be due to: 1) loss of S into large hydrothermal ,30 vol%) is similar to the percolation threshold at systems; 2) inef®ciency in the magma withdrawal which gas bubbles become suf®cientlyinterconnected process Spera et al., 1986); or 3) inef®ciencyin to allow permeable ¯ow of gas. This value is probably the tranfer of S from basaltic magma in the deep the maximum exsolved gas volume fraction that can roots of the system to highly differentiated magma occur at the roof zone of a magma reservoir because in the upper parts of the reservoir. anyadditional gas would be lost byadvective ¯ow through the permeable bubble network. Comparison of time±volume±eruption periodicityin individual 9. Conclusions volcanic systems, and the estimated rate at which basaltic magma supplies S into the base of such Most volcanic eruptions, whether basaltic, andesi- systems, suggests a relatively steady state ¯ux of S tic, dacitic or rhyolitic, are characterized by emissions from Earth's mantle to atmosphere through many of SO2 that are 10 times or more greater than the andesitic and dacitic magmatic systems. amounts that could have been derived from synerup- tive exsolution of dissolved S from the erupted volume of magma. The most notable exceptions to Acknowledgements this pattern are Hawaiian and Icelandic volcanoes; other basaltic systems Pacaya, Etna, Stromboli) are I would like to thank A.T. Anderson, T. Gerlach, characterized byhighlyvariable excess SO 2 emis- J. Lowenstern, and B. Scaillet for helpful discussions sions. A plausible hypothesis to explain excess SO2 on various aspects of the ideas presented here, and emissions is that magmas in crustal reservoirs are M. Carroll for his thoughtful comments on an early almost universallysaturated with a multicomponent draft of the manuscript. I would also like to thank gas phase consisting dominantlyof H 2O, CO2, and T. Gerlach, S. Self, and H. Shinohara for constructive S-species. During sustained Plinian and pyroclastic reviews that led to signi®cant improvements in the ¯ow-forming eruptions, signi®cant contributions of ®nal manuscript. SO2 from syneruptive degassing of unerupted magma are unlikelygiven the mass balance and low pressures in the deep source reservoir that would be References required for such gas loss. Using remote sensing data for SO emissions from 2 Allard, P., Carbonnelle, J., Metrich, N., Loyer, H., Zettwoog, P., recent explosive eruptions, it is possible to estimate 1994. Sulphur output and magma degassing budget of Stromboli the mass fractions of exsolved gas that are present in volcano. Nature 368, 326±330. P.J. Wallace / Journal of Volcanology and Geothermal Research 108 22001) 85±106 103

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