Fumarole emissions at Mount St. Helens volcano, June 1980 to October 1981: Degassing of a magma-hydrothermal system

Journal o f Volcanology and Geothermal Research, 28 (1986)141--160

141

Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands

FUMAROLE EMISSIONS AT MOUNT ST. HELENS VOLCANO, JUNE 1980 TO OCTOBER 1981: DEGASSING OF A MAGMA-HYDROTHERMAL SYSTEM*

T E R R E N C E M. GE R L A C H 1 and THOMAS J. CASADEVALL ~

1Division 1543, Sandia National Laboratories, Albuquerque, NM 8 7185, U.S.A. Cascades Volcano Observatory, U.S. Geological Survey, Vancouver, WA 98661, U.S.A. (Received October 9, 1985)

ABSTRACT Gerlach, T.M. and Casadevali, T.J., 1986. Fumarole emissions at Mount St. Helens volcano, June 1980 to October 1981: degassing of a magma-hydrothermal system. J. Volcanol. Geotherm. Res., 28: 141--160. This study is an investigation of the chemical changes in the Mount St. Helens fumarole gases up to October 1981, the sources of the fumarole gases, and the stability of gas species in the shallow magma system. These problems are investigated by calculations of element compositions, thermodynamic equilibria, and magmatic volatile-hydrothermal steam mixing models. The fumarole gases are treated as mixtures of magmatic volatiles and hydrothermal steam formed by magma degassing and boiling of local waters in a dryout zone near conduit and dome magma. The magmatic volatile fraction is significant in fumaroles with temperatures in excess of the magma cracking-temperature (~ 700°C) -- i.e., the temperature below which cracking is induced by thermal stresses during cooling and solidification. Linear composition changes of the fumarole gases over time appear to be the result of a steady decline in the magmatic volatile mixing fraction, which may be due to the tapping of progressively volatile-depleted magma. The maximum proportion of hydrothermal steam in the fumaroles rose from about 25--35% in September 1980 to around 50--70% by October 1981. Fractional degassing of magmatic CO2 and sulfur also contributed to the chemical changes in the fumarole gases. The steady chemical changes indicate that replenishment of the magma system with undegassed magma was not significant between September 1980 and September 1981. Extrapolations of chemical trends suggest that fumarole gases emitted at the time of formation of the first dome in mid-June 1980 were more enriched in a magmatic volatile fraction and contained a minimum of 9% CO 2. Calculations show H~S is the predominant sulfur species in Mount St. Helens magma below depths of 200 m. Rapid release of gases from magma below this depth is a plausible mechanism for producing the high H:S/SO2 observed in Mount St. Helens plumes during explosive eruptions. This study suggests that dacite-andesite volcanos may emit gases richer in COs during the earlier episodes of an eruptive cycle and burden the atmosphere with much more H2S than SOs during explosive eruptions.

*This work was performed in part at Sandia National Laboratories supported by the U.S. Department of Energy under contract DE-AC04-76DP00789.

0377-0273/86/$03.50

© 1986 Elsevier Science Publishers B.V.

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INTRODUCTION

In this paper, we make several interpretations of the Mount St. Helens fumarole gases over the period June 1980 to October 1981. The investigation focuses on interpreting the chemical changes in the fumarole gases over this period of time, inferring the source(s) and origin of the fumarole gases, and calculating the composition of the gas in shallow magma at depth. We have reported the analyses of 50 fumarole gas samples collected on the dome or crater floor during noneruption intervals between September 1980 and December 1981 (Gerlach and Casadevall, 1986). In that report, we provided a detailed evaluation of the gas data for the following sources of variability: partial analysis, analytical errors, alterations introduced by sampling procedures, and alterations from external sources (e.g., atmospheric gases, meteoric water, and organic materials). We also attempted to remove these effects from the analytical data and to derive several "improved compositions" for the emitted gases. The improved compositions show chemical changes characterized by increasing proportions of H20 and decreasing proportions of CO2 over the period September 1980 to September 1981 (fig. 6, Gerlach and Casadevall, 1986). The composition changes appear to form linear trends with time. They suggest that fumarole compositions could be inferred by interpolation and extrapolation over earlier periods when sampling was not possible. Relatively high scatter, however, in trends for H2, CO, SO2, and H~S discourages interpolation and extrapolation for these species. Much of the scatter reflects the effects of steadily falling fumarole temperatures on reaction equilibria involving these species (Gerlach and Casadevall, 1986). Here we resolve this problem by converting the molecular compositions {H20, CO~, H~, CO, H2S, SO~) to element compositions (C, O, H, S). The latter show linear trends in time that can be used to infer fumarole compositions from the inception of the first dome in mid-June 1980 to October 1981. The potential sources for the fumarole gases are: (1) degassing shallow magma; (2) local waters (i.e., local surface waters, groundwaters, hydrothermal waters, and brines); (3) sulfur and carbon in the host rocks; and (4) the atmosphere. Magma degassing was surely the source for the thousands of tons/day of SO2 and CO2 emitted during explosive eruptions from May through October 1980 (Casadevall et al., 1981, 1983; Harris et al., 1981). It also was the most probable source of fumarole CO2 and sulfur released in noneruptive emissions at rates of hundreds to thousands of tons/day between June and December 1980 (Casadevall et al., 1981, 1983). Evans et al. (1981) reported 613C of --10 to --11 per mill for carbon in the CO2 of fumarole samples collected in the crater during November 1980. Because these values are close to the --4.5 to --8.0 per mill range proposed for mantle carbon (Pineau et al., 1976; Moore et al., 1977; Barnes

143

et al., 1978), they inferred a predominantly mantle source for the CO2. They acknowledged, however, a possibility of some contamination (< 20%) from crustal organic carbon, which may also explain trace occurrences of CH4 in some samples (Gerlach and Casadevall, 1986). Isotope data for sulfur in the fumarole gases also indicate a magmatic source and imply temperatures near 900°C (H. Sakai, pets. commun., 1981). The source for fumarolic H20 is considerably more obscure. Current 6 D data are ambiguous on the question of a magmatic or a local source for the water. November 1980 fumarole samples contained water with a 5D of --33 per mill (Evans et al., 1981; Barnes, 1984). Water in samples from dome fumaroles in May 1981 had 6 D values of --64 to --77 per mill (T. Casadevall, unpublished data). These values are different from the D of --90 per mill for crater-pond water (Barnes et al., 1981; Barnes, 1984), and they are within the range observed for water from dacitic and rhyolitic magma (Mizutani, 1978; Taylor et al., 1983). Because of the enrichment of deuterium in steam relative to liquid water at temperatures above 220°C, however, boiling of local waters (as defined above) is also a potential source for the fumarole gases. For example, deep boiling of a briny fluid like that in Pigeon Springs about 38 km southwest of Mount St. Helens with 6 D of --43 per mill (Barnes et al., 1981, 1984) could have been responsible for the higher 5 D of the water in the November 1980 samples, as suggested b y Evans et al. (1981) and Barnes (1984). Because of the evidence for magmatic carbon and sulfur and the possibility of multiple sources for the water in the fumarole gases, we adopt the working hypothesis that the m o u n t St. Helens fumarole gases are mixtures of magmatic volatiles and steam from waters of local origin. We develop a mixing model for estimating the fraction of fumarole gas contributed by boiling and superheating local waters in a dry-out zone near shallow magma (Hardee, 1982; Carrigan, 1986). Application of the model to the Mount St. Helens fumaroles indicates that the local water component in the fumarole gases increased appreciably from September 1980 to October 1981. We also use least-squares mass balance calculations to determine whether the chemical changes observed in the fumarole gases are entirely explained by incorporation of an increasing proportion of local water. The results show that progressive incorporation of local water alone, while important, does not account for all the changes; fractional degassing of magma appears to be involved as well. The predominant sulfur species in the Mount St. Helens plume normally is SO2 (Casadevall et al., 1981). During explosive eruptions in 1980, however, H2S was observed to exceed SO2 in the plume, sometimes by as much as an order of magnitude (Casadevall et al., 1981; Hobbs et al., 1982). We use the fumarole gas compositions and thermodynamic calculations to infer the stable sulfur species in gases at the temperatures and pressures of Mount St. Helens magma at depth. The calculations predict that H2S is the predominant sulfur species for gas present in magma at depths greater

144 than 200 m below the crater floor. Rapid release of gas from magma below these depths during explosive eruptions is, therefore, a plausible mechanism for producing the high H2S/SO2 in the plume at such times. BACKGROUND The magma system

A variety of evidence suggests that a magma b o d y was emplaced beneath Mount St. Helens crater by mid~lune 1980. At that time, inflation of the volcanic edifice ended (Dvorak et al., 1981; Swanson et al., 1981), and the first occurrences of dome lava were observed in the crater (Moore et al., 1981). Large increases in the gas emission rates coincided with these m i d ~ u n e events. Emission rates for SO2, which had been at levels of ~< 250 tons/day since March (except for the May 18 eruption), suddenly increased fivefold and remained at high levels until the end of 1980 (Casadevall et al., 1981, 1983). We assume that the magma b o d y is deeper than 1 km under the crater floor. There is seismic, ground deformation, and tiltmeter evidence for magma at depths of 1 to 3 km (Malone et al., 1983; Weaver et al., 1983; Chadwick et al., 1983; Dzurisin et al., 1983). The geophysical evidence for a magma b o d y of significant size, however, is strongest for a deep reservoir at 7--14 km (Scandone and Malone, 1985). A narrow vertical conduit is ir, ferred between the magma b o d y and the dome (e.g., Weaver et al., 1983; Scandone and Malone, 1985). A 15--25 m diameter section through the conduit was observed from the air during an early episode of dome building in October 1980 (D.A. Swanson, pets. commun., 1984). By the end of the period of interest in this study (October 1981), the top of the dome was approximately 200 m above the crater floor. The average bulk density estimated for the Mount St. Helens cone material is 2.2 g/cm 3 (Friedman et al., 1981; Voight et al., 1981). This density gives a lithostatic pressure gradient of approximately 200 bar/km, which we have used to estimate pressures in magma beneath the crater floor. Magma pressures inside the dome are not known, but we assume they are less than that given by the 200 bar/km gradient. We have used 1000°C to characterize the magma temperature. Several investigators have reported temperatures in the range 1050°C to 920°C for Mount St. Helens magma erupted between 18 May 1980 and early 1982 (Friedman et al., 1981; Melson and Hopson, 1981; F u k u y a m a et al., 1981; Merzbacher and Eggler, 1983, 1984; Rutherford et al., 1985). The hydrotherrnal dry-out zone

Near shallow magma, temperatures rise in adjacent wallrocks and at some point the phase change from liquid water to steam produces a dry

145

steam zone or " d r y - o u t zone" (Hardee, 1982). Hardee's investigations of dry-out p h e n o m e n a in magma-hydrothermal systems focused on horizontal dry steam layers overlying shallow sills or large magma bodies with broad shallow tops (Hardee, 1982). The heat transfer and fluid dynamics of vertical dry-out zones, which can form near bodies such as the Mount St. Helens conduit, have been investigated b y Carrigan (1986). The thickness of vertical dry-out zones adjacent to dikes or conduits is inversely proportional to permeability. It can range from tens of meters at 1 Darcy to several hundreds of meters at 0.01 Darcy. The surrounding liquid water column exerts a hydrostatic head on the vertical dry-out zone and causes a pressure gradient that forces the steam column upward. The upward transported steam is replaced by vaporization of groundwater that flows in towards the dry-out zone. A liquid--vapor phase change will form the external boundary of the dry-out zone associated with the Mount St. Helens feeder conduit and dome. The temperature on this boundary can range from near the local boiling point {96--97°C), at shallow depths, to the critical temperature {374°C), at depths greater than 1 km. The dry-out zone adjacent to the shallower portion of the Mount St. Helens conduit may be in unsaturated medium above the water table. For this condition, the location of the phase change and the size of the dry-out zone will depend in part on the rate at which downward percolating meteoric water absorbs heat at the liquid--vapor phase boundary. Circulation of air through the unsaturated medium and into the dry-out zone could also occur to some extent. The cracking-front of magma in the conduit and base of the d o m e forms the internal boundary of the Mount St. Helens dry-out zone. The cracking-front defines a sharp permeability discontinuity separating impermeable plastic material from permeable brittle material. The brittle material fractures in response to thermal stresses associated with cooling and solidification of magma. Magma in a plastic condition is impermeable and readily relaxes thermal stresses. The cracking-temperature along the cracking-front caused by thermal stresses during cooling is not known for the Mount St. Helens magma. We shall assume that 700°C is a reasonable upper limit, based on previous theoretical and experimental work (Lister, 1974; Ryan and Sammis, 1981).

Fumarole temperatures and gas compositions We have shown previously (Gerlach and Casadevall, 1986) that the maxim u m temperatures measured in the fumaroles on a given date over the period September 1980 to March 1982 decreased linearly with time according to the relation: Tma x = 865. -- 0.37 t

(1)

where t is the number o f days since 31 May 1980. The temperatures given b y eqn. (1) are in good agreement with the last equilibrium temperatures

146

(i.e., quench-temperatures) calculated for the sampled gases (Gerlach and Casadevall, 1986). The best representation of the fumarole gas chemistry over the period September 1980 to September 1981 is provided by the 16 improved compositions shown in Table 1. Nearly all samples for Table 1 came from the "radial fumaroles" site along a N40°W-trending radial fissure on the crater floor adjacent to the southeast margin of the dome. This site lasted until October 1981 when it was covered by a rock slide. We have previously described in detail the analytical data and the procedures used to derive the improved compositions (Gerlach and Casadevall, 1986). The most accurate compositions given in Table 1 are those for samples 800925-710, CNR, and CNRS. These samples were collected by techniques that preserved the water and sulfur content of the gas. The other samples were collected by procedures that neglected water and produced sulfur alterations. We were able to make satisfactory corrections for neglected water in most of these samples; sulfur corrections, however, were either impossible or very approximate and may significantly under or over estimate true S {Gerlach and Casadevall, 1986). Despite the problems with correcting the TABLE 1

Improved

c o m p o s i t i o n s a o f the M o u n t St. Helens fumarole gases, S e p t e m b e r 1 9 8 0 -

September 1981

Sample

800925-710 CQ243IB80 CQ244IB80 810210-2 810527-1 810527-3 810702-5 810702-6 810727-1 810727-3 810727-4 810831-3 810831-4 810831-5 CNR CNRS

Date

25Sep80 4Nov80 4Nov80 10 F e b 81 27 M a y 81 27 M a y 81 2Jul81 2Jul81 27 J u l 8 1 27 J u 1 8 1 27 J u l 8 1 31Aug81 31 A u g 81 31 A u g 81 16Sep81 17 S e p 8 1

m o l e %b H20

CO~

H~

CO

H2S

SO2

St c

91.58 93.53 91.53 94.14 95.99 96.37 96.32 96.38 97.7 97.7 97.6 97.80 97.76 97.1 98.52 98.6

6.942 5.40 7.43 5.02 3.30 2.84 1.98 2.50 1.94 1.93 2.10 1.90 1.92 2.50 0.913 0.886

0.8542 0.823 0.739 0.76 0.46 0.57 0.30 0.33 0.31 0.33 0.33 0.299 0.309 0.38 0.269 0.39

0.06 0.037 0.049 0.034 0.006 0.009 0.003 0.005 0.004 0.004 0.004 0.0044 0.0047 0.006 0.0013 0.0023

0.3553 0.165 0.178 0.030 0.23 0.20 0.92 0.57 -. . . . 0.0004 0.001 -0.137 0.099

0.2089 0.037 0.072 0.013 0.005 0.011 0.49 0.22 ---

------~ -0.02 0.006 0.01 --

0.0019 0.0049 0.073 0.067

0.01 --

aDetailed descriptions o f analytical data and procedures used to obtain improved comp o s i t i o n s are given in Gerlach and Casadevall ( 1 9 8 6 ) . b M i n o r a m o u n t s o f C O S , $2, a n d HC1 were neglected for s o m e samples in this tabulation. CTotal sulfur (S t = SOs + H : S ) .

147

sulfur data, we are confident that S is always present at a low level because of the consistently small concentrations derived for H2S and SO2 (Table 1), including for the three most reliable samples. This characteristic of the gases agrees with the low S (100 ppm) reported for Mount St. Helens glass inclusions (Sigurdsson, 1982). WORKING HYPOTHESIS

We assume that the fumaroles of the dome and crater floor occur in areas of high permeability along intrusion-related fractures that penetrate the dry-out zone. Consequently, we suppose the fumaroles to contain steam from dry-out zone boiling and superheating of local waters {surface waters, groundwaters, brines, etc.). We refer to steam of such origin as hydrothermal steam. The evidence for the presence of magmatic volatiles as well in the fumaroles was n o t e d previously. The high temperatures (850-700°C) of the fumaroles before October 1981 exceed the probable cracking-temperature {from thermal stresses) of cooling Mount St. Helens magma and, therefore, also suggest magmatic volatile contributions. We propose that the make-up of the fumarole gases involves the processes of (1) vapor generation by exsolution and glass devitrification in conduit and dome magma, (2) outgassing of magmatic volatiles through the cracking-front and into the dry-out zone, and (3) mixing and chemical equilibration of outgassed magmatic volatiles with hydrothermal steam. Injection of magmatic volatiles into the dry-out zone is probably most intense near the top of the conduit and base of the dome where confining pressure is low. We concluded in our previous investigation that any local water present in the fumarole gases must have reached chemical equilibrium with admixed magmatic volatiles at temperatures above the observed fumarole temperatures (Gerlach and Casadevall, 1986). PROCEDURES AND METHODS

E l e m e n t c o m p o s i t i o n calculations

Element compositions were calculated from mole percent gas compositions and the equation: N

ey = iZ=l aiyn i

(2)

The quantity n i is the mole percent concentration of species i in the gas; aii is the number of moles of element j in a mole of species i; N is the total n u m b e r of species; and ej is the total moles of element j. The use of mole percent quantities for n i is a convenience that allows direct calculation from tabulated mole percent gas data. Consequently, e i is normalized to

148

100 moles of gas. For example, a gas with the composition 90% H20, 9% CO2, and 1% SO2 has the following element composition: ec = eo = eH= es =

(1)(9) = 9 moles C/100 moles gas (1)(90) + (2)(9) + (2)(1) = 110 moles O/100 moles gas (2){90) = 180 moles H/100 moles gas (1)(1) = 1 mole S/100 moles gas.

In this report "%" means mole percent, the same as volume percent in the case of gas data.

Thermodynamic calculations Equilibrium gas compositions were calculated with a steepest descent, free,energy minimization algorithm (Van Zeggeren and Storey, 1970). The calculation converges on the equilibrium species distribution for a defined element composition (i.e., mass balance) subject to temperature, total pressure, and free,energy minimization constraints. All thermochemical data for the gas species are from the 1971 J A N A F tables (Stull and Prophet, 1971) and subsequent supplements to the J A N A F tables (Chase et al., 1982). The calculations use the Redlich-Kwong equation to approximate nonideal gas effects. These effects are small or insignificant at the high temperatures and low pressures of interest in this investigation.

Least-squares mass balance calculations We used X L F R A C , a computer code for interactive least-squares mass balance calculations (Stormer and Nicholls, 1978), to test models for the composition changes observed over time in the fumarole gases. The models were evaluated on the basis of their fit, as indicated by residuals (ri), to gas composition data (Table 1).

Magmatic volatile--hydrothermal steam mixing model Mixing of magmatic volatiles with hydrothermal steam from the dryo u t zone forms the basis of a simple heat-transfer model for estimating hydrothermal steam contributions to the fumarole gases. The hydrothermal steam in the portion of the dry-out zone injected with magmatic volatiles is assumed to have an average temperature Ths less than that of the magmatic volatiles (Tmv). The temperature of the mixture after thermal equilibration is T e. The heat transferred from the magmatic volatile mixing c o m p o n e n t to the hydrothermal steam mixing c o m p o n e n t is given by: Te AHhs = nhs

f Ths

CH~ O d T

(3)

149 where nhs is the quantity of hydrothermal steam in moles per 100 moles of fumarole gas, and CH~O is the molar heat capacity of steam. The heat removed from the magraatic volatile mixing c o m p o n e n t is given by: Te

AHmv=(nH~O--nhs)

f

Te

CH~O d T + ~

Tmv

i

ni f

C idT

(4)

Tmv

where nH20 and n i refer respectively to the moles of water and other species (CO2, H2, H2S, etc.) per 100 moles of fumarole gas, and C i is the molar heat capacity of the ith species. Because AHhs = --AHmv, eqns. (3) and (4) can be equated and simplified to derive the following expression for nhs: Tinv

nH~O

f

fmv

C H ~ Od T + E n i i

Te

nhs =

(5)

Tiny

f

Ci dT Te

CH2o dT

Th s

The quantities nhs, nH20, ni are normalized to 100 moles of fumarole gas as a convenience that allows direct use of mole percent gas data. Heat capacity terms in eqn. (5) can be expanded with polynomial equations (e.g., Robie et al., 1978). We have specified parameters in eqn. (5) in such a way as to maximize nhs. The quantity of hydrothermal steam (nhs) is most sensitive to the thermal equilibration temperature (Te); it increases sharply as T e decreases. We use maximum fumarole temperatures from eqn. (1) to estimate T e. Because of thermal losses to wallrock before fumarole emission, this procedure gives minimum values for T e and, therefore, maximum values for nhs. Increases in the temperature of magmatic volatiles (Troy) or in the temperature of hydrothermal steam (Ths) cause nhs to increase, but the response is less sensitive than for changes in T e. We set Tmv = 1000°C as an upper limit for the average temperature of the magmatic volatile mixing component. A range of plausible values is used for the hydrothermal steam temperature (Ths) in the dry-out zone. Temperature gradients across the dry-out zone are approximately linear (Carrigan, 1986). The hydrothermal steam temperatures in the dry-out zone near the t o p of the conduit would range from 700°C along the cracking-front to the local boiling point (96-97°C); the average temperature would be a b o u t 400°C. At depths greater than 1 kin, temperatures in the dry-out zone would range from 700°C to near the water critical point, with an average value of approximately 550°C. Accordingly, we use temperatures of 400°C and 550°C to characterize Ths , the temperature of the hydrothermal steam c o m p o n e n t that mixes with the magmatic volatiles injected from the conduit. We also use a temperature of 700°C for Ths as an extreme upper limit that would apply

150

if all the hydrothermal steam mixing with the magmatic volatiles is from the cracking-front. This choice for Ths may not be realistic, but it gives the m a x i m u m hydrothermal steam contribution that can be calculated from eqn. (5). RESULTS

Changes in fumarole gas compositions, June 1980--October 1981 The element compositions for the Mount St. Helens fumarole gases were calculated from the compositions in Table 1 and eqn. (2). The results I08.0

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Fig. 1. Elemental compositions for C, O, H, and S in moles per 100 moles of gas are shown for improved compositions o f 16 samples collected from Mount St. Helens fumaroles between September 1980 and September 1981 (Tables 1 and 2). The eight square symbols, four of which nearly coincide on most of the diagrams, refer to the more reliable improved compositions as discussed in Gerlach and Casadevall (1986). The eight × symbols, two of which coincide, are based on less reliable improved compositions of Gerlach and Casadevalt (1986). The dashed lines are composition trends based on the three most reliable compositions (September 25, 1980; September 16 and 17, 1981) as discussed in the text. The composition trend lines are extrapolated back to June 1980 to estimate gas compositions for the earlier fumaroles. The restricted number of sample points for sulfur is explained in the text.

151

are given in Table 2 for the period September 1980 to September 1981. The element compositions for carbon, oxygen, and hydrogen show linear trends when plotted against time of collection (Fig. 1). The element compositions for sulfur scatter considerably (Table 2), as a result of the sampling related errors discussed above and elsewhere (Gerlach and Casadevail, 1986). The three samples with the most reliable compositions (800925-710, CNR, and CNRS) are free of significant sampling errors and define a complementary trend for sulfur (Fig. 1). We use these samples, which fortunately were collected at the beginning and end of the sampling period, to fit linear composition trend lines through the points in Fig. 1. We extrapolate the trend lines back to June 1980 (Fig. 1) in order to infer the element compositions of the earlier dome fumarole gases. The following equations describe the element composition trend lines: e c = 9.00 -- 0.0171 t

(6)

eo = 107.73--0.0153 t

(7)

e H = 181.65 -- 0.0348 t

(8)

e s = 0.699 + 0.0011 t

(9)

The quantity t is the number of days since 31 May 1980. Each equation has a correlation coefficient of 0.99. TABLE 2 Element compositions of the Mount St. Helens fumarole gases, September 1980--September 1981 Sample

800925-710 CQ243IB80 CQ244IB80 810210-2 810527-1 810527-3 810702-5 810702-6 810727-1 810727-3 810727-4 810831-3 810831-4 810831-5 CNR CNRS

Date

25 4 4 10 27 27 2 2 27 27 27 31 31 31 16 17

Sep 80 Nov 80 Nov 80 Feb 81 May 81 May 81 Jul 81 Jul 81 Jul 81 Jul 81 Jul 81 Aug 81 Aug 81 Aug 81 Sep 81 Sep 81

moles/100 moles gas a C

O

H

S

7.002 5.437 7.479 5.054 3.306 2.849 1.983 2.505 1.944 1.934 2.104 1.9044 1.9247 2.506 0.9143 0.8883

105.9426 104.4412 106.5834 104.2400 102.6061 102.0811 101.2633 101.8253 101.584 101.564 101.804 101.6082 101.6145 102.106 100.4933 100.5083

185.579 189.036 184.894 189.860 193.360 194.280 195.080 194.560 196.020 196.060 195.860 196.1988 196.140 194.960 197.852 198.178

0.5642 0.202 0.250 O.043 0.235 0.211 1.41 0.79 0.02 0.006 0.01 0.0023 O.0O59 0.01 0.210 0.167

aCalculated from eqn. (2) and mole % compositions in Table 1.

152

A 1-atm equilibrium model for the noneruptive fumarole gases over the period June 1980 to October 1981 is shown in Fig. 2. Equations (6)-(9) provide the element mass balance constraints for the model. Temperatures are constrained by eqn. (1). The model reproduces the characteristic features observed in the fumarole gases over the period of collection. Model 02 fugacities are close to those of the Ni-NiO buffer reaction, in agreement with observation (Gerlach and Casadevall, 1986), and range from log fo2 = --13.2 bar at 865°C in early June 1980 to --16.3 bar at 685°C in early October 1981. In addition to the species shown in Fig. 2, the fumarole gases contain a b o u t 0.1% HC1 and 0.05% HF (Gerlach and Casadevall, 1986). Of particular interest are the model compositions for mid-June 1980 when the first dome lava was observed. The model suggests the midJune fumarole gases had temperatures of about 865°C and contained 89% H20, 8.8% CO2, 1.2% H2, 0.1% CO, 0.4% H~S, 0.3% SO2, and 0.1% $2. 0.0

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M

R

M

J

i

i

i

J

R

S

O

Fig. 2. Model fumarole gas compositions for Mount St. Helens during noneruptive periods between June 1980 and October 1981. The model compositions are based on equilibrium calculations constrained by the elemental composition trends (Fig. 1 and eqns. (6)-(9)) and temperatures from eqn. (1).

Hydrothermal steam in the fumarole gases The results of the calculated hydrothermal steam contributions to the fumarole gases, based on the magraatic volatile--hydrothermal steam mixing model, are summarized in Table 3. The results are obtained from eqn. (5), the gas compositions in Fig. 2, a magmatic volatile temperature of IO00°C, and three hydrothermal steam temperatures (400°C, 550°C, 700°C).

153 TABLE 3 C o n t r i b u t i o n s of h y d r o t h e r m a l steam to the fumarole gases a Date

1 J u n e 1980 1 S e p t e m b e r 1980 1 October 1981

Ths (°C) 400

550

700

20 b 26 52

27 34 69

40 52 100

aAll results calculated from eqn. (5). Te was estimated f r o m eqn. (1). Tmv = 1000°C. C o m p o s i t i o n terms (nil: O, hi) evaluated f r o m fumarole m o d e l (Fig. 2). bResults in m o l e percent (absolute).

Recall that we use eqn. {1) to estimate Te, the temperature of the magmatic volatile--hydrothermal steam mixture after thermal equilibration. Because eqn. (1) tends to underestimate Te, as discussed above, hydrothermal steam contributions are overestimated by this procedure. For example, if eqn. (1) underestimates T e by 20°C, the results in Table 3 overestimate hydrothermal steam contributions by 5--8% (absolute). Our choice of 1000°C for the magmatic volatile temperature (Tmv) is an upper limit deliberately selected to maximize the calculated hydrothermal steam contributions. Experimental studies on Mount St. Helens pyroclastic eruption products indicate magma temperatures of 920 ° to 940°C (Merzbacher and Eggler, 1984; Rutherford et al., 1985}. If temperatures in this range are used for Tmv in eqn. (5), the calculated hydrothermal steam contributions are 10--15% (absolute) lower than those in Table 3. Thus, the results in Table 3 should be viewed as maximum values for the hydrothermal steam component. The results of the mixing model calculations (Table 3) indicate a progressive increase in the proportions of hydrothermal steam in fumarolic gases. We used least-squares mass balance calculations to test the hypothesis that the fumarole chemical changes (Tables 1 and 2, Figs. 1 and 2) are entirely due to progressive increase in hydrothermal steam. All attempts to generate the compositions of the later gases merely by adding water to the earliest gases failed to give satisfactory solutions; large residuals (Y~ri2 = 2--4) were obtained. Successful solutions required increases in water and decreases in CO2 and sulfur. For example, the 17 September 1981 gas (CNRS, Table 1) differed from the 25 September 1980 gas (800925-710, Table 1) by relative changes of +140% H20, - 7 2 % CO2, and - 3 7 % total sulfur ( Z r i 2 < 0.01). C o m p o s i t i o n o f magmatic gas at depth, Fall 1 9 8 0

Figure 3 shows the effects of increasing pressure on the equilibrium composition of a Mount St. Helens magmatic gas as approximated by sample

154

800925-710 (Table 1) at 1000°C. The analysis for this sample was used because it is the most accurate available for the early samples and was the least contaminated by hydrothermal steam, which became significant in the later fumarole gases (Table 3). Estimated amounts of HC1 (0.1%) and HF (0.05%) based on data we reported elsewhere (Gerlach and Casadevall, 1986) were added to the sample and included in the calculations. The principal effect of increasing pressure is on reaction equilibria involving sulfur species, as shown in the first 50 bars (250 m depth) of pressure increase (Fig. 3). The changes reflect right shifts in the following equilibria: 3H2 + SO2 ~ 2H=O

+ H2S

2H: + CO + SO2 e- 2H20 + COS 2H20 + ~ S2.

2H2 + SO2 ~

The predominant sulfur species at depths greater than 200 m (40 bar) is H2S. At pressures over 100 bar (500 m), the equilibrium composition remains relatively constant. If 35% of 800925-710 is assumed to originate from hydrothermal steam (Table 3) and if this contamination is removed before calculating the pressure effects, the results are similar to those in Fig. 3. Of course, the proportion of H20 decreases, and the proportions of all other species increase. The sulfur speciation, however, again follows I

O.O

---

~l 5

I

1.0

:~.

]

1.5

2.0

30'0.0

400.0

OEPTH :Kbl) O.0 I

7 ~

C_D

H20

-I.aL

H2

2.0

~>~25

L

3.0-

[J

co2

~HCL.

/5o2

. . . .

Hr

C~ >\ -4.0

52

J -

~

COS

/

-6.0

O.0

100.0

20~0.O

PRESSURE (BFIRS} Fig. 3. Calculated d i s t r i b u t i o n o f e q u i l i b r i u m species w i t h increasing pressure at 1000°C for a f u m a r o l e gas c o m p o s i t i o n o n S e p t e m b e r 25, 1980. D o t t e d lines s h o w e s t i m a t e d HCI and H F c o n c e n t r a t i o n s . The d e p t h - p r e s s u r e relation assumes a l i t h o s t a t i c gradient o f 200 b a r / k m . Curves c o n n e c t results calculated at 50-bar intervals.

155

the pattern in Fig. 3 with a crossover in the predominant sulfur species from SO2 to HsS at a b o u t 30 bar (150 m depth), instead of 40 bar. The 1000°C temperature for the gas is an upper limit and acts to minimize HsS, which is increasingly stable at lower temperatures. If the magma temperature is taken to be 920°C, which better agrees with recent experimental studies (Merzbacher and Eggler, 1984; Rutherford et al., 1985), HsS/SO2 increases slightly at all pressures, and the SO2 to H2S crossover occurs at lower pressures and shallower depths than shown in Fig. 3. Pressure increase alone causes the higher H2S/SO2 for all reasonable choices of temperature and candidate gases that we have investigated. The fo2 of the gas increases up to an order of magnitude in the first 500 bar, and b y itself would tend to increase SO2, rather than H2S. The rise in fo~ occurs because of equilibria shifts that consume H2 and CO. The results indicate that the fO~ of the gases at depth will be higher than that of the fumarole gases (Gerlach and Casadevall, 1986) and agree better with fo2 obtained from Fe-Ti oxides in Mount St. Helens eruption products (Melson and Hopson, 1981; Rutherford et al., 1985), which consistently give values a b o u t an order of magnitude higher than the fumarole values. DISCUSSION

The element compositions of the Mount St. Helens fumarole gases unambiguously d o c u m e n t a steady change in the gas chemistry over the period September 1980 to September 1981. The changes define approximately linear trends with time. They are not caused by the decline in fumarole temperatures with time, because element compositions are n o t affected by shifts in reaction equilibria. The changes in element compositions reflect increasing proportions of H20 and decreasing proportions of COs, HsS, and SO2 in the fumarole gases over time (Fig. 2). Mizutani and Sugiura (1982) have described similar chemical and temperature trends in fumaroles after emplacement of a dacite lava dome at Showashinzan volcano. Changes displayed by the fumarole gases up to September 1981 record in part the decline of the high-temperature magmatic volatile fraction and the rise of the hydrothermal steam fraction. As a result, temperatures of fumaroles dropped while the proportion of hydrothermal steam in the fumarole gases rose. The decline of the magmatic volatile fraction in the fumarole gases agrees with the observed trend of decreasing COs and SOs emission abundances since July 1980 (Casadevall et al., 1981, 1983; Harris et al., 1981). Although the maximum proportion of hydrothermal steam in the fumaroles rose from a b o u t 25--35% in September 1980 to around 50--70% in October 1981 (Table 3), mass balance calculations show that a progressive increase in hydrothermal steam alone does n o t completely account for chemical changes in the fumarole gases over this period. A decrease in the proportion of CO2 and sulfur in the magmatic volatile fraction is also called for.

156 We conclude from the steady uninterrupted chemical trends of the fumarole gases and the decline of the magmatic volatile fraction that significant replenishment of the magma reservoir with relatively undegassed magma did n o t occur over the period September 1980 to September 1981. The processes responsible for the decline of the magmatic volatile fraction are still matters of considerable conjecture, b u t explanations involving the tapping of progressively deeper, volatile~lepleted portions of the magma reservoir and a diminishing of the magma supply rate during noneruption periods seem reasonable. Chemical changes of the sort exhibited b y the Mount St. Helens fumaroles also suggest progressive infiltration of meteoric water deeper into solidifying magma (Mizutani, 1978; Mizutani and Sugiura, 1982). Infiltration, however, would become most important after movement of magma toward the surface becomes so diminished that the rate of heat supplied by intrusion of magma into the conduit and dome is exceeded by the rate of heat removed in the liquid--vapor hydrothermal system (Carrigan, 1986). Because of the frequent eruptions up to September 1981 (and after), we favor the interpretation that intrusion into the conduit-dome complex of magma progressively depleted in total volatiles (i.e., fluid + dissolved load) was primarily responsible for the decline in the magmatic volatile fraction. To the extent that water is a significant volatile in the magma, this interpretation is supported by evidence of decreasing water in glass inclusions and matrix glasses of pyroclastic lapilli and dome rocks formed during events from 18 May 1980 to 22 March 1982 (Meison, 1983; Merzbacher and Eggler, 1984). Scandone and Malone (1985) similarly attribute the sharp decrease in magma supply rate for the explosive eruptions since 18 May 1980 to an increase in magma viscosity caused by tapping of progressively deeper, water-depleted magma. The simplest explanation for the decrease in the CO2 and sulfur contributions to the magmatic volatiles is the fractional degassing of these volatiles and their accumulation in and removal from the shallower portions of the magma b o d y during the earlier stages of the current cycle of volcanism. Early degassing of CO: from magma at these depths is consistent with its limited solubility in silicate melts at crustal pressures (Mysen et al., 1975; Holloway, 1 9 7 6 ) . Our results indicate that magmatic water may have become insignificant in fumarole emissions after, but not before, October 1981. The composition changes in the fumarole gases up to this time are nevertheless consistent with the suggestion of Evans et al. (1981) that the fumaroles contained significant amounts of nonmagmatic water even as early as November 1980. However, we do not agree with the conclusions of Barnes (1984) that the only Mount St. Helens volatiles of magmatic origin are CO: and sulfur gases and that water, even in the early emissions, was derived entirely from boiling metamorphic brine in an underlying meta-andesite. The presence of amphibole phenocrysts in eruption products, the occurrence of water in glass inclusions (Melson, 1983; Rutherford et al., 1985), and the experimental investigations of phase relations and melt chemistry for

157

representative dacite samples (Merzbacher and Eggler, 1984; Rutherford et al., 1985) leave virtually no doubt of the presence of magmatic water in the current cycle of Mount St. Helens volcanism. The 9% CO2 predicted for the fumaroles in mid-June 1980 by extrapolation of linear composition trends may be an underestimate. Because of the fractional degassing of CO2 (and sulfur), trends based on samples obtained more than four months after the paroxysmal eruption of May 18, 1980, are likely to be biased towards residual magmatic volatiles. Indeed, airborne data suggest that CO2 was the predominant volatile during the Plinian phase of the May 18 eruption (Hobbs et al., 1982). We therefore urge caution in applying the linear extrapolations of the composition trends to times much earlier than September 1980. We also caution that the trends should not be extrapolated forward beyond September 1981. The trends terminate after this time; the available post-September 1981 samples are monotonously water-rich with minor CO2, H:S, and SO2 (Gerlach and Casadevall, 1986) that are probably derived to a substantial degree from devitrification of glass in the conduit and dome. Experimental studies of phase relations and melt chemistry for dacite from the 18 May 1980 eruption (Rutherford et al., 1985) strongly support the conclusion that CO2 was an important magmatic volatile in the early stages of renewed volcanism at Mount St. Helens. Observed phase relations and liquid compositions are not experimentally reproducible in dacite-H20 systems but are closely simulated in dacite-H20-CO2 systems when PH~O is made less than Pfluid by addition of CO2. Close experimental analogs are obtained at 920--940°C and 2.2 kbar (7.2 km) for fluid phases containing 30--50 mole % CO2. These conditions duplicate the natural liquid composition and phase assemblage, except for amphibole, which requires lower temperatures by 15--40°C. Rutherford et al. (1985) suggest that amphibole was unstable in the magma or that its stability was enhanced by F. The presence of HF in the fumarole gases (Gerlach and Casadevall, 1986) lends credence to the latter alternative. From our calculations constrained by fumarole gas compositions, magmatic temperatures, and estimated pressures for shallow Mount St. Helens magma, we conclude that H2S is the predominant sulfur species (H2S/SO2 > > 1) in gases below depths of 150--200 m. Accordingly, rapid release of gases from magma at such depths is a plausible cause of the high H2S/SO2 observed in Mount St. Helens plumes during explosive eruptions (Casadevall et ai., 1981; Hobbs et al., 1982). Near-surface degassing at high temperatures and low pressures favors SOs as the predominant sulfur species (Fig. 3). Hence, extrusions of dome lava will produce gases with a low H2S/SO: ( < ~ 1). Noneruptive degassing of magma in the dome and at depths shallower than 150--200 m in the feeder conduit will also produce gases with low H2S/SO2. Cooling and reequilibration of these gases with hydrothermal steam in the dry-out zone, however, will give rise to fumarole gases in which H2S is two to three times SO: (Table 1, Fig. 2).

158 It is n o t e w o r t h y that in the 1982 eruptions at E1 Chichon, H2S also appears to have predominated SO2 in the vapor phase of magma with an 02 fugacity above the Ni-NiO buffer (Luhr et al., 1984). In conclusion, this study leads us to stress the following general points concerning the degassing of dacite-andesite volcanic systems: (1) A significant magmatic volatile fraction will be present in fumaroles with temperatures above the cracking-temperature (i.e., the temperature at which thermal stresses induce cracking during cooling). Thus, the lava cracking-temperature offers a practical monitoring guide for readily estimating the magmatic volatile fraction from temperature measurements on fumaroles gases. Many more experimental data are needed, however, for the cracking-temperatures of appropriate magmas. (2) Current fumarole gas data for dacite-andesite volcanic systems may be biased towards residual magmatic volatiles. This concern arises because the gases from these systems are generally not collected during the more hazardous early stages of an eruptive cycle. The work reported here suggests that the initial stages of dacite-andesite eruptions may emit gases much richer in CO2 than is generally recognized, as was initially suggested by Tazieff {1970). (3) Rapid unloading of magma at pressures over 100 bars during explosive episodes of dacite-andesite eruptions may burden the atmosphere with gases that initially have a high H2S/SO2 ( > > 1), even though the magma involved may have an f02 up to an order of magnitude higher than the Ni-NiO buffer. Much attention of late has been given to the possible climatic significance of volcanic SO2 released during explosive volcanism {e.g., Pollack et al., 1983). We suggest that in explosive daciteandesite volcanism, this environmental impact may often require an intermediate step of post-eruption oxidation of H2S to SO2. Emissions of significant quantities of SO2 directly to the atmosphere may be largely restricted to the lower-pressure (and higher-temperature) shallow degassing of basaltic volcanism (Gerlach, 1982). ACKNOWLEDGMENTS We acknowledge E. Corazza, J.C. Eichelberger, R. Fournier, F. LeGuern, D.A. Swanson, and B.E. Taylor for critically reviewing this report. We also thank C.R. Carrigan and H.C. Hardee for discussions of the role of dry-out zones in shallow magma-hydrothermal systems. REFERENCES Barnes, I., 1984. Volatiles of Mount St. Helens and their origins. J. Volcanol. Geotherm. Res., 22: 133--146. Barnes, I., Irwin, W.P. and White, D.E., 1978. Global distributions of carbon dioxide discharges and major zones of seismicity. U.S. Geol. Surv., Water Res. Invest. 78-39, 12 pp.

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