Undegassed Magma Research Articles

Prediction of magmatic water contents via measurement of H2O in clinopyroxene phenocrysts

Water is fundamental to magma genesis, evolution, and eruption. Few direct measurements of magmatic H 2 O exist, however, because rocks found at the surface have extensively degassed upon eruption. Olivine-hosted melt inclusions provide a standard approach to measur ing volatiles in undegassed magma, but many volcanic deposits do not contain melt inclusions large enough for analysis (>30 μm), or olivine at all. Here we use an Al IV -dependent partitioning relationship to calculate magmatic H 2 O from direct measurements of H 2 O in clinopyroxene phenocrysts. We test this approach using phenocrysts from four arc volcanoes (Galunggung, Irazu, Arenal, and Augustine) that span the global range in H 2 O contents as measured in olivine-hosted melt inclusions (from 0.1 to 7 wt% H 2 O). The average and maximum magmatic H 2 O contents calculated from the clinopyroxene measurements agree within 15% of the melt inclusion values for most of the samples. The evolutionary paths recorded in H 2 O-Mg# variations overlap in some clinopyroxene and olivine-hosted melt inclusion populations, and in others, the clinopyroxenes record a larger portion of the liquid line of descent or a different portion of the magma system. Thus, the use of phenocrysts to estimate magmatic H 2 O contents creates a new and powerful tool in igneous petrology and volcanology.

Probing Stromboli volcano from the mantle to paroxysmal eruptions

Abstract We investigated the plumbing system of Stromboli volcano from the upper mantle to the surface. Thermobarometric estimates indicate that the deeper detected part of the plumbing system is located in the upper mantle, at approximately 34–24 km depth where, during their ascent, primitive Stromboli basalts (HKCA to shoshonitic) interact with peridotitic materials. In this region magma flow is probably channelled along fracture zones that may converge into a feeder dyke that crosscuts the Moho at about 17 km depth. During their ascent, basaltic magmas will interact with lower crust materials represented by cumulates of earlier Stromboli-type basalts at 13–10 km depth. This zone is also the section of the plumbing system where the feeder dyke is entering the chamber. Thermobarometric estimates, obtained by constructing a grid of selected reactions, indicate that current primitive Stromboli basalts equilibrate at 0.3–0.15 GPa and temperatures approaching 1200 °C, and progressively crystallize and degas before being erupted. Crystal size distributions on lavas and juvenile tephra erupted in 2002–2003 give very variable residence times. Based on average bubble distances, the estimated times for the exsolution of the gaseous phases range from 2–7 days to 45 min for the lavas and scorias, down to about 15 h to 12 min for the pumices erupted during paroxysmal explosions. Estimated syneruptive viscosities range from 10 2 Pa s for the anhydrous basaltic pumices at 1200 °C, to 10 3 –10 4 Pa s for lavas approaching their effusion temperatures (1100–1150 °C). In turn, viscosities for the hydrous basaltic melt that led to the formation of the basaltic pumices may be around 10 Pa s or lower. In the light of the above, we discuss the possible shapes and volumes of Stromboli magma chamber by considering a sphere, an ellipsoid (geometrically concordant with the regional stress distribution) and a feeder dyke, the last two being more likely. In the light of volcanological, structural and geophysical data on conduit thickness, we propose an alternative model that takes into account the volumes of recently erupted lavas. This model consists of a convective ellipsoidal magma chamber ‘injected’ by an active feeder dike of undegassed magma of higher temperature, lower density and lower viscosity. This dyke will evolve into a magma column inside the chamber and will separate the reservoir into two lateral, nearly symmetric convective regions. Crystallization would occur preferentially in the proximity of the wallrocks, particularly where the chamber is entering the conduit. The onset of paroxysmal explosions during major effusive cycles may be explained by a drastic increase in the intrusion rates at the base of the chamber that will produce a progressive inflation of the magma column dynamically transferred to the chamber walls. The ceasing of ‘anomalous’ intrusion rates at the base of the chamber, coupled with higher discharge rates, will progressively depressurize the chamber to a critical threshold, until the stress transferred to the walls is dynamically released: at this point the walls themselves will undergo a nearly instantaneous elastic rebound and contract in the attempt to recover their original pre-eruptive geometry. These dynamics will squeeze up portions of the undegassed magma column, triggering a paroxysmal explosion with the ejection of ‘golden pumices’.

Textural effects of steady state behaviour of the Stromboli feeding system

The exceptionally persistent activity of Stromboli volcano has lasted for at least 1400 years. The normal strombolian activity is periodically interrupted by more energetic explosions (1–2 per year) and by sporadic effusive episodes (every 10–20 years). Normal activity and effusive episodes are characterized by crystal-rich high-K to shoshonitic basalts issuing from a volatile-poor shallow system. Crystal-poor pumice are emitted only during more violent explosions, and are thought to derive from deep pulses of volatile-rich magma. Shallow level degassing induces massive crystallization of deep pulses of feeding magma that, continuously mixing with the resident one, produces the crystal-rich shoshonite of the persistent activity. We examined the crystallization history of the crystal-rich, shallow reservoir using plagioclase Crystal Size Distribution (CSD) analysis of scoriae and lavas emitted in the past twenty years. CSDs show a linear dependence from crystal size in the size interval 0.06–1.2 mm; number density of larger crystals is biased by right hand truncation effects. CSDs slopes and intercepts are quite constant during the whole considered time span revealing a system that is close to the equilibrium also from a kinetic point of view. The linear crystal size distribution are reached by the system through episodes of growth and resorption, respectively occurring in the degassed and undegassed magma during the continuous mixing in the feeding system. Plagioclase net growth rate (2 × 10 − 11 cm/s) results from a balance of growth (10 − 10 cm/s) and resorption episodes which induce spectacular zoning and resorption textures in crystals larger than 200 μm. CSDs of mafic phases cannot be accurately acquired on each single sample due to poor counting statistics; the evaluation of pyroxene and olivine CSD on the whole data set, however, confirms the conclusions acquired from plagioclase CSDs.

Geochemistry of rainfall at Stromboli volcano (Aeolian Islands): Isotopic composition and plume‐rain interaction

The chemical and isotopic compositions of the precipitation at Stromboli Island, Italy, were investigated between October 2003 and October 2005. We employed a rain gauge network designed to cover the range in exposures and elevations of the volcanic edifice. The hydrogen and oxygen isotopic ratios vary greatly on a seasonal basis and correlate with air temperature. Deuterium excess values show a positive correlation with altitude. No direct contribution of volcanogenic H or O is evident in the isotopic composition of the rainwater. The chemical composition of the rainwater is principally controlled by the sea aerosol contribution at the coastal sites, whereas it is significantly influenced by volcanic activity near the summit vents. Interaction with volcanic acid gases is indicated by the pH, which is usually 1–2 units lower near the craters than at the coastal sites. The S/Cl, Cl/F, and S/F molar ratios in rainwater 1.5 km from the craters are consistent with those measured in the volcanic plume using other methods (diffusive tubes and Fourier transform infrared spectroscopy). Rising of undegassed magmas changes these molar ratios because of the differential degassing of sulphur, chlorine, and fluorine from the magma. We therefore propose that the chemical composition of precipitation, within 1.5 km of the craters, provides additional information that is useful for monitoring volcanic activity at Stromboli Island. Moreover, this paper presents estimates of the fluxes of F, Cl, S, Na, K, Ca, and Mg to the soil that could be useful for geochemical studies on groundwater.

Volatiles in glasses from the HSDP2 drill core

H2O, CO2, S, Cl, and F concentrations are reported for 556 glasses from the submarine section of the 1999 phase of HSDP drilling in Hilo, Hawaii, providing a high‐resolution record of magmatic volatiles over ∼200 kyr of a Hawaiian volcano's lifetime. Glasses range from undegassed to having lost significant volatiles at near‐atmospheric pressure. Nearly all hyaloclastite glasses are degassed, compatible with formation from subaerial lavas that fragmented on entering the ocean and were transported by gravity flows down the volcano flank. Most pillows are undegassed, indicating submarine eruption. The shallowest pillows and most massive lavas are degassed, suggesting formation by subaerial flows that penetrated the shoreline and flowed some distance under water. Some pillow rim glasses have H2O and S contents indicating degassing but elevated CO2 contents that correlate with depth in the core; these tend to be more fractionated and could have formed by mixing of degassed, fractionated magmas with undegassed magmas during magma chamber overturn or by resorption of rising CO2‐rich bubbles by degassed magmas. Intrusive glasses are undegassed and have CO2 contents similar to adjacent pillows, indicating intrusion shallow in the volcanic edifice. Cl correlates weakly with H2O and S, suggesting loss during low‐pressure degassing, although most samples appear contaminated by seawater‐derived components. F behaves as an involatile incompatible element. Fractionation trends were modeled using MELTS. Degassed glasses require fractionation at p ≈ 5–10 bars. Undegassed low‐SiO2 glasses require fractionation at p ≈ 50 bars. Undegassed and partially degassed high‐SiO2 glasses can be modeled by coupled crystallization and degassing. Eruption depths of undegassed pillows can be calculated from their volatile contents assuming vapor saturation. The amount of subsidence can be determined from the difference between this depth and the sample's depth in the core. Assuming subsidence at 2.5 mm/y, the amount of subsidence suggests ages of ∼500 ka for samples from the lower 750 m of the core, consistent with radiometric ages. H2O contents of undegassed low‐SiO2 HSDP2 glasses are systematically higher than those of high‐SiO2 glasses, and their H2O/K2O and H2O/Ce ratios are higher than typical tholeiitic pillow rim glasses from Hawaiian volcanoes.

Volatiles in glasses from Mauna Loa Volcano, Hawai'i: implications for magma degassing and contamination, and growth of Hawaiian volcanoes

Glasses from Mauna Loa pillow basalts, recent subaerial vents, and inclusions in olivine were analyzed for S, Cl, F, and major elements by electron microprobe. Select submarine glasses were also analyzed for H2O and CO2 by infrared spectroscopy. The compositional variation of these tholeiitic glasses is dominantly controlled by crystal fractionation and they indicate quenching temperatures of 1,115–1,196 °C. Submarine rift zone glasses have higher volatile abundances (except F) than nearly all other submarine and subaerial glasses with the maximum concentrations increasing with water depth. The overwhelming dominance of degassed glasses on the submarine flanks of Mauna Loa implies that much of volcano's recent submarine growth involved subaerially erupted lava that reached great water depths (up to 3.1 km) via lava tubes. Anomalously high F and Cl in some submarine glasses and glass inclusions indicate contamination possibly by fumarolic deposits in ephemeral rift zone magma chambers. The relatively high CO2 but variable H2O/K2O and S/K2O in some submarine rift zone glasses indicates pre-eruptive mixing between degassed and undegassed magma within Mauna Loa's rift system. Volatile compositions for Mauna Loa magmas are similar to other active Hawaiian volcanoes in S and F, but are less Cl-rich than Lō'ihi glasses. However, Cl/K2O ratios are similar. Mauna Loa and Lō'ihi magmas have comparable, but lower H2O than those from Kilauea. Thus, Kilauea's source may be more H2O-rich. The dissimilar volatile distribution in glasses from active Hawaiian volcanoes is inconsistent with predictions for a simple, concentrically zoned plume model.

Radionuclides and sulfur content in Mount Etna plume in 1983–1995: new constraints on the magma feeding system

Radionuclide activities (210Pb, 210Bi, 210Po) were investigated in Mount Etna plume from 1983 to 1995. At SE crater the long-term observation (12years) of the 210Po/210Pb ratio shows that it can behave as a degassing vent not directly related to the main magma reservoir depending on the magma level inside the volcano. Since 1992, the simultaneous determination of radionuclides and sulfur in the main plume results in new constraints on the degassing model of Lambert et al. (Earth Planet. Sci. Lett., 76 (1986) 185). The 210Po/SO2 and 210Pb/SO2 ratios enable us to identify two sources of 210Po in the plume: one is magmatic, correlated with SO2, the other one is an additional component issued from the decay of 210Pb in the shallow degassing cell, and depends directly on the residence time of the gases before their emission. Estimations of the volume of degassing magma, the residence time of the gases and the proportion of undegassed magma renewing the shallow degassing cell are given for the period 1992–1995. During the 1992 eruption, the rate of degassing magma volume is estimated to have been as high as 5×106m3/day, and the volume of the shallow degassing magma reservoir about 0.5km3. In 1994 and 1995 the rate of non-erupted degassing magma volume was estimated to have been about 0.18km3/year. During the entire 1983–1995 period, only 15–20% of the degassed magma was erupted.

Sulfur and chlorine degassing from primitive arc magmas: temporal changes during the 1982–1983 eruptions of Galunggung (West Java, Indonesia)

The 1982–1983 eruptions of Galunggung represent a nine-month period of intermittent volcanic activity with significant changes in explosivity and emission of volatiles. Eruptions started with Vulcanian explosions but changed gradually to Strombolian activity. Compositions of juvenile material changed from basaltic andesite to high-Mg basalt, which are among the most primitive rock types known in the Indonesian arc system. Although bulk compositions suggest a single evolution trend, we infer from the compositions of melt inclusions in olivine phenocrysts that the magmas represent derivatives of a complex spectrum of primary melts. Primitive inclusions in olivine phenocrysts from magma erupted during the Strombolian phase contain up to ∼2000ppm sulfur, but concentrations decrease rapidly with increasing SiO2 down to matrix glass values (50–100ppm). ‘Vulcanian’ inclusions appear to be degassed before eruption (∼200ppm S). Chlorine concentrations increase from 750 to 2200ppm in Strombolian, and from 800 to 1500 in Vulcanian magmas, whereas matrix glass contains about 1000 ppm in both cases. Ash leachates show two cycles of decreasing S/Cl ratios: from 9.7 to 5.6 at the start of the activity, and from 12.2 to 2.0 after four months. As the second cycle follows upon increased seismic activity at shallow depth, it probably reflects degassing of fresh sulfur-rich magma arriving in the shallow Galunggung reservoir. In contrast to the degassed state of Vulcanian magma, the significant amounts of adsorbed sulfur on the ashes point to an excess source of sulfur, which was most likely derived from intruding Strombolian magma. Hence, the observed sulfur flux of 2Mt is not in accordance with a petrologic estimate of 0.09Mt. Using a published value of 550Mt of erupted material about 0.34km3 fresh undegassed magma is needed to account for the observed sulfur flux. This is close to the erupted volume of Vulcanian magma (0.26km3), which presumably was replaced completely by Strombolian magma during the eruption. Using the petrologic method, we calculate a total release of 0.3Mt chlorine, which agrees well with an output of 0.47Mt estimated independently from S/Cl ratios of the ash leachates and TOMS sulfur yields. Ash leachates show that about 35% of the sulfur and 30% of the chlorine was scavenged from the eruption plumes. Our results suggest that sulfur and chlorine were largely decoupled during degassing, which resulted in considerable variations in S/Cl ratios during the Galunggung eruptions. We infer that sulfur degassing reflects the arrival of fresh magma at shallow depth, whereas chlorine is largely derived from simultaneously erupted material. As a consequence, the petrologic estimates are more consistent with observed emissions for chlorine than for sulfur.

Degassing processes at Stromboli volcano inferred from short-lived disequilibria (210Pb–210Bi–210Po) in volcanic gases

Volcanic aerosols and gases released by three active craters at Stromboli volcano have been regularly collected since 1985. In this paper, we present new evidence of the high volatility of some nuclides among radon daughters (210Pb, 210Bi and 210Po), which are strongly fractionated, leading to significant radioactive disequilibria in volcanic exhalations. The very low volcanic activity in October 1996 allowed a separate sampling of each crater plume for the first time; remote sampling of the bulk plume were also performed. These data show that the chemical composition of volcanic aerosols remains constant within the first few hundred meters from their source vents, ensuring the validity of remote sampling when the activity does not allow one to approach the active craters. Moreover, it appears that there is no differentiation of gases from one crater to another suggesting that the geometry of the upper plumbing system of the volcano is rather simple, gases being directly emitted from a shallow magma chamber without significant cooling inside the edifice. On the basis of the assumption of a continuously replenished shallow magma reservoir in steady state, we propose a dynamic model of degassing accounting for the variations of radionuclide contents and ratios observed in the gas phase since 1985. This model allows us to relate these variations to changes in the magma chamber dynamics, namely the magma residence time inside the chamber and the escape time of gases from it, both parameters being closely linked to the volcanic activity. While gases are always emitted within a few hours after bubble nucleation, suggesting that the chamber is no deeper than a few hundred meters, magma residence time varies from less than 20days during eruptive periods (highly explosive or effusive periods) to more than 200days before the 1985 eruption. The latter figure is explained by the storage at shallow depth of a poorly renewed magma batch that fed the eruption. The variations of the residence time observed over the studied period suggest that the steady-state dynamics of the Stromboli magma chamber is sustained for rather short periods of a few months at most. On basis of the knowledge of the deep undegassed magma supply and the magma residence time, we estimate the volume of the shallow magma chamber at about 7±2×105m3. The good agreement of our results with previous estimates based on both geochemical and geophysical studies suggests that the measurement of radioactive disequilibria in the gas phase provides a robust tool for a deeper understanding of the volcanic activity and the magma dynamics beneath active volcanoes.

210Pb– 226Ra radioactive disequilibria in recent lavas and radon degassing: inferences on the magma chamber dynamics at Stromboli and Merapi volcanoes

U-series and especially 210Pb- 226Ra disequilibria have been measured in recently erupted lavas from the Stromboli (Aeolian Islands, Italy) and Merapi (Java, Indonesia) volcanoes. Both volcanoes display 230Th- 238U disequilibria (with a Th enrichment for Stromboli and a U enrichment for Merapi, respectively), strong 226Ra enrichments over 230Th (whereas 228Ra- 232Th equilibrium is ubiquitous), and a contrasted evolution of their ( 210Pb/ 226Ra) activity ratios calculated at the time of eruption. While these ratios are always close to the equilibrium value of 1 at Stromboli, they show significant variations at Merapi (from 0.75 to 1). It is suggested that the low ( 210Pb/ 226Ra) activity ratios result from continuous degassing of 222Rn, the precursor of 210Pb in the decay series, from a shallow magma chamber. A model assuming a physical steady-state of the shallow magma reservoir is proposed, linking the ( 210Pb/ 226Ra) ratio of the magma to the fraction of 222Rn lost and to the renewal rate (or residence time) of the magma in the shallow reservoir. The near equilibrium ( 210Pb/ 226Ra) ratios at Stromboli imply a high renewal rate of the shallow reservoir (in the range 0.7–3 yr −1)in agreement with its persistent explosive activity. The volume of the reservoir is estimated at 1.3 ± 1.2 × 10 7 m 3. In contrast, the variability of ( 210Pb/ 226Ra) ratios at Merapi is interpreted by successive phases of nearly closed-system evolution of the shallow magma chamber (with continuous Rn degassing) and episodes of reinjections and mixing of a deep undegassed magma in this reservoir. Its volume is estimated at 1.6 × 10 7 m 3.

Gondwana dispersion and Asian accretion: IGCP 321 final results volume : I. Metcalfe, ed I. Metcalfe, (A.A. Balkema), Gondwana dispersion and Asian accretion. IGCP 321 final results volume, ISBN 9054104465, 1999, (361p)

We present new 238U-230Th-226Ra-210Pb-210Po, 87Sr/86Sr and 143Nd/144Nd isotopic data of whole-rock samples and plagioclase separates from volcanic deposits of the 2006 and 2010 eruptions at Merapi volcano, Java, Indonesia. These data are combined with available eruption monitoring, petrographic, mineralogical and Pb isotopic data to assess current theories on the cause of a recent transition from effusive dome-building (2006) to explosive (2010) activity at the volcano, as well as to further investigate the petrogenetic components involved in magma genesis and evolution. Despite the significant difference in eruption style, the 2006 and 2010 volcanic rocks show no significant difference in (238U/232Th), (230Th/232Th) and (226Ra/230Th) activity ratios, with all samples displaying U and Ra excesses. The 226Ra and 210Pb excesses observed in plagioclase separates from the 2006 and 2010 eruptions indicate that a proportion of the plagioclase grew within the decades preceding eruption. The 2006 and 2010 samples were depleted in 210Po relative to 210Pb ((210Po/210Pb)i < 1) at the time of eruption but were variably degassed (69%–100%), with the degree of 210Pb degassing strongly related to sample texture and eruption phase. In good agreement with several activity monitoring parameters, 210Po ingrowth calculations suggest that initial intrusion into the shallow magma plumbing system occurred several weeks to a few months prior to the initial 2010 eruption. The 2006 and 2010 samples show a wide range in (210Pb/226Ra) activity ratio within a single eruption at Merapi and are largely characterised by 210Pb deficits ((210Pb/226Ra) < 1). Assuming a model of complete radon degassing, the 210Pb deficits in the 2006 volcanic rocks indicate relatively longer degassing timescales of ∼2–4 years than those given by the 2010 samples of ∼0–3 years. The uranium-series and radiogenic isotopic data do not support greater crustal assimilation of carbonate material as the explanation for the more explosive behaviour of Merapi in 2010 (as has been previously suggested) and instead indicate that relatively rapid ascent of a more undegassed magma was the primary difference responsible for the transition in explosive behaviour. This interpretation is in good agreement with gas monitoring data, previous petrological studies (mineral, microlite and melt inclusion work) and maximum calculated timescale estimates using Fe-Mg compositional gradients in clinopyroxene, that also suggest more rapid movement of relatively undegassed magma in 2010 relative to 2006.

The volatiles record of a “popping” rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles

The popping rock dredged at 13.5°N and 3770 m deep on the Mid Atlantic Ridge by the ship Akademik Boris Petrov (sample 2πD43), has the largest gas content so far reported in MOR basalts. Given its vesicularity of 17% and gas chemical composition, this rock contained, when erupted, 7.56 cm 3 STP g −1 of gas, most of which is CO 2. The corresponding carbon concentration is 3850 ppm, mostly contained in the vesicles. The water content, mostly from the glass, is 5400 ppm, giving a CO 2H/ 2O molar ratio of 1.07. The initial helium and argon contents were 2.46 and 1.80 10 −4 cm 3 STP g −1 respectively. The average chemical composition of the vesicle gas is CO 2 = 94.95%, H 2O = 4.93%, N 2 = 0.12% and He = 32.6 ppm, Ar = 23.9 ppm, H 2 = 28.8 ppm, CO = 7.2 ppm and CH 4 = 1.8 ppm. Because of its very low degree of outgassing and very typical MORB rare gas isotopic composition [3] ( 3He/ 4He= 8 R a , 40Ar/ 36Ar up to 28,000 ), this rock provides probably the best sample so far of upper mantle gas, with negligible (3–7%) atmospheric and seawater contamination ( 40Ar/ 36Ar> 6000 in all aliquots). The isotopic compositions of carbon and nitrogen are −3.68‰ PDB, and −3.54‰ ATM, respectively. The average C/N ratio is 400, similar to the lower end of the diamond range (400–10,000). The ratios C/He of 29 000 ( C/ 3He= 2.7 10 9 ) and He/Ar of 1.36 are similar to the computed radioactive production ratios for the upper mantle. However, within this single sample significant variations of these ratios occur, and are more or less related to the vesicle size: a factor of 2 for C He, 40% for He Ar and 30% for C N, probably reflecting outgassing effects. Whether it is considered as a concentrate of MORB volatiles at the top of a magma column or chamber, or as a virtually undegassed magma sample, this sample provides new estimates of upper mantle isotopic compositions: δ 13C around −4‰ PDB, significantly higher than previous estimates, and δ 15N around −3.5‰ ATM, a little higher than the average of ordinary diamonds sampled so far. If the sample's carbon ( ∼ 4000 ppm) and nitrogen ( ∼ 12 ppm) contents are representative of primary upper mantle magmas, they correspond to mantle concentrations of ∼ 300 and 1 ppm respectively, and suggest a mantle carbon flux around 1.5 10 13 mole y −1.

"Variations in trace metal and halogen ratios in magmatic gases through an eruptive. Cycle of the Pu'u O'o Vent, Kilauea, Hawaii: July–August 1985""

Particle and gas samples were obtained before and after eruptive episode 35 in July and August 1985 at the fuming Pu'u O'o vent, Kilauea volcano, Hawaii. The sampling system employed consisted of a particle filter followed by four 7LiOH treated filters to collect acidic gases. The filters were analyzed using instrumental neutron activation analysis (INAA). The results indicate that Br/Cl and Re/Cl ratios do not fluctuate through an eruption cycle but the F/Cl, F/Br and metal/Cl ratios (In and Cd) do change through the cycle. An inverse relationship between F/Cl and metal/Cl was observed. The changes are probably due to influxes of relatively undegassed magma during the repose period releasing fume with lower F/Cl, F/Br and higher metal/Cl ratios. As the magma in the Pu'u O'o conduit gradually degasses either before or several days after an eruptive episode, F/Cl and F/Br ratios increase and the metal/Cl ratios decrease. One sample collected on July 24, two days before eruptive episode 35, did not follow this general trend. This can be explained by a gas pulse from a deeper, less degassed portion of magma making its way to the top of the conduit.

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

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 CO 2 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 2S 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 2S/SO 2 observed in Mount St. Helens plumes during explosive eruptions. This study suggests that dacite-andesite volcanos may emit gases richer in CO 2 during the earlier episodes of an eruptive cycle and burden the atmosphere with much more H 2S than SO 2 during explosive eruptions.