T241 Experimental and Petrologic Investigation of Halogens, Sulfur, and Other Volatile Species in Igneous Systems: In Honor of Jim Webster

Friday, 30 October 2020
10:00 AM - 2:00 PM
Meeting Rooms

MODELING S DEGASSING DURING MAGMA ASCENT

Paper number: T241-5
Program: Technical Sessions
Time: 10:50 AM – 11:05 AM

Authors

Abstract

The degassing of CO2 and S from arc volcanoes is fundamentally important to global climate, eruption forecasting, and cycling of volatiles through subduction zones. While the behavior of CO2-H2O is relatively well understood from thermodynamic/empirical models1, significant discrepancies exist in predicting the behavior of sulfur. In this study, we aim to develop a degassing model to track the evolution of volatiles and redox state in melt and co-existing vapor in ascending mafic-intermediate magma.

This degassing model takes an approach similar to previous studies2, by combining an existing H2O-CO2 degassing model1 with sulfur partitioning between vapor and silicate melt (KdSfluid-melt), assuming the magma is not sulfide or sulfate saturated. With known H2O, CO2 in the melt and vapor, and initial S in the melt, S in the vapor can be calculated from KdSfluid-melt. Mass fractions of vapor, melt and crystals are constrained using mass balance of H2O or CO2, and K2O as a perfectly incompatible element in both crystals and vapor. Sulfur evolution in the melt can then be tracked along the degassing path by mass balance.

However, because sulfur exists as S2- and S6+ in the melt and H2S and SO2 in the vapor, it is difficult to develop a bulk KdSfluid-melt that applies over the range of fO2 encountered in arc magmas. We thus consider separate Kds for the four relevant reactions: I. S2-(m)→H2S (v), Ia. S2- (m)→SO2 (v), II. SO42-(m)→SO2 (v), and IIa. SO42-(m)→H2S (v). We examined experimental studies relevant to parental arc magmas and parameterized Kd(I), Kd(Ia), and Kd(II) as functions of T, P, fO2, H2O and melt composition, and also used constraints on H2S/SO2 calculated from the equilibrium gas reaction H2S + (3/2)O2 → SO2 + H2O. There are no experiments that constrain Kd(IIa), but it can be calculated from the three other Kds, as can the Kdtotal Sfluid-melt. Applying the Kds to the 1974 eruption of Fuego gives between 20-80, agreeing with that derived from fitting the Fuego melt inclusion CO2-S-H2O data trend3. The advantage of considering all four S reactions is that we can track fO2, S isotopes and Fe3+/FeT using the S6+ and S2- in the melt4. We plan to develop such an open source model that can be applied broadly to degassing magmas.

1. Ghiorso and Gualda (2015), CMP; 2. Sisson and Layne (1993) EPSL; 3. Rasmussen et al. (2020), Am.Min., in press. 4. Nash et al. (2019) EPSL.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-357963

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