TY - JOUR

T1 - Kinetic vs. thermodynamic control of degassing of H2O-S±Cl-bearing andesitic melts

AU - Fiege, Adrian

AU - Behrens, Harald

AU - Holtz, François

AU - Adams, Franziska

N1 - Funding Information: This project was supported by the German Science Foundation (BE1720/25-1) and the German National Academic Foundation. We thank Bruno Scaillet, Roberto Moretti and J. Maarten de Moor for their valuable and constructive reviews that helped to improve the manuscript. Finally, we would like to thank Otto Dietrich (preparation), Eric Wolff (EMP), and Jörg Göttlicher (XANES). Copyright: Copyright 2013 Elsevier B.V., All rights reserved.

PY - 2014/1/15

Y1 - 2014/1/15

N2 - The sulfur (S) distribution between andesitic melts and fluids, both under near-equilibrium conditions and during fast decompression (disequilibrium conditions), has been experimentally investigated. Isothermal decompression experiments were conducted at ~1030°C and variable oxygen fugacity (fO2; log(fO2) from ~QFM+0.8 to ~QFM+4.2; QFM=quartz-fayalite-magnetite buffer) in internally heated pressure vessels (IHPV) using synthetic, H2O- and S-bearing andesitic melts (~4-8wt% H2O, ~140-2700ppm S). Selected glasses were doped with chlorine (Cl; 500-1000ppm) to study the influence of Cl on S partitioning. The starting pressure varied from 300 to 500MPa, and pressure (p) was released continuously to reach 150, 100, 70, or 30MPa. The decompression rate (r) ranged from 0.0005 to 0.17MPa/s and samples were either directly quenched to preserve disequilibrium conditions or annealed for various times (annealing time (tA)=1-72h) at final p and 1030°C to achieve near-equilibrium conditions.The directly quenched experiments revealed a strong increase of the S(fluid)/S(melt) ratio (S(fluid)=wt% S in the fluid; S(melt)=wt% S in the melt) with increasing r, from ~30 at 0.02MPa/s to ~300 at 0.2MPa/s at oxidizing conditions (log(fO2)>QFM+3), i.e., when sulfate (S6+) was the only S species. After fast decompression (~0.1MPa/s) subsequent annealing for ≥5h resulted in a decrease of S(fluid)/S(melt) by a factor of ~6, indicating that part of the S present in the fluid at tA=0h diffuses back into the melt during annealing. This behavior is explained by the positive correlation between p and sulfate solubility in aqueous fluids. In contrast to oxidizing conditions, the S content in the melt remained almost constant with varying r and was independent of tA at low fO2 (QFM+1 to QFM+1.5), when sulfide (S2-) became abundant. Thus, the different behaviors of S2- and S6+ during kinetically-controlled degassing need to be considered when modeling the volatile release of ascending magma.The addition of >500ppm Cl to the system slightly increased the S(fluid)/S(melt) under near-equilibrium conditions by a factor of ~2 at QFM+1.8. Furthermore, Cl(fluid)/Cl(melt) (wt% Cl in fluid/wt% Cl in melt) showed a positive correlation with initial Cl content of the melt and ranged from 1 to 13, largely independent of r and tA. The interaction between S and Cl as well as the dependence of Cl(fluid)/Cl(melt) on Cl content in the melt may have a significant influence on S/Cl ratios in volcanic gasses.

AB - The sulfur (S) distribution between andesitic melts and fluids, both under near-equilibrium conditions and during fast decompression (disequilibrium conditions), has been experimentally investigated. Isothermal decompression experiments were conducted at ~1030°C and variable oxygen fugacity (fO2; log(fO2) from ~QFM+0.8 to ~QFM+4.2; QFM=quartz-fayalite-magnetite buffer) in internally heated pressure vessels (IHPV) using synthetic, H2O- and S-bearing andesitic melts (~4-8wt% H2O, ~140-2700ppm S). Selected glasses were doped with chlorine (Cl; 500-1000ppm) to study the influence of Cl on S partitioning. The starting pressure varied from 300 to 500MPa, and pressure (p) was released continuously to reach 150, 100, 70, or 30MPa. The decompression rate (r) ranged from 0.0005 to 0.17MPa/s and samples were either directly quenched to preserve disequilibrium conditions or annealed for various times (annealing time (tA)=1-72h) at final p and 1030°C to achieve near-equilibrium conditions.The directly quenched experiments revealed a strong increase of the S(fluid)/S(melt) ratio (S(fluid)=wt% S in the fluid; S(melt)=wt% S in the melt) with increasing r, from ~30 at 0.02MPa/s to ~300 at 0.2MPa/s at oxidizing conditions (log(fO2)>QFM+3), i.e., when sulfate (S6+) was the only S species. After fast decompression (~0.1MPa/s) subsequent annealing for ≥5h resulted in a decrease of S(fluid)/S(melt) by a factor of ~6, indicating that part of the S present in the fluid at tA=0h diffuses back into the melt during annealing. This behavior is explained by the positive correlation between p and sulfate solubility in aqueous fluids. In contrast to oxidizing conditions, the S content in the melt remained almost constant with varying r and was independent of tA at low fO2 (QFM+1 to QFM+1.5), when sulfide (S2-) became abundant. Thus, the different behaviors of S2- and S6+ during kinetically-controlled degassing need to be considered when modeling the volatile release of ascending magma.The addition of >500ppm Cl to the system slightly increased the S(fluid)/S(melt) under near-equilibrium conditions by a factor of ~2 at QFM+1.8. Furthermore, Cl(fluid)/Cl(melt) (wt% Cl in fluid/wt% Cl in melt) showed a positive correlation with initial Cl content of the melt and ranged from 1 to 13, largely independent of r and tA. The interaction between S and Cl as well as the dependence of Cl(fluid)/Cl(melt) on Cl content in the melt may have a significant influence on S/Cl ratios in volcanic gasses.

UR - http://www.scopus.com/inward/record.url?scp=84888027005&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2013.10.012

DO - 10.1016/j.gca.2013.10.012

M3 - Article

AN - SCOPUS:84888027005

VL - 125

SP - 241

EP - 264

JO - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

SN - 0016-7037

ER -