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

Abstract

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.