Kinetic vs. thermodynamic control of degassing of H2O–S ± Cl-bearing andesitic melts
Kinetic vs. thermodynamic control of degassing of H2O–S ± Cl-bearing andesitic melts
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H2OâS ± Cl 含安山岩熔体脱气的动力学与热力学控制
DOI：
10.1016/j.gca.2013.10.012
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发表时间：
2014
影响因子：
5
通讯作者：
Adams F.
中科院分区：
文献类型：
--
作者：
Fiege A;Behrens H;Holtz F;Adams F.
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 (∼4–8 wt% H2O, ∼140–2700 ppm S). Selected glasses were doped with chlorine (Cl; 500–1000 ppm) to study the influence of Cl on S partitioning. The starting pressure varied from 300 to 500 MPa, and pressure (p) was released continuously to reach 150, 100, 70, or 30 MPa. The decompression rate (r) ranged from 0.0005 to 0.17 MPa/s and samples were either directly quenched to preserve disequilibrium conditions or annealed for various times (annealing time (tA) = 1–72 h) at finalpand 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 increasingr, from ∼30 at 0.02 MPa/s to ∼300 at 0.2 MPa/s at oxidizing conditions (log(fO2) > QFM+3), i.e., when sulfate (S6+) was the only S species. After fast decompression (∼0.1 MPa/s) subsequent annealing for ≥ 5 h resulted in a decrease of S(fluid)/S(melt)by a factor of ∼6, indicating that part of the S present in the fluid attA= 0 h diffuses back into the melt during annealing. This behavior is explained by the positive correlation betweenpand sulfate solubility in aqueous fluids. In contrast to oxidizing conditions, the S content in the melt remained almost constant with varyingrand was independent oftAat lowfO2(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 >500 ppm 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 ofrandtA. 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.
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