S-CO2-H2O-Cl partitioning and mixing in rhyolitic melt and fluid - Implications on S degassing in rhyolite
Abstract
Magmatic degassing involving multiple volatile components (C, O, H, S, Cl, etc.) is one of the key factors influencing the timing and nature of volcanic eruptions, and the chemistry of volcanic gases released to the surface. In particular, knowledge of sulfur behavior in the felsic magmatic system in the crust is essential to understand the ore deposit formation, to evaluate potential environmental impacts of volcanic eruptions, and to estimate budgets of other volatiles. Previous experiments have shown that S solubility is limited by the sulfide and anhydrite saturation in the FeO- and CaO-bearing system, depending on the fO2 [1-3], however, quantitative evaluation of S partitioning between felsic magma and fluid remains ambiguous due to the lack of experiments on high silica magma with extremely low FeO and CaO at various fO2.
Rhyolite-fluid equilibria experiments were conducted in an internally heated pressure vessel at 200-300 MPa and 800 ̊ C to determine the COHS±Cl fluid-melt partitioning of S in the oxygen fugacity (fO2) range of FMQ-0.4 to FMQ+2.5. Rhyolite dissolved 80-1600 ppm S and 3.2-6.6 wt.% H2O, varying with melt composition and fO2. The integrated bulk fluid contains 1-4 mol% S, up to 91 mol% H2O, 47 mol% CO2, and 1mol% Cl. Molar aluminosity of the rhyolite varies between 0.65-1.0. At pressure of 300 MPa, S concentrations and S partition coefficients in the meta-aluminous rhyolite vary with fO2 following an "U" pattern with the lowest S content in the melt and highest partition coefficient around FMQ+0.85. At more reduced (<FMQ) and more oxidized conditions (>FMQ+1), S contents in the meat-aluminous rhyolite decreases as H2O concentration in the melt increases. No effect of the addition of CO2 and Cl is observed. At a given reduced fO2, rhyolite with lower molar aluminosity can dissolve more sulfur (up to 1600 ppm) and derive lower sulfur partition coefficient. The experimental data can be applied to massive rhyolitic eruptions of Bishop tuff, Toba tuff and Pinatubo to understand the volatiles degassing. The enhanced sulfur concentration in the melt due to melt composition at relatively reduced condition could be used to imply magmatic conditions in favor of ore deposit formation. [1] Clemente et al., 2004, J. Petrology; [2] Huang and Keppler, 2015, American Minerologist.; [3] Binder et al., 2018, CMP.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V11F0072D
- Keywords:
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- 1043 Fluid and melt inclusion geochemistry;
- GEOCHEMISTRYDE: 3630 Experimental mineralogy and petrology;
- MINERALOGY AND PETROLOGYDE: 8430 Volcanic gases;
- VOLCANOLOGYDE: 8488 Volcanic hazards and risks;
- VOLCANOLOGY