The Stability of Igneous Anhydrite: Experimental Results and Implications for Sulfur Behavior in the 1982 El Chichon Trachyandesite and Other Evolved Magmas |
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| Authors: | CARROLL MICHAEL R; RUTHERFORD MALCOLM J |
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| Institution: | Department of Geological Sciences, Brown University Providence, Rhode Island 02912 |
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| Abstract: | Hydrothermal experiments on natural samples of trachyandesiteand dacite bulk composition show that anhydrite (CaSO4) mayoccur as a stable phenocryst phase at oxygen fugacities greaterthan or equal to 1.0 to 1.5 log fO2 units above the Ni–NiOequilibrium. The dissolved sulfur concentration in anhydritesaturated melts from MnO–Mn3O4 buffered experiments decreaseswith decreasing temperature, from approximately 2300 p.p.m.Sat 1025?C to 250 p.p.m.S at 850?C (all at 2 kb Pfluid = Ptotal).In FeS-saturated melts equilibrated at the Ni–NiO bufferand 2 kb pressure, the concentration of dissolved sulfur alsodecreases with decreasing temperature, varying from approximately400 p.p.m. S at 1025?C to less than 100 p.p.m. S at 850?C. AtNNO or lowerfO2s, decreasing melt FeO content due to crystalfractionation may explain the observed decrease in sulfur solubilitywith decreasing temperature. Sulfur solubility values equivalent to the approximately 0.6wt. per cent S present in fresh bulk pumice samples from the1982 eruptions of El Chichon volcano are not readily achievedunder any reasonable combinations of pressure, temperature,and oxidation state. Dissolved sulfur contents approaching 0.6wt. per cent might occur if the source regions of melts parentalto the El Chichon trachyandesite were at an fO2 several logunits above the Ni–NiO equilibrium. Because such elevatedoxidation states are far greater than the generally acceptedvalues for mantle-derived partial melts we believe the highsulfur content of the El Chichon pumices is not a primary feature;it reflects reaction with sulfur enriched material at some unknowndepth beneath the volcano. Published sulfur isotopic and petrologicdata suggest that hydrothermally altered rocks similar to thepyrite- and anydritebearing lithic fragments found in the 1982pumices could have provided a source of sulfur for crystallizationof magmatic anhydrite. The anhydrite was an important sourceof sulfur for evolution of a sulfur-rich vapor phase duringeruption of the magma. Although many calc-alkaline dacites and rhyolites appear toattain oxidation states high enough to stabilize anhydrite,the characteristically low bulk sulfur contents of these rockswill limit anhydrite abundances to less than approximately 0.1wt. per cent, assuming sufficient sulfur is present to achievesaturation. Such small amounts of a water soluble mineral couldbe easily removed by subaerial weathering processes, dissolvedduring vapor exsolution from a magma, or simply overlooked duringroutine petrographic examination. |
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