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Petrologic and sulfur isotopic significance of highly oxidized and sulfur-rich magma of Mt. Pinatubo, Philippines

¹ Geological Institute, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
² Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113, Japan

Dacitic pumices from pyroclastic-flow deposits and air-fall tephra of the June 14-15,1991, eruption of Mt. Pinatubo, Philippines, are characteristically rich in sulfur. The presence of microphenocrystic anhydrite suggests that sulfur existed mostly as oxidized species in the magma. This supposition is corroborated by unusually high sulfur contents (up to 0.78 wt% as SO₃) in apatite microphenocrysts and apatite inclusions in other phenocrystic minerals and by the highly oxidized state of the magma, near or slightly below the hematite-magnetite buffer, on the basis of extrapolation from biotite compositions and the two-oxide method. This highly oxidized state possibly caused the extraordinarily high sulfur content of Mt. Pinatubo dacite through prohibition of sulfide fractionation and because of increasing solubility of sulfur as oxidized species with increasing oxygen fugacity. Hornblende geobarometry indicates a pressure of 2 kbar for phenocryst formation. Among the two pumice types, cummingtoniterimmed hornblende phenocrysts are typically found in type 1 (white and crystal-rich) pumice, whereas rare hypersthene has been observed as discrete microphenocrysts in type 2 (tan and crystal-poor) pumice. These observations indicate conditions at least near if not at vapor saturation in type 1 magma and undersaturation in type 2 magma prior to eruption. Vapor saturated type 1 dacitic magma probably caused the explosive June 14-15,1991, eruption. The highly oxidized condition and magmatic water saturation are similar to those of intrusions related to porphyry copper deposits.

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