著者

Keiko MATSUMOTO Michihiko NAKAMURA

出版者

一般社団法人日本鉱物科学会

雑誌

Journal of Mineralogical and Petrological Sciences (ISSN:13456296)

巻号頁・発行日

vol.107, no.5, pp.206-211, 2012 (Released:2012-11-17)

参考文献数

23

被引用文献数

3

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Pyrrhotite (Po) occurs as inclusions and as isolated crystals in pumice from the 1914-15 eruption of the Sakurajima volcano, Kyushu, Japan. The Po crystals have partly reacted to form spongy Fe oxides. A similar texture has been reported in some previous studies (Hattori, 1993), but the mineral phases and formation processes of the spongy Fe oxides have not been clarified. Our quantitative and compositional map analyses with electron probe microanalysis (EPMA) reveal that the spongy Fe oxides are mostly magnetite (Mt), with a thin rim (<3 μm) of hematite on rare occasions. The spongy texture includes unreacted regions of Po, mesh-like pores, and S-rich spots, showing that it was formed by desulfidation of Po. No Ti was detected, even in the outermost rim; this indicates that the reaction occurred syn-eruptively. According to diffusion calculations, the spongy Mt was formed during the 4 h preceding quenching. Thermodynamic calculations showed that Po is stable at log fO2 < NNO + 2 at a pressure of 1 bar and magmatic temperature, which is 1-2 log units higher than the usual magmatic fO2. These constraints on the timing and oxidation condition of desulfidation lead to the conclusion that the reaction was caused by oxidation of the magma in a shallow volcanic conduit, not in magma chamber processes. The pumice groundmass consists mostly of glass, indicating that the rate of the desulfidation reaction is faster than the decompression-induced crystallization of microlites in the andesitic magma. Therefore, the desulfidation reaction of Po has the potential to be used as a geospeedometer for very fast magma ascent in vigorous explosive eruptions.