Corundum and zincian spinel from the Kakkonda geothermal system, Iwate Prefecture, northeastern Japan

概要

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Corundum and zincian spinel were found in a drill core of thermally metamorphosed rock at 2843 m in depth of the deep research well, WD-1, in the Kakkonda geothermal area, northeastern Japan. Corundum is surrounded by K-feldspar, and the occurrence suggests that corundum was formed by the decomposition of muscovite. Andalusite occurs in the same sample, without sillimanite. The petrography and previous works on phase relations between aluminous minerals suggest that corundum was formed at 600-700°C and 100 MPa at the peak of contact metamorphism. The ZnO content of spinel exceed 20 wt.%, and has a gahnitic composition. Zincian spinel is surrounded by K-feldspar as well as corundum, and it is assumed that spinel was formed by a similar reaction to the case of corundum. Two-phase vapor-rich and liquid-rich fluid inclusions occur in corundum, and they probably represent fluids formed at the metamorphic stage. A laser Raman microprobe analysis detected the presence of CO2, H2S and N2 in the fluid inclusions. The occurrence and analyses of the fluid inclusions indicate that the fluid produced during the thermal metamorphism at Kakkonda was low-salinity with gases and water.
日本鉱物科学会の論文