「岡山県荒戸山のアルカリ玄武岩中のざくろ石を含むハルツバージャイト捕獲岩（山元ら，１９９９）に対するコメント（阿部・荒井，２００１）」に対する回答

Abstract

In order to determine their Sr isotopic ratios, we have manually selected clinopyroxene grains taken from mantle peridotite xenoliths. The Arato-yama harzburgite sample was all crushed for hand-picking because it was too small to allow preparation of thin section. The garnet grains in this sample were found under a binocular microscope. The clinopyroxene of the above Arato-yama sample is relatively low in CaO and appears to have relatively high equilibrium temperature within the low temperature group peridotites (Fig. 2 of Yamamoto et al., 1999). The Arato-yama mantle peridotites as a whole, however, still belong to the low temperature group in the same way as do the Kurose, On-yama and Megata peridotites (Fig. 3 of Yamamoto et al., 1999). This result is consistent with the temperatures estimated by Arai and Hirai (1983) and Abe and Arai (2001) who applied the same geothermometer to the Noyama-dake (high temperature group), Kurose and Arato-yama mantle peridotites. Orthopyroxenes of the low temperature group peridotites (Arato-yama and other peridotites) are characteristically low in Al2O3. This suggests a different geothermal gradient from that of the Fukue-jima peridotites which include high Al2O3 orthopyroxenes even in shallow seated origin.