Upper mantle can in-situ fracture: an implication from a cataclastic peridotite xenolith from Megata, Northeast Japan arc

作者：Takeuchi Miyuki^*; Arai Shoji

来源：Mineralogy and Petrology, 2015, 109(2): 283-294.

DOI：10.1007/s00710-015-0365-7

A peculiar peridotite xenolith with cataclastic texture was found from Ichinomegata crater, Megata volcano, the Northeast Japan arc. This peridotite xenolith is the same in mineral assemblage and mineral chemistry (olivine, Fo(90); spinel, Cr/(Cr + Al) atomic ratio, 0.2) to some fertile mantle lherzolites, but quite different in texture from all the documented mantle peridotite xenoliths from Ichinomegata and other localities on Earth. The peridotite is a mixture of coarse and fine grains of olivine, orthopyroxene, clinopyroxene, chromian spinel. The coarse mineral grains are angular but never elongated like those in mylonite. All minerals have been fragmented, and the fragmented pyroxenes and chromian spinel form thin streaks in fine-grained olivine-rich matrix. These features indicate that brittle fracturing does occur even in the upper mantle possibly along pre-existing deep-seated faults that have been frequent in the Northeast Japan arc. Some of the fine grains (< 100 mu m across) of olivine and pyroxenes display strong intra-grain and inter-grain chemical variations; some are more refractory and the others are more evolved in chemistry than the coarse grains. This suggests a possibility of very small degree frictional melting of peridotite upon cataclastic fracturing.

• 出版日期2015-4