Where are noble gases trapped in Yamato-74063 (unique) ?

概要

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Yamato-74063,a primitive achondrite with affinities to Acapulco, has been analyzed for noble gas isotopes by laser ablation mass spectrometry. Some grains released huge amounts of trapped Xe as well as trapped Ar and Kr. Among them, an orthopyroxene grain containing abundant inclusions of tiny metal spherules released equally large amounts of Xe as found for carbon residues of ureilites and carbonaceous chondrites, with ^<36>Ar/^<132>Xe=11. In contrast, other grains including silicates, Fe-Ni metal and troilite released negligible amounts of trapped gases. Mineralogical studies of polished thin sections, prepared from the same chip as was analyzed for noble gases, indicate that the silicate grains containing the tiny metal inclusions also contain abundant tiny voids, one to several μm across. Originally these voids must have been filled with gases or/and fluids. We propose bubbles as a candidate of hosts that trap large amounts of noble gases in the silicate. With data on noble gas behavior in bubbles, although available only for andesite melt, the extremely low ^<36>Ar/^<132>Xe ratio in the silicate phase of Y-74063 can be understood in terms of the bubble hypothesis.
国立極地研究所の論文