Petrology of the Yamato 980459 shergottite

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Y980459 is a new type of shergottite, consisting mainly of olivine, orthopyroxene with augite mantles, and glassy groundmass. The constituent minerals crystallized under a rapid cooling condition. The cores of olivine phenocrysts are Fo85-73, and Fo85 is the most magnesian composition among all martian meteorites. The magnesian olivine may be mostly phenocrysts which crystallized from the parent magma having the Y980459 bulk composition. Many magmatic inclusions occur in olivine phenocrysts around Fo74. They consist mainly of glass, and the trapped melt composition was estimated from the inclusion glass by addition of the wall olivine. The trapped melt represents the early- to middle-stage melt of the Y980459 crystallization sequence. The cores of pyroxene are orthopyroxene, which are surrounded by augite mantles showing remarkable normal zoning. The glassy groundmass contains quenched crystals of ferroan olivine and clinopyroxenes set in a glassy groundmass. Shock-melt pockets occur in Y980459, and the shock degree, which was originally defined for ordinary chondrites (D. Stoffler et al.; Geochim. Cosmochim. Acta, 55, 3845, 1991), corresponds to S3 for Y980459. A new stage classification for lithological types of the shergottite group is proposed, and the Y980459 meteorite is classified as an olivine-bearing glassy basaltic type.
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