Yamato-793605: A new lherzolitic shergottite from the Japanese Antarctic meteorite collection

Abstract

Y-793605 is a new martian meteorite from Antarctica that can be classified as a lherzolitic shergottite. Y-793605 mainly shows a poikilitic texture (large pyroxene oikocryst with enclosed olivine and chromite), but partly contains non-poikilitic areas (mainly maskelynite, olivine, and pigeonite). Olivine in the non-poikilitic area is more Fe-rich and shows a narrower compositional distribution than that in the poikilitic area. Low-Ca pyroxenes in the non-poikilitic area are also more Fe-rich (En_Fs_Wo_7∿En_Fs_Wo_) than those in the poikilitic area (En_Fs_Wo_3∿En_Fs_Wo_). Augites in the poikilitic area are usually present rimming the oikocrysts (En_Fs_Wo_ to En_Fs_Wo_). The crystallization sequence of minerals in Y-793605 is considered to have begun by initial crystallization of cumulus phases (olivine and chromite) from the parent magma. Then, low-Ca pyroxenes and later augite poikilitically enclosed cumulus phases, and became a large oikocryst. Due to accumulation of phases, small interstitial melts formed between the oikocryst boundaries, and plagioclase crystallized from the Ca-Fe-rich melt along with pigeonite. After minor augite crystallization in the non-poikilitic area, all phases experienced re-equilibration (e. g., homogenization of olivine). Y-793605 shows a close relationship to previously known lherzolitic shergottites ALH77005 and LEW88516. Especially, olivine composition of Y-793605 is nearly identical to that of LEW88516. Pyroxene and maskelynite compositions are almost the same among these three meteorites. Although it is hardly possible to consider that Y-793605 is paired with ALH77005 or LEW88516 in the sense that Y-793605 was in the same fall with them, it can be concluded that Y-793605 originated from the same igneous body or rock in Mars as ALH77005 and LEW88516