Northwest Africa 1950: Mineralogy and comparison with Antarctic lherzolitic shergottites

Abstract

NWA 1950 is a new lherzolitic shergottite recently recovered from Morocco and is the first sample of this group found outside Antarctica. Major constituent phases of NWA 1950 are olivine, pyroxenes, and plagioclase glass ("maskelynite") and the rock shows a two distinct textures: poikilitic and non-poikilitic typical of lherzolitic shergottites. In poikilitic areas, several-millimeter-sized pyroxene oikocrysts enclose cumulus olivine and chromite. In contrast, pyroxenes are much smaller in non-poikilitic areas, and olivine and plagioclase glass are more abundant. Olivine in non-poikilitic areas is more Fe-rich (Fa29-31) and shows a narrower distribution than that in poikilitic areas (Fa23-29). Pyroxenes in non-poikilitic areas are also more Fe-rich than those in poikilitic areas that show continuous chemical zoning suggesting fractional crystallization under a closed system. These observations indicate that pyroxene in non-poikilitic areas crystallized from evolved interstitial melts and olivine was re-equilibrated with such melts. NWA 1950 shows similar mineralogy and petrology to previously known lherzolitic shergottites (ALH 77005, LEW 88516, Y-793605 and GRV 99027) that are considered to have originated from the same igneous body on Mars. Olivine composition of NWA 1950 is intermediate between those of ALH 77005-GRV 99027 and those of LEW 88516-Y-793605, but is rather similar to ALH 77005 and GRV 99027. The subtle difference of mineral chemistry (especially, olivine composition) can be explained by different degrees of re-equilibration compared to other lherzolitic shergottites, perhaps due to different location in the same igneous body. Thus, NWA 1950 experienced a high degree of re-equilibration, similar to ALH 77005 and GRV 99027.