A textural examination of the Yamato 980459 and Los Angeles shergottites using crystal size distribution analysis

Abstract

The basaltic shergottite group is the most plentiful of the Martian meteorite types. Within that compositional category are three distinct textural groups, each suggesting distinct crystallization histories. We present results of a textural study, using crystal size distribution (CSD) analysis, of Yamato (Y) 980459 and Los Angeles, the most primitive and evolved shergottites respectively, and we compare these results to previous CSD work on basaltic shergottites. Y980459 resembles picritic shergottites (e.g. DaG 476), with large zoned olivine set in a groundmass dominated by orthopyroxene. It is unique in having a glassy mesostasis with dendritic olivine and pyroxene, rather than maskelynite. Los Angeles resembles other co-saturated shergottites (e.g. QUE 94201) with a subophitic intergrowth of zoned clinopyroxene and maskelynite. CSD results show Y980459 pyroxenes grew in one stage of steady-state nucleation and growth, cooling at 3-7°C /hr. A CSD of the olivine population suggests slower cooling rates during megacryst formation with an increase during groundmass olivine growth, probably reflecting magma ascent. A CSD plot of Los Angeles pyroxenes shows a smooth downward curvature, also noted in previous analyses of QUE 94201 and EETA79001B. The plot reflects co-crystallization of plagioclase and pyroxene, and supports a single continuous interval of growth