A combined ToF-SIMS and EMP/SEM study of a three-phase symplectite in the Los Angeles basaltic shergottite

Abstract

A rare three-phase symplectite consisting of Ca-rich pyroxene, Fe-rich olivine, and a silica phase is frequently found rimming pyroxene in the Martian meteorite Los Angeles. This assemblage is usually interpreted as the breakdown product of metastable pyroxferroite, a very rare pyroxenoid mineral itself. However, its origin is not entirely understood, mainly because the extremely small average size of the constituent phases represents a challenge for precise high-resolution analysis. In addition to electron microbeam methods, the present study uses time-of-flight secondary ion mass spectrometry (ToF-SIMS) to overcome the limits of spatial resolution and to comprehensively study this mineral assemblage. The prevailing breakdown hypothesis is supported by the following results: (1) The three symplectite phases are very homogenous in composition from 100 μm down to the micrometer scale. (2) The silica phase could be shown to be almost pure SiO2. (3) The symplectite bulk composition is consistent with pyroxferroite. Sub micrometer sized Ti-oxide grains are found within the symplectite (but not within the Ca-rich pyroxene) and probably represent a minor breakdown phase in addition to the three main phases.