Los Angeles: A tale of two stones

Abstract

We compare and contrast the mineralogy and petrology of the 2 stones of the extremely ferroan and Cr-poor martian meteorite, Los Angeles. The 2 stones are similar in many characteristics, strongly suggesting that they originated from a single flow or shallow intrusion. However, stone 2 is more ferroan and enriched in late-stage materials than its larger, and more widely studied, sibling. Stone 2 has a far higher abundance (~25 vol%) than stone 1 (10 vol%) of combined opaques, meaning not only conventional opaque minerals but also, and more abundantly, fine-grained symplectitic intergrowths of fayalite + ferroan augite + silica (interpreted as pyroxferroite breakdown material, PBM). The bulk composition of the PBM is close to that of stoichiometric pyroxferroite, with roughly 45 wt% FeO. Extensive zonation within the pyroxenes of both stones is consistent with origin by closed-system fractional crystallization of the parent basaltic melt(s). However, the compositional and modal disparity between the two stones suggests that they formed in an environment where at least mild multi-cm-scale differentiation occurred. Probably, in both stones, crystallization began from similar melts with mg ~27-28 mol%, but during crystallization, significant migration of the melt component occurred, perhaps by crystal settling and/or filter pressing. Stone 2 acquired an enhanced proportion of residual melt and, thus, higher proportions of late-stage materials such as PBM, oxides, and phosphates. Within the PBM, clinopyroxene poikiloblastically encloses fayalite and silica. At least some of the PBM had already formed by decomposition of pyroxferroite before the major shock that caused the very scarce brecciation within Los Angeles. However, the low abundance of fractures within PBM, in comparison to pyroxene and some other minerals, may be an indication that the textures of PBM regions typically did not assume their final detailed configuration until after the last major shock. The steep slope of a pyroxene mg-Cr correlation suggests that igneous crystallization occurred at higher fO2 in Los Angeles than in otherwise similar shergottites such as QUE 94201, Shergotty, and Zagami.