Los Angeles: A tale of two stones

Abstract

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 fO 2 in Los Angeles than in otherwise similar shergottites such as QUE 94201, Shergotty, and Zagami.

Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars

“…Because MIF sulphur signatures are not produced by magmatic processes, these observations suggest that sulphur was fractionated by ultraviolet photochemistry in the Martian atmosphere 5,6 , deposited onto the surface and ultimately assimilated into melts, where it was reduced and incorporated into igneous sulphides. NWA 2990, NWA 5960 and Los Angeles also exhibit trace element and radiogenic isotopic characteristics that some workers have interpreted to indicate incorporation of crustal material [14][15][16][17] . The coincidence of anomalous sulphide with other geochemical characteristics, including evidence for crystallization under relatively oxidizing conditions [14][15][16][17] , suggests a link between anomalous sulphur in igneous minerals and assimilation of crustal material into the melts.…”

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

“…Of these, (3) can almost certainly be ruled because Los Angeles demonstrably crystallized rather near the surface (Rubin et al 2000;Xirouchakis et al 2002) than in a depth of ~100 km, where a pressure of 1 GPa, necessary for pyroxferroite formation in equilibrium, would be reached on Mars. As pointed out above, (4) and (5) have been strongly rejected in previous studies (Rubin et al 2000;Aramovich et al 2002;Warren et al 2004).…”

“…Some symplectite might also have been formed by direct decomposition of the primary pyroxene. Warren et al (2004) rebutted this new interpretation in great detail, with the main arguments being that Xirouchakis et al (2002) did not acknowledge the ubiquitous existence of the silica phase within the symplectites and that the modal abundance of Ca-rich pyroxene, Fe-rich olivine, and silica, when reconstructed into a single phase, is always consistent with a pyroxenoid precursor.…”

“…Moreover, since the breakdown conditions discovered by Lindsley et al (1972) require temperatures greater thañ 990 °C for more than three days and because shock events are very rapid on the order of seconds, Aramovich et al (2002) precluded shock heating in general from being a viable mechanism for creating the symplectites. Yet Warren et al (2004) discussed the possibility that the symplectites in Los Angeles did not assume their final detailed texture until after the last shock.…”

“…Rubin et al (2000) and Warren et al (2004) argued against a shock related breakdown in case of Los Angeles because the symplectite contains shock induced faults. Moreover, since the breakdown conditions discovered by Lindsley et al (1972) require temperatures greater thañ 990 °C for more than three days and because shock events are very rapid on the order of seconds, Aramovich et al (2002) precluded shock heating in general from being a viable mechanism for creating the symplectites.…”