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Journal of Geophysical Research: Solid Earth

Petrology and chemistry of hyperferroan anorthosites and other clasts from lunar meteorite ALHA81005

Authors

  • Cyrena Anne Goodrich,

  • G. Jeffrey Taylor,

  • Klaus Keil,

  • William V. Boynton,

  • Dolores H. Hill


Abstract

Thirteen previously undescribed clasts from lunar meteorite Allan Hills A81005 include one that is a cataclastic, “hyperferroan” noritic anorthosite. Mafic minerals in this clast (low-Ca pyroxene with mg′ 52 and olivine with Fo 41) have very low mg compared to most lunar anorthosites described before the discovery of 81005. Siderophile element concentrations are low and indicate that the clast is compositionally pristine. Rare earth elements (REE) are 2.5–10 times more abundant and slightly more fractionated than in previously described ferroan anorthosites. Another probably pristine clast is a granulitic gabbroic norite that has low-Ca pyroxene with mg′ 53. Hyperferroan anorthosites are a significant component of 81005. Extreme iron enrichment and high REE abundances indicate that these rocks formed late in the evolution of the magma from which the ferroan anorthosite suite formed. Plagioclase was not enriched in Na even at these late stages. Another clast is a cataclastic ferroan anorthosite (Fo 66) that might be pristine. Of the remaining 10 clasts, nine are granular to cataclastic rocks (primarily anorthositic troctolites or troctolitic anorthosites), and two are impact melt breccias, all of which probably represent mixtures of pristine rock types. The former have REE that are essentially unfractionated relative to chondrites and range in abundance from 1.5–3×Cl, and the latter are similar to bulk 81005 (REE 5–8×Cl). Siderophile element abundances are high and indicate the presence of a significant meteoritic component. Our data indicate that 81005 has only a small KREEP (potassium, rare earth elements, phosphorus) component. This is consistent with previous results.

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