Fragments of ancient lunar crust: Petrology and geochemistry of ferroan noritic anorthosites from the Descartes region of the Moon

Clasts of ferroan noritic anorthosite from Apollo 16 breccia 67016 have mineralogical and geochemical affinities with pristine ferroan anorthosites and therefore may represent primary igneous rocks from the lunar crust. The textures of these clasts document a complex history involving magmatic crystallization, brecciation, subsolidus recrystallization, and sulfide metasomatism. Nonetheless, the major, minor, and trace element compositions of their minerals, as determined by electron and proton microprobe, show that the clasts are essentially monomict and belong to the ferroan anorthositic suite of lunar highlands rocks. The low bulk Ni contents, low Ni Co ratios, and ancient SmNd isotopic age (≥4.5 Ga; Alibert et al., 1994) of the clasts are consistent with this interpretation. Strontium and gallium contents of plagioclase in the clasts show that their parental magma was depleted in Ga relative to Sr by a factor of about 30 relative to carbonaceous chondrites. This depletion of Ga, which is both moderately volatile and moderately siderophile, probably reflects volatile loss from the Moon rather than extraction of metal. The parental magma from which these clasts originally crystallized was enriched in Sr by about 4-6X relative to proposed bulk Moon compositions, which implies either that ferroan anorthosites crystallized late in the evolution of a lunar magma ocean, or that their parental magma was a basaltic partial melt. Bulk major and trace element compositions of these clasts are remarkably similar to that of the average upper crust of the Moon, which suggests that igneous ferroan noritic anorthosite such as that found in the North Ray Crater breccias and soils may be a significant primary component of the lunar crust. The ferroan noritic anorthosite clasts in 67016 are unusual among lunar samples for their abundance of troilite, and for the evidence of metasomatic introduction of volatile chalcophile elements such as Cu, Zn, Ni, Sb, and Se. Anhydrous COSCl vapors are the most likely transport agent for these metals, which would be more consistent with an indigenous lunar origin for the volatiles rather than a cometary or meteoritic source.