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 hav e mineralogical and geochemical affinities with pristine ferroan anort hosites and therefore may represent primary igneous rocks from the lun ar crust. The textures of these clasts document a complex history invo lving magmatic crystallization, brecciation, subsolidus recrystallizat ion, and sulfide metasomatism. Nonetheless, the major, minor, and trac e element compositions of their minerals, as determined by electron an d proton microprobe, show that the clasts are essentially monomict and belong to the ferroan anorthositic suite of lunar highlands rocks. Th e low bulk Ni contents, low Ni/Co ratios, and ancient Sm-Nd isotopic a ge (greater than or equal to 4.5 Ga; Alibert et al., 1994) of the clas ts are consistent with this interpretation. Strontium and gallium cont ents of plagioclase in the clasts show that their parental magma was d epleted in Ga relative to Sr by a factor of about 30 relative to carbo naceous chondrites. This depletion of Ga, which is both moderately vol atile and moderately siderophile, probably reflects volatile loss from the Moon rather than extraction of metal. The parental magma from whi ch these clasts originally crystallized was enriched in Sr by about 4- 6X relative to proposed bulk Moon compositions, which implies either t hat 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 remark ably similar to that of the average upper crust of the Moon, which sug gests that igneous ferroan noritic anorthosite such as that found in t he North Ray Crater breccias and soils may be a significant primary co mponent 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 chalcoph ile elements such as Cu, Zn, Ni, Sb, and Se. Anhydrous C-O-S-Cl vapors are the most likely transport agent for these metals, which would be more consistent with an indigenous lunar origin for the volatiles rath er than a cometary or meteoritic source.