Abstract

Apollo 12 sample 12032,366-19 is a 21.3 mg granite fragment that is distinct from any other lunar granite or felsite. It is composed of barian K-feldspar, quartz, sodic plagioclase, hedenbergite, fayalite, and ilmenite, with trace amounts of zirconolite, baddeleyite, apatite, and merrillite. The texture of 12032,366-19 is largely a micrographic intergrowth predominantly of K-feldspar and quartz and, to a lesser extent, plagioclase and quartz. Hedenbergite, fayalite, and ilmenite are present in minor but significant quantities—6.0, 3.1, and 1.7 wt%, respectively—and are scattered throughout the feldspar-quartz intergrowths. Trace amounts of Zr-bearing phases are found including zirconolite (0.6 wt%) and baddeleyite (0.04 wt%). Incompatible trace-element concentrations are high in 12032,366-19, particularly the high-field-strength elements, e.g., Zr, Sm, and Th (1500, 25, and 61 μg/g, respectively). The chondrite-normalized, rare-earth-element concentrations form a “V-pattern” that is characteristic of other lunar granitic material. By modeling 12032,366-19 as a derivative from a KREEP-like parent melt, the composition and mineral assemblage can be obtained by extended fractional crystallization combined with separation of the low-density minerals plus trapped melt components prior to final solidification. However, this model cannot quantitatively account for the relatively sodic composition of the plagioclase (An34–50) and requires that the starting melt has Na2O of 1.2–1.4 wt%, which is higher than most KREEP compositions. Formation of this assemblage by silicate-liquid immiscibility is neither required nor indicated by petrogenetic modeling.

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