Apollo 17 KREEPy basalt: A rock type intermediate between mare and KREEP basalts

The Apollo 17 KREEPy basalt is a unique lunar volcanic rock, observed only as clasts in the light friable breccia matrix (72275) of Boulder 1, Station 2 at Taurus-Littrow. Its status as a volcanic rock is confirmed by the absence of any meteoritic contamination, a lack of cognate inclusions or xenocrystal material, and low Ni contents in metal grains.The basalt was extruded 4.01 ± 0.04 b.y. ago, approximately contemporaneously with the high-alumina mare basalts at Fra Mauro; shortly afterwards it was disrupted, probably by the Serenitatis impact, and its fragments emplaced in the South Massif. The basalt, which is quartz-normative and aluminous, is chemically and mineralogically intermediate between the Apollo 15 KREEP basalts and the high-alumina mare basalts in most respects. It consists mainly of plagioclase and pigeonitic pyroxene in approximately equal amounts, and 10–30% of mesostatis. Minor phases outside of the mesostatis are chromite, a silica mineral, Fe-metal, and rare olivine; the mesostatis consists primarily of ilmenite, Fe-metal, troilite, and ferroaugite, set in a glassy or microcrystalline Si-rich base.Chemical and isotopic data indicate that an origin by partial melting of a distinct source region is more probable than hybridization or contamination of magmas, and is responsible for the transitional composition of the basalt. The moon did not produce two completely distinct volcanic groups, the KREEP basalts and the mare/mare-like basalts; some intermediate rock types were generated as well. A corresponding spectrum of source regions must exist in the interior of the moon.