KREEP cumulates in the western lunar highlands: Ion and electron microprobe study of alkali-suite anorthosites and norites from Apollo 12 and 14

Alkali suite anorthosites and norites are the second most common plutonic r ock association in the western lunar highlands (after the magnesian suite), but their origin poses an enigma for most petrologic models of lunar crust al evolution. Some models suggest that the alkali and magnesian suites form ed from distinct, unrelated parental magmas, whereas other models propose t hat both suites formed from the same parental magma. The contrast in major element chemistry of the cumulus phases in each suite is difficult to recon cile with their similar incompatible element chemistry. We present herein a detailed ion microprobe (SIMS) and electron microprobe study of seven alkali suite rocks. Our data show that most alkali suite ano rthosites preserve major and trace element characteristics acquired during their formation as igneous cumulate rocks, and that these characteristics c an be used to reconstruct the parental-magma composition. The data indicate that cumulates of the alkali suite crystallized from magmas with rare-eart h element (REE) contents similar to 0.6-2.0x high-K KREEP, and small but co nsistently positive Eu anomalies (Eu/Eu* similar to 2) relative to KREEP. S nyder and others (1995a) have proposed that the alkali suite parental magma is similar to Apollo 15 pristine KREEP basalt. Our model suggests that the major element composition of cumulus plagioclase in most alkali suite rock s is too sodic for the calculated crystal line-of-descent of pristine KREEP basalt, and that assimilation of pre-existing calcic anorthosite is requir ed. This conclusion is supported by the REE patterns of th alkali-suite par ental magma determined here. We propose that alkali suite anorthosites formed as flotation cumulates in KREEPy plutons that may have formed norites as complementary bottom cumulat es. The alkali flotation cumulates reflect fractional crystallization of th eir parental pluton, local equilibrium crystallization, assimilation of pla gioclase-rich roof rock, and episodic magma-mixing during convective overtu rn of the crystallizing magma bodies. Texturally pristine alkali anorthosit es exhibit petrographic characteristics that are consistent with their orig in as cumulates in a KREEPy pluton, including abundant modal plagioclase, p ost-cumulus pyroxenes (both augite and pigeonite) that generally lack exsol ution lamellae and that have equilibration temperatures of 950-1100 degrees C, relict igneous textures and, in some cases, igneous lamination. The lac k of cumulus mafic phases in rocks that should be pyroxene-saturated sugges ts separation of the plagioclase by flotation, not sinking. Assimilation of plagioclase from older, anorthositic highlands crust is indicated by the h igh Eu contents of the cumulates and by the positive Eu anomalies in their calculated parental melts relative to high-K KREEP. Mixing of the evolved a lkali-suite parental magma with primitive melt occured episodically, as sho wn by reverse zoning profiles in some cumulus plagioclase. Injection of thi s primitive, hot magma into the crystallizing pluton may have induced conve ctive overturn of the magma chamber.