Meteoritic Evidence for Extensive Compositional Variability on the Moon

Introduction: Crystallization of the lunar magma ocean occurred ~4.4 Ga [e.g., 1-4] and produced three principal magma source regions [5-6]. These sources are characterized chemically by low titanium (Ti), high Ti, and high potassium, rare earth element, and phosphorous (KREEP) abundances. The KREEP component was produced after nearly complete solidification of the magma ocean and is highly enriched in all incompatible elements not partitioned into the main mineral phases [7-9]. Compositional variability of lunar basalts [Fig. 1] is interpreted to reflect mixing of materials derived from these three sources [5,6,9]. New lunar meteorites, however, have compositions and ages that differ from the basalts in the Apollo and Luna collections, suggesting that there is greater diversity on the Moon than was previously recognized. Below we discuss some of the compositional, chronological, and petrogenetic differences between these two groups of basalts and assess to what extent the magma ocean model of lunar differentiation can accommodate these differences. Ultimately this exercise offers insights into the advancement of lunar geologic science that will result from a lunar exploration program in which new samples are returned to Earth. Compositions of basaltic meteorites: Several of the basaltic lunar meteorites have mineralogical and/or geochemical characteristics that differ from previously collected samples. Although these basalts share some compositional characteristics with other lunar basalts [Fig. 1], their geochemical and isotopic systematics are distinctive. Representative examples that we have completed, or are in the process of completing, detailed chronologic investigations on and are discussed below and include: Northwest Africa (NWA) 032, NWA 773, NWA 4898. In addition, we include the unusual meteorite Asuka 881757. Each of these samples has unique mineralogical and/or compositional characteristics. Northwest Africa 032 has elevated Th concentration (1.9 ppm) and Th/Sm ratio (0.28) that is similar to many KREEP-rich samples [10-12]. However, unlike KREEP, this sample has a moderate TiO2 concentration [~3.0 wt. %; 10-11] indicating that its source region has not experienced the same extent of ilmenite fractionation. Northwest Africa 773 also has an elevated Th/Sm ratio, but unlike NWA 032 has many other KREEP-like geochemical characteristics [12,13]. This sample is mineralogically distinctive because it is the only KREEP-rich sample of basaltic origin dominated by olivine and containing a minor amount of Ba-K feldspar. Northwest Africa 4898 has a major element composition similar to Apollo 14 and Luna 16 highalumina basalts except that it has significantly lower Mg/Fe ratio suggesting that it is more evolved [14]. In