Kalahari 009: One of the Oldest Lunar Mare Basalts - Chronology, Chemical and Petrological Composition, and Source Region

The meteorite Kalahari 009 is a monomict mare basalt breccia consisting of fragments of basaltic lithologies embedded in a fine-grained, heterogeneous matrix. The basaltic clasts have a coarse-grained subophitic texture. The main constituents of the breccia are predominantly pyroxene followed by plagioclase with lesser amount of olivine. Accessory minerals include ilmenite, chromite, troilite, chromian ulvöspinel, baddelyite, phosphates and Fe, Ni metal (having about 0.6 wt% Ni) (Sokol and Bischoff, 2005). The modal abundance is as follows: 50-55% pyroxene, ~35% plagioclase, ~10% olivine and <5% of opaque accessories. Symplectitic intergrowths of hedenbergite\+fayalite\+SiO2, representing late-stage assemblages, are ubiquitous (Sokol and Bischoff, 2005). The lunar origin of this sample is documented by bulk rock chemistry, Fe/Mn ratios in olivines and low 87Sr/86Sr ratio of 0.699242±43 measured in the third leachate of plagioclases (Sokol et al. 2007). Chemically Kalahari 009 has lower chondrite-normalized REE abundances and the lowest Th than the majority of mare basalts (Sokol et al. 2007). This lunar basalt shows positive Eu anomaly, which was previously only observed in some Luna 24 basalts (Ryder and Marvin, 1978) and suggested by Neal and Taylor (1992) to be due to plagioclase contamination of the source, or contamination from the anorthositic wallrock during magma ascent, or due to differentiation of ferrobasalt magma (Laul et al., 1978). Pyroxene compositions show strong variation and follow a typical mare basalt fractionation trend showing compositions in the miscibility gap of the pyroxene quadrilateral as well as zoning indicating and initial rapid cooling. On the other hand, the complete breakdown of the pyroxferoite, exsolution lamellae in pyroxene and the coarse grained nature of the basalt indicate slow cooling (second cooling stage). A crystallisation age of 4.286±0.095 Ga (2σ) was determined by Lu-Hf systematics (Sokol et al. 2007). The Ar-Ar age suggests a major impact at 1.721±0.030 Ga (2σ), and combined with the petrologic observations, it must have been an impact of ~30 GPa followed by an extended post-shock high temperature (<900°C) that caused the resetting of the K-Ar systematics. The very low Th content of Kalahari 009 (lower than that in Luna 24 basalts) points to a possible source far from the Procellarum-KREEP terrain. It is possible that this lunar basalt may come from a source on the lunar farside (e.g. Mare Moscovensis).