The Multi-stage Cooling History of Lunar Meteorite Nwa 032 as Recorded by Phenocrystic Olivine and Pyroxene

Introduction – Lunar meteorite Northwest Africa (NWA) 032 is an unbrecciated mare basalt, discovered in 1999, whose modal mineralogy is characterized by phenocrystic olivine, pyroxene and chromite, with a groundmass containing feldspar, pyroxene, ilmenite, troilite and other trace metals [1]. Shock melt occurs in veins throughout the sample as well. Fagan et al. [1] observed that the major element chemistry of the sample is most similar to low-Ti basalts from Apollo 12 and 15, though it is enriched in the LREE, a characteristic more akin to Apollo 14 basalts. Radiogenic ages for this sample, based on Ar/Ar [1], and Rb/Sr and Sm/Nd [2] are concordant, ranging from 2.85 – 2.69 Ga. Fagan et al. suggest a relatively simple crystallization history for this sample, based on petrographic observations and thermodynamic modeling using the MELTS algorithm. This study reexamines the petrographic characteristics of NWA 032 using a combination of backscattered electron imaging (BSE), wavelength dispersive (WDS) X-ray mapping and targeted quantitative electron microprobe analyses (EMP). In doing so, we seek to build on the comprehensive dataset by [1], and frequently allude that that study throughout this abstract. Specifically, we have focused on the crystallization history of the NWA 032 magma as recorded by phenocrystic olivine and pyroxene grains (as opposed to later intermediate and groundmass pyroxene, as described by [1]). Our observations describe previously undocumented zoning features in phenocrystic pyroxene grains, and their implication for the magma crystallization sequence. Analytical Parameters – Initial inspection of NWA 032 was carried out using BSE on the JEOL 733 Superprobe, at the University of New Mexico. BSE images were used to