Petro-mineralogical and geochemical study of lunar meteorite NWA 13859

The composition of the lunar crust provides key information on the petrological evolution of the Moon. Studies mostly based on analyses of Apollo samples revealed the presence of two well distinct groups of rocks within the Highlands, Ferroan Anorthosites (FAN) and Mg-suite. The presence of these two well defined groups represents a key evidence for the Lunar Magma Ocean model (Warren & Taylor, 2014). Data from lunar meteorites, which may sample a larger portion of the lunar surface, have shown the presence of a more widespread compositional range leading to a re-evaluation of the Lunar Magma Ocean model (Gross et al., 2014). However, the “pristine” character of lunar meteorites has been questioned as they may represent impact- mixtures. In the framework of the MELODY project, we investigated the lunar meteorite NWA 13859, a polymict feldspathic breccia made up by several lithic clasts of variable lithologies (anorthosites, gabbroic anorthosites, troctolites, anorthositic norites, gabbroic norites) surrounded by a fine grained and sometimes glassy matrix. Major and trace elements of the mineral phases within the clasts and the matrix glass were analysed by Electron Microprobe (EMPA) and Laser-Ablation ICP-MS (LA-ICP-MS). Few clasts (mainly gabbroic anorthosites and anorthositic norites) fall within the field of FAN, whilst gabbroic norites have mafic minerals with high Mg#, sometimes reaching the composition typical of Mg-rich suites. However, most of the data fall at intermediate Mg# values. Matrix glasses have major elements compositions broadly resembling the average of the mineral phases of the clasts. Rare earth elements (REE) contents in plagioclase and pyroxene show large variations, with values in the range of previous data on feldspathic lunar meteorites, arguing against simple fractional crystallization from a single Lunar magma Ocean (Russel et al., 2014). Every plagioclase exhibits a positive Eu-anomaly whilst pyroxene have ubiquitous Eu negative anomalies. Matrix glasses have REE abundances and Eu positive anomalies similar to those of plagioclase with LREE higher, on average, than those of all the other mineral phases hosted in the lithic clasts. Similarly, Ni contents of matrix glasses are high, similar to those of olivine. The trace element composition of matrix glasses is compatible with an origin as impact melts derived from the sum of the various mineral phases making up the clasts. However, their high Ni and REE contents indicate a contribution from extra-lunar material related to the impactor that blasted the meteorite off the Moon, or to micrometeorites associated with regolith gardening and maturation. Overall, our data suggest that the contribution of exogenous material in the meteorite NWA 13859 is mainly concentrated in the matrix glass, whilst the single clasts (and the minerals within them) may preserve the composition of the original lunar crust.