Unique spinel-rich lithology in lunar meteorite ALHA 81005: Origin and possible connection to M³ observations of the farside highlands

A new lunar rock type, rich in (Mg, Fe)Al spinel and lacking abundant olivine and pyroxene, was recently detected by near-infrared reflectance spectroscopy by the M³ instrument on the Chandrayaan-1 spacecraft. No such material has been described from lunar rocks, either returned samples or meteorites. Here we describe a fragment of rock containing ∼30% (Mg, Fe)Al spinel from the lunar meteorite ALHA 81005. Although the fragment is not identical to the material detected by M³ (it contains ∼20% olivine + pyroxene), both share the defining feature of an unusual enrichment in spinel. The fragment, 350 × 150 m, is so fine grained that it reasonably could represent a larger rock body; it is not spinel-rich merely by chance incorporation of a few spinel grains. The fragment is so rich in spinel (and consequently in Al₂O₃) that it could not have formed by melting a peridotitic mantle or a basaltic lunar crust. The clast's small grain size and its apparent disequilibrium between spinel and pyroxene suggest fairly rapid crystallization at low pressure. It could have formed as a spinel cumulate from an impact melt of troctolitic composition or from a picritic magma body that assimilated crustal anorthosite on its margins. The latter mechanism is preferred because it not only explains the petrographic and chemical features of our clast but is also consistent with the regional setting of the Moscoviense spinel deposit. In that area, M³ spectra have defined areas rich in olivine and in orthopyroxene; these could represent igneous cumulate rocks formed during crystallization and differentiation of a picritic magma body and thus suggests a possible link between the analyzed clast and the observed spinels at Moscoviense.