Geocheminar fall-2014

New gravity data (GRAIL) and recent seismic inferences have important implications for the bulk composition of the Moon. The volume and density of the crust imply that contrary to old prejudices, the bulk Moon is not much enriched in refractory lithophile elements (Al2O3, CaO, etc.). In its present much-cooled state, a remarkably high proportion of the lunar interior has dense garnet and/or spinel, not low-density plagioclase, as the dominant Al-silicate; a circumstance which implies a relatively low density for the mafic silicates (olivine + pyroxene). Density modeling suggests the bulk-Moon composition probably has a mildly lower mg than bulk Earth; mg = 85 mol% implies an [FeO] enrichment of approx. 1.36 times bulk-Earth. The greatest uncertainty in this approach is mantle temperature. Both the refractory-lithophile component of the bulk-Moon composition, and its FeO content, could be constrained by augmenting the current meager heat flow data.