Comparative geochemistry of basalts from the Moon, Earth, HED asteroid, and Mars: Implications for the origin of the Moon

Most hypotheses for the origin of the Moon (rotational fission, co-accretio n, and collisional ejection from the Earth. including "giant impact") call for the formation of the Moon in a geocentric environment. However, key geo chemical data for basaltic rocks from the Moon, Earth, the howardite-eucrit e-diogenite (HED) meteorite parent body (probably asteroid 4-Vesta), and th e shergottite-nakhlite-chassignite (SNC) meteorite parent body (likely Mars ), provide no evidence that the Moon was derived from the Earth, and sugges t that some objects with lunar-like compositions were produced without invo lvement of the Earth. The source region compositions of basalts produced in the Moon (mare basalts) were similar to those produced in the HED asteroid (eucrites) with regard to volatile-lithophile elements (Na, K, Rb, Cs, and T1), siderophile elements (Ni, Co, Ga, Ge, Re, and Ir), acid ferromagnesia n elements (Mg, Fe, Cr, and V), and less similar to those in the Earth or M ars. Mare and eucrite basalts differ in their Mn abundances, Fe/Mn values, and isotopic composition, suggesting that the Moon and HED asteroid formed in different nebular locations. However, previous claims that the Moon and HED parent body differ significantly in the abundances of some elements, su ch as Ni, Co, Cr, and V, are not supported by the data. Instead, Cr-ME-Fe-N i-Co abundance systematics suggest a close similarity between the source re gion compositions and conditions involved in producing mare and eucrite bas alts, and a significant difference from those of terrestrial basalts. The d ata imply that the Moon and HED asteroid experienced similar volatile-eleme nt depletion and similar fractionation of metallic and mafic phases. Among hypotheses of lunar origin, rotational fission, and small-impact collisiona l ejection seem less tenable than co-accretion, capture, or a variant of gi ant-impact collisional ejection in which the Moon inherits the composition of the impactor. Both the Moon and HED asteroid may have been derived from a class of objects that were common in the early solar system. Copyright (C ) 2001 Elsevier Science Ltd.