Basaltic magmatism and the bulk composition of the moon

Abstract

Abundance patterns of siderophile and volatile elements imply that the Moon was derived from the Earth's mantle after the core had segregated. The relative abundances of siderophile and volatile elements in the Moon and in the Earth's mantle are obtained from a comparison of their abundances in terrestrial ocean-floor basalts and lunar low-Ti mare basalts. The abundances of a group of siderophile elements Ni, Co, W, Ir, Os, P, S and Se are found to be very similar in ocean-floor tholeiites and low-Ti mare basalts and this similarity is believed to extend to their respective source regions in the Earth's mantle and lunar interior. The abundances of the above siderophile elements in the Earth's mantle have been determined by the interaction of several complex processesunique to the Earth, which relate to core formation and non-equilibrium distribution of elements between metallic and silicate phases. Since these factors could not possibly have operated separately within the Moon, the similarity in siderophile element abundances therefore implies thatthe Moon was derived from the Earth's mantle after the Earth's core was formed. Abundance patterns of volatile elements in the Moon differ dramatically from those in ordinary chondrites and from those to be expected from condensation of a nebula of solar composition. These differences imply that the Moon was not formed from components which themselves had condensed directly from the solar nebula. For these reasons, current versions of the capture and binary planet hypotheses of lunar origin, which maintain that the Moon formed independently by accretion in the solar nebula, can be rejected. The drastic depletion of many volatile elements in the Moon compared to the Earth implies that the separation of material from the Earth's mantle to form the Moon occurred at very high temperatures. Depletions of this magnitude are best explained by the hypothesis that material from the Earth's mantle was totally evaporated, and then selectively recondensed, whilst the more volatile components were lost. The Moon then formed in Earth orbit by accretion from this volatile-depleted, mantle-derived condensate.