W-Hf isotope abundances and the early origin and evolution of the Earth-Moon system

The decay of the short-lived isotope Hf-182 to the isotope W-182 early in s olar system history provides clues to the origin and evolution of the Earth and Moon. Current time-scales for the formation of proto-planets in the so lar system require early accretion of the proto-Earth, accompanied with cor e formation, which should lead to non-chondritic radiogenic W isotope ratio s (W-182/W-184) in the Earth's mantle. Surprisingly, the observed ratio of W-182/W-184 in the Earth's mantle is chondritic, which requires special cir cumstances to explain (Halliday et al., 1996; Lee and Halliday, 1997; Lee e t al., 1997). Previous explanations include delayed core formation until 50 M yrs. after the origin of the solar system, or a very long time-scale for accretion of the Earth (Halliday et al., 1996; Lee and Halliday, 1997; Lee et al., 1997). In this paper, we explore models consistent with the early formation of the proto-Earth; a giant impact origin of the Moon, and substa ntial additional accretion to the Earth following the impact. The post-impa ct processes responsible for the chondritic W isotopes may include late cor e-mantle equilibration in the hot Earth following the giant impact, and lat er metal segregation and accretion. In contrast to the Earth, various igneous rocks on the Moon exhibit non-cho ndritic, positive anomalies in the ratio of W-182/W-184. The explanation fo r the lunar samples with chondritic W isotopes may also involve late metal segregation and accretion, as in the Earth. The positive anomalies in lunar samples may be derived in part or whole through cosmogenic processes on th e lunar surface, Hf-W fractionation during core formation, or Hf-W fraction ation during lunar magma ocean crystallization. If the latter, the positive anomaly places constraints on the early differentiation of the lunar mantl e and the duration of the crystallization of the lunar magma ocean. The obs erved W-Hf systematics are best explained if the W anomalies are carried;by late-crystallizing ilmenite and high-Ca pyroxene, which is later mixed int o the source regions of the mare basalts. In this model, the crystallizatio n of the magma ocean must occur in less than 40 million years. This rapid c rystallization implies that the very early lunar crust was not as stable an d insulating as previously suggested, and that the fundamental change from magmasphere to serial magmatism processes occurred very early in the evolut ion of the Moon. Copyright (C) 2000 Elsevier Science Ltd.