Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon
Résumé
Characterization of the hafnium-tungsten systematics (Hf-182 decaying to W-182 and emitting two electrons with a half-life of 8.9 million years) of the lunar mantle will enable better constraints on the timescale and processes involved in the currently accepted giant-impact theory for the formation and evolution of the Moon, and for testing the late-accretion hypothesis. Uniform, terrestrial-mantle-like W isotopic compositions have been reported(1,2) among crystallization products of the lunar magma ocean. These observations were interpreted to reflect formation of the Moon and crystallization of the lunar magma ocean after Hf-182 was no longer extant-that is, more than about 60 million years after the Solar System formed. Here we present W isotope data for three lunar samples that are more precise by a factor of = 4 than those previously reported(1,2). The new data reveal that the lunar mantle has a well-resolved W-182 excess of 20.6 +/- 5.1 parts per million (+/- 2 standard deviations), relative to the modern terrestrial mantle. The offset between the mantles of the Moon and the modern Earth is best explained by assuming that the W isotopic compositions of the two bodies were identical immediately following formation of the Moon, and that they then diverged as a result of disproportional late accretion to the Earth and Moon(3,4). One implication of this model is that metal from the core of the Moon-forming impactor must have efficiently stripped the Earth's mantle of highly siderophile elements on its way to merge with the terrestrial core, requiring a substantial, but still poorly defined, level of metal-silicate equilibration.