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Science 12 January 2007:
Vol. 315. no. 5809, pp. 217 - 219
DOI: 10.1126/science.1133355
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Reports

Highly Siderophile Element Constraints on Accretion and Differentiation of the Earth-Moon System

James M. D. Day,1* D. Graham Pearson,2 Lawrence A. Taylor1

A new combined rhenium-osmium– and platinum-group element data set for basalts from the Moon establishes that the basalts have uniformly low abundances of highly siderophile elements. The data set indicates a lunar mantle with long-term, chondritic, highly siderophile element ratios, but with absolute abundances that are over 20 times lower than those in Earth's mantle. The results are consistent with silicate-metal equilibrium during a giant impact and core formation in both bodies, followed by post–core-formation late accretion that replenished their mantles with highly siderophile elements. The lunar mantle experienced late accretion that was similar in composition to that of Earth but volumetrically less than (~0.02% lunar mass) and terminated earlier than for Earth.

1 Department of Earth and Planetary Science, University of Tennessee, Knoxville, TN 37996, USA.
2 Department of Earth Sciences, University of Durham, Durham, DH1 3LE, UK.

* To whom correspondence should be addressed. E-mail: jday13{at}utk.edu

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