Earth and Planetary Science Letters;
OCT 15 2014.
Citações Web of Science:
The timing of core formation is essential for understanding the early differentiation history of the Earth and the Moon. Because Hf is lithophile and W is siderophile during metal-silicate segregation, the decay of Hf-182 to W-182 (half-life of 9 Ma) has proven to be a useful chronometer of core-mantle differentiation events. A key parameter for the interpretation of Hf-182/W-182 data is the Hf/W ratio of the primitive (i.e. undepleted) mantle. Since W is incompatible during mantle melting, its ratio relative to U and other similarly incompatible elements in basalts (e.g. Th, La) may be used as proxies for their mantle sources. However, the assumption that W and U are equally incompatible may be flawed for petrological systems that equilibrated over a large range of oxygen fugacity (fO(2)). Although W is typically perceived as being homovalent, evidence suggests that U is heterovalent over the range of fO(2) inferred for the silicate mantles of the Earth and the Moon. Here we report new partitioning data for W, U, high-field-strength elements (HFSE), and Th between clinopyroxene, orthopyroxene, olivine, plagioclase and silicate melt. In agreement with previous studies, we show that these elements behave as homovalent elements at fO(2) characteristic of Earth's upper mantle. However, both W and U become more compatible at low fO(2), indicating a change in their redox state, with W becoming more compatible at progressively lower fO(2). This result for W is particularly unexpected, because this element was thought to be hexavalent even at very low fO(2). The much higher compatibility of W4+ (the species inferred here at low fO(2)) relative to W6+ means that even a small fraction of W4+ will have a significant effect on the overall compatibility of W. Our results imply that over the range of reducing conditions in which lunar differentiation is thought to have taken place (i.e. similar to IW-2 to IW-0.5), W is likely to become fractionated from U. When our partitioning data are applied to model the fractional crystallization of a lunar magma ocean, lunar trends for U/W, Hf/W and Th/W are well reproduced. The result of this model carries with it the implication that the Hf/W of the bulk silicate fractions that comprise the Earth and the Moon are virtually identical. (C) 2014 Elsevier B.V. All rights reserved. (AU)