Hf-W constraints on metal-silicate mixing during Earth accretion

Details Hf-W constraints on metal-silicate mixing during Earth accretion Hf-W constraints on metal-silicate mixing during Earth accretion

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p14a..01d&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P14A-01

Physics

[1009] Geochemistry / Geochemical Modeling, [1040] Geochemistry / Radiogenic Isotope Geochemistry, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution

Scientific paper

We revisit the problem of partial metal-silicate re-equilibration during core-mantle differentiation and its effect on Hf-W chronometry. Previous models have focalized on the possibility that only a fraction of the core of impacting bodies equilibrates with the Earth mantle, but it was generally assumed that this fraction of impactor cores equilibrated with the whole mantle. We relax this probably unrealistic assumption and consider incomplete re-equilibration of the mantle. We first study the effect of partial mantle re-equilibration on Hf-W chronometry in a continuous accretion model with exponentialy decreasing accretion rate. Partial mantle re-equilibration during accretion leads to higher ɛ W for a given accretion timescale, which imply that a longer accretion (possibly by several tens of My) is required to explain the 182W excess of the Bulk Silicate Earth (BSE). Alternatively, using estimates of the age of the Moon as a constraint on Earth's accretion timescale provides usefull bounds on the amount of metal-silicate mixing. If the metal phase is assumed to fully equilibrate, we find that it must have mixed and equilibrated with a volume of silicate more than 10 times larger in order to explain the 182W excess of the BSE. If the metal phase equilibrates partially, the amount of mixing required is even larger. From a dynamical point of view, this implies that dispersion of a significant fraction of the metallic phase in the mantle must have been the norm during Earth accretion. In order to avoid the arbitrary choice of a particular form of the accretion history, we perform Monte-Carlo simulations of accretion and evolution of Hf-W abundances. This generates an envelope for acceptable accretion curves, and provides bounds on the mean amount of metal-silicate mixing and equilibration during accretion.

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