HR: 08:45h
AN: U21D-04 INVITED    [Abstracts]
TI: Accretion and Core Formation: Constraints From Metal-Silicate Partitioning Experiments
AU: * Wood, B J
EM: berniew@earth.ox.ac.uk
AF: Oxford University, Earth Sciences, Oxford, ox1 3pr, United Kingdom
AB: The concentrations of siderophile elements in the Earth's mantle provide, in principle, when compared to chondritic meteorites, the means to estimate the conditions under which the Earth's core segregated from the mantle. The additional requirements are experimental metal-silicate partitioning data for as many elements as possible over appropriate ranges of pressure, temperature and oxygen fugacity. The purpose of this presentation is to use data for a large number of elements of widely differing behavior (e.g V, Ni, Cr, Nb, Co, W, Si, Ga, Pb, Cu, Ag) to constrain the conditions under which the Earth accreted and segregated its core. If we assume that the core segregated continuously during accretion at the base of a homogeneous magma ocean then results indicate that the magma ocean would have, on average, extended to one-third of mantle depth on the growing planet. In order to match the V and Cr concentrations of the mantle, however, accretion must have begun under strongly reducing conditions with progressive oxidation occurring as the Earth grew. This model leads to a slightly subchondritic Nb/Ta ratio of the silicate Earth and an Si content of the core of about 5per cent. The latter is in accord with recent Si isotopic data.Modifications of the model lead to similar conclusions. If we assume that the magma ocean is not well-stirred and that metal equilibrates only locally with melt, then pressures and temperatures increase but progressive oxidation is still required. If we assume that metal equilibrates with a crystal-rich mush instead of melt, oxidation is required and it becomes very difficult to match the mantle concentrations of strongly incompatible elements such as Nb and W. Given that the Earth underwent an early reduced phase before becoming more oxidised, oxidation could have occurred by addition of more oxidised materials in the later stages of accretion. Another possibility is that the mantle "self-oxidised" through disproportionation of ferrous iron to ferric plus metal in the lower mantle. Whatever the explanation, the results imply that Si and S are the major light elements in the core and that, because of the strongly reducing conditions, the oxygen content of the core is low.
DE: 3630 Experimental mineralogy and petrology
DE: 5749 Origin and evolution
SC: Union [U]
MN: 2007 Fall Meeting