Core formation in the Earth and Shergottite Parent Body (SPB): Chemical evidence from basalts

Abstract

Constraints on processes of core formation in terrestrial planets may be inferred from the abundances of siderophile and chalcophile elements in their mantles. Of particular interest is a comparison of processes of core formation in the Earth and in the Shergottite Parent Body (SPB), a terrestrial planet tentatively inferred to be Mars. To this end, we (i) present new INAA and RNAA analyses of the non-Antarctic SNC meteorites, (ii) infer the composition of the SPB mantle from the compositions of the SNC meteorites, (iii) infer the composition of the Earth's mantle from the compositions of terrestrial basalts, and (iv) deconvolve the effects of volatile depletion, core formation, and mineral/melt fractionation on the abundances of siderophile and chalcophile elements in the SPB and the Earth. Element abundances in the mantles of the SPB and the Earth are estimated from element/element correlations observed among basalt samples. In basalts from the SPB (the SNC meteorites), four groups of covariant elements are observed: highly incompatible, moderately incompatible, indifferent and compatible. From correlations within these groups, the SPB mantle is found to be depleted in volatile elements, and strongly depleted in siderophile and chalcophile elements. For the Earth, the element/element correlation method gives a mantle composition similar to previous estimates. Compared to the Earth, the SPB mantle is richer in moderately siderophile elements (e.g., W, P), consistent with its inferred higher oxidation state. Chalcophile elements in the SPB mantle are more depleted than in the Earth's mantle, particularly when compared to estimates of the original abundances of volatile chalcophile elements in the two planets. In the SPB mantle, the NiCo ratio is nonchondritic, in contrast to the chondritic ratio in the Earth's mantle. Abundances of siderophile and chalcophile elements in the SPB mantle may be modelled by equilibrium with solid metal and metallic sulfide liquid, with some metal and sulfide trapped in the mantle (i.e., homogeneous accretion and inefficient core formation). Neither this model nor a heterogeneous accretion model is satisfactory in explaining element abundances in the Earth's mantle, particularly the abundances of Ni, Co, Mo, and W. Nevertheless it appears that core formation in the SPB and the Earth left quite different chemical signatures in their planetary mantles.