Siderophile elements in IVA irons and the compaction of their parent asteroidal core

Abstract

The highly siderophile element concentrations of IVA irons constitute an excellent benchmark for asteroidal core processes and, so far, the paradigm, based on the negative correlation between Ni contents and crystallographic cooling rates, has been that they represent a suite of cumulates crystallizing inwards from a molten asteroidal core. It has, however, been recognized that fractional crystallization does not account for all of the siderophile concentration patterns. Here we use the experimental parameterization of siderophile element fractionation with respect to the sulfur content of molten iron (Chabot and Jones, 2003), least-squares techniques, and Monte Carlo error propagation to assess whether incremental changes in the series of IVA irons can be accounted for by fractional crystallization or rather by partial melting. We show that the apparent order of incompatibility during solid–melt segregation deduced from binary plots of siderophile elements is misleading as a result of the strong dependency of partition coefficients on the sulfur content of the melt. All models of fractional crystallization of an Fe–S melt corrected for this effect result in negative sulfur contents. The effect of interstitial melt on fractionation hence is negligible because the high sulfur content of the melts makes all the elements compatible. In contrast, residues left by the compaction of a molten asteroidal core that crystallized with traces of sulfides and silicates according to the incremental form of batch melting provide a successful representation of the IVA suite for a large number of elements. Misfit for some elements may be due to the presence of carbon or reflects memory of variations acquired during the crystallization of the core. It is likely that melting was triggered by 26Al and that melt extraction was enhanced by the high strain-rate of impacts. The calculated S content of the liquid reproduces the experimental value of the Fe–S eutectic (45% S). Inward crystallization of the molten IVA asteroidal core is no longer a requirement. By invoking fractionation between silicate, metal, and sulfide melts, this compaction model also offers a satisfactory explanation for the low Th/U of IVA irons.