Isotope tracers of core formation

The study of siderophile element isotope compositions in planetary mantles offers a new methodology to constrain the temperatures of core formation, provided there is an appropriate calibration of the temperature-dependence and possibly pressure-dependence of isotope fractionation between metal and silicate and of the metal-silicate partitioning for these elements. In this review, we examine recent studies that have shown that Si, Fe, Mo, Cr, Cu, Ni, N and C could potentially be used to constrain the temperature of metal-silicate equilibration using single stage or continuous models of core formation, yielding contrasted results. Such an approach requires assumptions about the building blocks of the Earth and it is generally considered that the composition of some chondrites is representative of bulk Earth. This is obviously more complex for volatile elements such as Cu, N or C, as the isotope composition of the building blocks of the Earth could have been affected by thermal processing. On the basis of a chondritic bulk composition, one can estimate a temperature of core formation assuming a model for this process. If the metal-silicate equilibration is incomplete, as is likely the case for giant impacts, then the composition of the mantle of the impactor and the fraction of metal that equilibrates needs to be assessed carefully. It has been shown recently that the degree of equilibration will be a function of the metal-silicate partition coefficient and will be hence very different for Si, Cr, or Mo, an aspect that has not been considered in previous studies and may help explain differences in interpretation. In this context, the expected temperatures of equilibration are quite variable and are a function of the impactor's conditions of metal-silicate segregation. Another complication arises when considering continuous models of core formation: the most siderophile elements will be sensitive to the last episodes of core formation, while the budget of less siderophile elements will reflect its integrated accretion history (e.g. Cr or Si). A model including Si, Cr and Mo isotope data that takes into account these aspects has been constructed and shown to be consistent with scenarii that were derived from siderophile element data.