THEORETICAL ESTIMATES OF STABLE ISOTOPE FRACTIONATION DURING CORE FORMATION (Invited Presentation)

Although stable isotope fractionations tend to diminish as temperature increases, multiple lines of evidence now show that they can provide useful petrogenetic and compositional information about igneous and planetary differentiation processes. However, much work remains to be done to understand which stable isotope systems, materials, and processes are the most promising candidates for study. This presentation will focus on the use of theoretical calculations, including but not limited to electronic structure methods, to provide a framework for understanding fractionations at these extreme conditions. Among the strengths of electronic structure calculations are the ability to rather easily (if crudely) model extreme pressure conditions that are difficult to obtain in the laboratory, and the ability to quantify very small fractionation effects that challenge the present analytical limits of mass spectrometry. Of particular interest are signatures in the isotopic compositions of silicon, iron and other moderate-mass elements imparted by the segregation of metallic planetary and proto-planetary cores. First-principles calibrations of these effects are in good agreement with models based on vibrational spectroscopic measurements, fractionation experiments, and carefully selected natural samples, in at least some cases. Reconnaissance theoretical studies of heavier, strongly siderophile elements such as platinum suggest that detection of a core-formation signature will be more difficult, even when non-mass dependent equilibrium fractionation phenomena (i.e., nuclear field shift effects) are considered.