Isotopic fractionation during planetary-scale processes

Mass-dependent fractionation of isotopes has rapidly grown as a tool to study a large variety of geological/-chemical processes since the advent of mass spectrometry. Traditionally, the isotopes of relatively low mass elements such as hydrogen, oxygen and carbon were used to study dominantly low-temperature processes. Improvements in measurement precision have, however, enabled us to resolve fractionation occurring in the isotopes of nearly all elements in the periodic table during low- or high-temperature processes.

Isotopic fractionation can take place under two extreme conditions. Unidirectional, or kinetic, fractionation is typically relatively large and roughly proportional to the square-root of the mass ratio of the isotopes. Under chemical equilibrium, fractionation is typically smaller and decreases with the square of temperature as differences in zero-point vibrational energies control the isotopic fractionation between phases. These two scenarios are just one of the complexities that need to be unravelled when analysing isotopic fractionation, particularly when isotopic variations are small, as is often the case for relatively high-mass elements and when attempting to study high-temperature processes.

I will present a case where careful sample selection and analysis combined with theoretically or experimentally constrained isotopic fractionation factors and multiple chemical proxies allow isotopic fractionation to be a valuable tool to understand large-scale, planetary formation/differentiation processes.