An assessment of iron isotope fractionation during core formation

Abstract

Iron is a ubiquitous element in terrestrial and extra-terrestrial settings and can provide clues as to the conditions during which planetary scale processes occurred. One example of this is determining the conditions accompanying metal core formation in rocky planets and planetesimals. For at least two decades there has been a growing database of experimental and natural data aimed at understanding whether iron isotopes fractionate during the separation of silicate and metal. While it has been argued that the data are not in agreement with one another, it is apparent that once criteria are established that prove equilibrated samples, there is good agreement amongst the different studies when looked at as a function of the metallic composition. Proving equilibrium is critical in these types of experiments. The three-isotope experimental technique for establishing equilibrium is found to be both mathematically and fundamentally sound. Further it is clear that the question of whether there is an equilibrium iron isotope fractionation between metal and silicate is not straightforward and that it can vary significantly as a function of temperature, pressure, metallic composition, oxygen fugacity, and silicate composition.