Copper isotope fractionation during asteroid core solidification

Moderately volatile elements (MVE) and their isotopic compositions are powerful tools to understand the origin of volatiles on terrestrial planets, including Earth. The 65Cu/63Cu ratio of Cu, one of the MVEs, has been found to be relatively high in the bulk silicate Earth (BSE), which potentially was caused by either evaporative loss or partitioning into Earth’s inaccessible core. Iron meteorites are the accessible cores of differentiated planetesimals whose creation involved processes similar to Earth’s in the early Solar System. Measurements of their Cu isotope composition currently yield a large range of values that reveal little about core-forming processes. Here, we determine the equilibrium Cu isotope fractionation between solid and liquid metal and quantify the partitioning of Cu between troilite and metal, showing that the latter is a more significant factor in fractionating the Cu isotopes when planetesimal cores cool. Our experiments also call for verification of existing data for equilibrium Cu isotope fractionation between silicate–sulphide and silicate–metal to support current models using sulphide segregation to explain the heavy Cu isotope enrichment in bulk silicate Earth and Moon.