IRON ISOTOPE CONSTRAINTS ON PLANETESIMAL CORE FORMATION IN THE EARLY SOLAR SYSTEM

Stable isotopes of iron can constrain the history of core formation within planetesimals of a variety of sizes in the early solar system. A necessary ingredient for such constraints is the equilibrium ⁵⁷Fe/⁵⁴Fe fractionation between metal and silicate. We measured this fractionation in two aubrite-like meteorites with known temperatures of crystallization. We find that the heavy isotopes of iron are concentrated in metal relative to silicate, consistent with recent experiments by Shahar and others. In order to verify that Rayleigh distillation during fractional crystallization was not a cause of iron isotope fractionation in iron meteories, we also measured iron isotope ratios in a suite of iron meteorites representing a large range of degrees of fractional crystallization. We find no clear variation in ⁵⁷Fe/⁵⁴Fe among these samples. Silicate materials in HED meteorites, representing planetesimal silicates in general, are chondritic in ⁵⁷Fe/⁵⁴Fe while iron meteorites have relatively high ⁵⁷Fe/⁵⁴Fe. These results, in combination with the measured fractionation between silicate and metal, suggest that the cores represented by iron meteorites comprised a small fraction of the total iron budget for their parent bodies compared with that for Earth. We consider that core formation may have been incomplete in these bodies; core formation may have been quenched in these objects as a result of catastrophic planetesimal disruption by collisions.