Oxidized conditions in iron meteorite parent bodies

Overview & Synopsis

Magmatic iron meteorites are thought to originate from the cores of differentiated planetary embryos, but it is unclear wherethey formed in the circumstellar disk. Oxygen fugacity (fO2) is thought to have varied radially across the disk, affecting planetaryaccretion and core formation. Metal–silicate equilibration experiments show that the chromium concentration in metallicmelt increases with decreasing fO2. Chromium can therefore be used to reconstruct fO2 during metal segregation. Here, wereport mass-dependent fractionation of stable chromium isotopes measured for several groups of magmatic iron meteorites.We find that chromium concentrations are variable and most samples are isotopically heavy compared with the bulk silicateEarth and chondrites. Although it is evident that the chromium isotopes were systematically affected by fractional crystallization,we use the mass-dependent fractionation behaviour to derive initial chromium concentrations for the planetary embryocores. We find that the cores were diverse in fO2 and demonstrate that core formation in some planetary embryos occurredunder relatively oxidizing conditions (fO2 relative to iron–wüstite (IW) buffer (Δ IW) = − 0.95 ± 0.15). It is difficult to reconcilethis with the view that iron meteorite parent bodies formed closer to Earth and subsequently migrated outwards. Instead,we suggest that these parent bodies formed at greater distances, beyond Mars.