Abundance, distribution, and origin of 60Fe in the solar protoplanetary disk

Abstract

Meteorites contain relict decay products of short-lived radionuclides that were present in the protoplanetary disk when asteroids and planets formed. Several studies reported a high abundance of 60Fe (t1/2=2.62±0.04Myr) in chondrites (60Fe/56Fe∼6×10−7), suggesting that planetary materials incorporated fresh products of stellar nucleosynthesis ejected by one or several massive stars that exploded in the vicinity of the newborn Sun. We measured 58Fe/54Fe and 60Ni/58Ni isotope ratios in whole rocks and constituents of differentiated achondrites (ureilites, aubrites, HEDs, and angrites), unequilibrated ordinary chondrites Semarkona (LL3.0) and NWA 5717 (ungrouped petrologic type 3.05), metal-rich carbonaceous chondrite Gujba (CBa), and several other meteorites (CV, EL H, LL chondrites; IIIAB, IVA, IVB iron meteorites). We derive from these measurements a much lower initial 60Fe/56Fe ratio of (11.5±2.6)×10−9 and conclude that 60Fe was homogeneously distributed among planetary bodies. This low ratio is consistent with derivation of 60Fe from galactic background (60Fe/56Fe≈2.8×10−7 in the interstellar medium from γ-ray observations) and can be reconciled with high 26Al/27Al∼5×10−5 in chondrites if solar material was contaminated through winds by outer layers of one or several massive stars (e.g., a Wolf–Rayet star) rich in 26Al and poor in 60Fe. We present the first chronological application of the 60Fe–60Ni decay system to establish the time of core formation on Vesta at 3.7−1.7+2.5Myr after condensation of calcium–aluminum-rich inclusions (CAIs).