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 (t<inf>1/2</inf>=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). © 2012.