Low 60Fe abundance in semarkona and Sahara 99555

Abstract

Iron-60 (t<inf>1/2</inf> = 2.62 Myr) is a short-lived nuclide that can help constrain the astrophysical context of Solar System formation and date early Solar System events. A high abundance of ⁶⁰Fe(⁶⁰Fe/⁵⁶Fe ≈ 4 × 10^-7) was reported by in situ techniques in some chondrules from the LL3.00 Semarkona meteorite, which was taken as evidence that a supernova exploded in the vicinity of the birthplace of the Sun. However, our previous multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) measurements of a wide range of meteoritic materials, including chondrules, showed that ⁶⁰Fe was present in the early Solar System at a much lower level (⁶⁰Fe/⁵⁶Fe ≈ 10^-8). The reason for the discrepancy is unknown but only two Semarkona chondrules were measured by MC-ICPMS and these had Fe/Ni ratios below ∼2× chondritic. Here, we show that the initial ⁶⁰Fe/⁵⁶Fe ratio in Semarkona chondrules with Fe/Ni ratios up to ∼24× chondritic is (5.39 ± 3.27) × 10^-9. We also establish the initial ⁶⁰Fe/⁵⁶Fe ratio at the time of crystallization of the Sahara 99555 angrite, a chronological anchor, to be (1.97 ± 0.77) × 10^-9. These results demonstrate that the initial abundance of ⁶⁰Fe at Solar System birth was low, corresponding to an initial ⁶⁰Fe/⁵⁶Fe ratio of (1.01 ± 0.27) × 10^-8.