* Astronomy

Title: Low 60Fe abundance in Semarkona and Sahara 99555
Author: Haolan Tang, Nicolas Dauphas

Iron-60 (t_1/2=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= 4x10^-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 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 ~2x chondritic. Here, we show that the initial ⁶⁰Fe/⁵⁶Fe ratio in Semarkona chondrules with Fe/Ni ratios up to ~24x chondritic is (5.4±3.27) x10^-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) x10^-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) x10^-8.

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