Low 60Fe abundance in Semarkona and Sahara 99555

TL;DR

This study measures the initial 60Fe abundance in meteorite samples, revealing it was much lower than previously reported, which impacts understanding of solar system formation and nearby supernova influence.

Contribution

The paper provides new MC-ICPMS measurements of 60Fe/56Fe ratios in Semarkona and Sahara 99555, clarifying the initial abundance of 60Fe in the early solar system.

Findings

Initial 60Fe/56Fe ratio in Semarkona chondrules is 5.4x10^-9.

Initial 60Fe/56Fe ratio in Sahara 99555 is 1.97x10^-9.

Overall initial 60Fe/56Fe ratio at solar system birth is approximately 1.01x10^-8.

Abstract

Iron-60 (t1/2=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 5.4x10-9. We also establish the initial 60Fe/56Fe ratio at the time of crystallization of the Sahara 99555 angrite, a chronological anchor, to be 1.97x10-9. These results demonstrate that the initial abundance of 60Fe at solar system birth was low, corresponding to an initial 60Fe/56Fe ratio of 1.01x10-8.