New Constraints on the Abundance of 60Fe in the Early Solar System

Abstract

Establishing the abundance of the extinct radionuclide ⁶⁰Fe (half-life 2.62 Ma) in the early solar system is important for understanding the astrophysical context of solar system formation. While bulk measurements of early solar system phases show a low abundance consistent with galactic background, some in situ measurements by secondary ion mass spectrometry (SIMS) imply a higher abundance, which would require injection from a nearby supernova (SN). Here we present in situ nickel isotopic analyses by resonance ionization mass spectrometry (RIMS) in a chondrule from the primitive meteorite Semarkona (LL3.00). The same chondrule had been previously analyzed by SIMS. Despite improved precision compared to SIMS, the RIMS nickel isotopic data do not reveal any resolved excesses of ⁶⁰Ni that could be unambiguously ascribed to in situ ⁶⁰Fe decay. Linear regression of ⁶⁰Ni/⁵⁸Ni versus ⁵⁶Fe/⁵⁸Ni yields an initial ⁶⁰Fe/⁵⁶Fe ratio for this chondrule of (3.8 ± 6.9) 10^-8, which is consistent with both the low initial value found by bulk measurements and the low end of the range of initial ratios inferred from some in situ work. The same regression also gives a solar initial ⁶⁰Ni/⁵⁸Ni ratio, which shows that this sample was not disturbed by nickel mobilization, thus agreeing with a low initial ⁶⁰Fe/⁵⁶Fe ratio. These findings agree with a re-evaluation of previous SIMS measurements of the same sample. Supernova injection of ⁶⁰Fe into the solar system or its parental cloud material is therefore not necessary to account for the measured solar system's initial amount of ⁶⁰Fe.