Fossil records of high level of 60Fe in chondrules from unequilibrated chondrites

•60Fe–60Ni systematics in 4 chondrules from Semarkona, Efremovka, Vigarano using SIMS.•In situ studies in silicates phases give excesses in 60Ni up to 75‰.•60Fe/56Fe ratios (×10−7×10−7) of 7.8±3.77.8±3.7, 3.8±1.63.8±1.6, 2.2±1.12.2±1.1 for Efk Ch 1, Sem Ch 12, J5.•Potential issues of biases 60Fe/56Fe ratios have insignificant effect.•Reason for the discordance between 60Fe/56Fe from ICPMS & SIMS not found; massive star source 60Fe.

The short-lived now-extinct nuclide (SLN) 60Fe, which decays to 60Ni with a half-life of 2.62 Ma, is uniquely of stellar origin. Hence, its Solar System initial abundance yields information about the source of SLNs and the astrophysical environment in which the Solar System was born. Only a few chondrules (∼19) from unequilibrated ordinary chondrites have reported resolved 60Ni excesses using in situ secondary ion mass spectrometry implying Fe60/Fe56>∼0.6×10−7 in the early Solar System, and among these very few (3) have higher excesses implying Fe60/Fe56∼7×10−7 (Mishra et al., 2010, Mishra and Goswami, 2014 and Telus et al., 2012). At variance, multi-collector inductively coupled plasma mass spectrometer studies of bulk samples and mineral separates from differentiated meteorites, angrites, achondrites, and chondrules suggest a low abundance of 60Fe/56Fe of ∼1.4×10−8∼1.4×10−8 which would rule out the need for an external seeding of the early Solar with stellar 60Fe (Quitté et al., 2011 and Tang and Dauphas, 2012). Two Semarkona chondrules and one Efremovka chondrule analyzed in the present study have mass fractionation corrected excess of up to ∼75 permil (‰) and give 60Fe isochrons with initial 60Fe/56Fe ratios of (7.8±3.7)×10−7(7.8±3.7)×10−7, (3.8±1.6)×10−7(3.8±1.6)×10−7, and (2.2±1.1)×10−7(2.2±1.1)×10−7 (2σ), for Efremovka Ch 1, Semarkona Ch 12, and Semarkona Ch J5 respectively. The higher values of 60Fe/56Fe ratios seen in the chondrules of these least altered meteorites samples concur with and lend greater credence to the suggestion of a massive star as the source of 60Fe, and possibly of other short-lived nuclides, to the early Solar System. However, no definitive explanation (e.g. sample bias, effects of metamorphism, 60Fe heterogeneity) to the apparent disagreement with studies of bulk chondrules and chondrule fragments has been found.