Iron 60 evidence for early injection and efficient mixing of stellar debris in the protosolar nebula

Abstract

Among extinct radioactivities present in meteorites, Fe-60 (t(1/2) = 1.49 Myr) plays a key role as a high-resolution chronometer, a heat source in planetesimals, and a fingerprint of the astrophysical setting of solar system formation. A critical issue with 60Fe is that it could have been heterogeneously distributed in the protoplanetary disk, calling into question the efficiency of mixing in the solar nebula or the timing of 60Fe injection relative to planetesimal formation. If this were the case, one would expect meteorites that did not incorporate 60Fe (either because of late injection or incomplete mixing) to show Ni-60 deficits (from lack of 60Fe decay) and collateral effects on other neutron-rich isotopes of Fe and Ni (coproduced with Fe-60 in core-collapse supernovae and AGB stars). Here, we show that measured iron meteorites and chondrites have Fe and Ni isotopic compositions identical to Earth. This demonstrates that 60Fe must have been injected into the protosolar nebula and mixed to less than 10% heterogeneity before formation of planetary bodies.