IRON-60 HETEROGENEITY AND INCOMPLETE ISOTOPE MIXING IN THE EARLY SOLAR SYSTEM

Abstract

Short-lived radionuclides (e.g., 26Al, 53Mn, 60Fe, 182Hf) are widely used to refine the chronology of the early solar system. They provide chronological information, however, only if they were homogeneously distributed in the source region of the objects under scrutiny at the time of their formation. With the high level of precision now achieved on isotopic measurements, very short time intervals can in principle be resolved and a precise evaluation of the initial homogeneity degree becomes increasingly crucial. High-precision nickel isotope data for differentiated meteorites (angrites, ureilites) and chondritic (CB) components allow us to test the initial distribution of radioactive 60Fe and stable Ni isotopes. Although these meteorites appear to have formed nearly contemporaneously, they yield variable initial 60Fe/56Fe ratios. Besides, the CB metal nodules and ureilite silicates show nucleosynthetic anomalies. The new data presented here do not confirm the recently inferred late injection of 60Fe into the protoplanetary disk. Instead, live 60Fe was present, but heterogeneously distributed, from the start of the solar system, revealing an incomplete mixing of material from various nucleosynthetic sources and restricting the use of the 60Fe-60Ni system as a chronometer.