Fe–Ni and Al–Mg isotope records in UOC chondrules: Plausible stellar source of ~(60)Fe and other short-lived nuclides in the early Solar System

摘要

The short-lived now-extinct nuclide ~(60)Fe, present in the early Solar System, is a unique product of stellar nucleosynthesis. Even though the first hint for its presence in the early Solar System was obtained more than two decades back, a robust value for Solar System initial (SSI) ~(60)Fe/~(56)Fe is yet to be established. A combined study of ~(26)Al–~(26)Mg and ~(60)Fe–~(60)Ni isotope systematics in chondrules from unequilibrated ordinary chondrites of low petrologic type, Semarkona (LL3.0), LEW 86134 (L3.0), and Y 791324 (L3.1), has been conducted to infer the value of SSI ~(60)Fe/~(56)Fe. Seven of the analysed chondrules host resolved radiogenic excess in both ~(60)Ni and ~(26)Mg resulting from in situ decay of the short-lived nuclides ~(60)Fe and ~(26)Al, respectively. The initial ~(26)Al/~(27)Al values for these chondrules range from (6.9 ± 5.8) × 10~(-6) to (3.01 ± 1.78) × 10~(-5) that suggest their formation between 2.1 and 0.6 Ma after CAIs. The initial ~(60)Fe/~(56)Fe at the time of formation of these chondrules ranges from (3.2 ± 1.3) × 10~(-7) to (1.12 ± 0.39) × 10~(-6) and show a good correlation with their initial ~(26)Al/~(27)Al values suggesting co-injection of the two short-lived nuclides, ~(60)Fe and ~(26)Al, into the protosolar cloud from the same stellar source. Considering ~(26)Al as a reliable early Solar System chronometer, this data set yield a SSI ~(60)Fe/~(56)Fe value of (7.0 ± 1.2) × 10~(-7), if we adopt a half-life value of 2.6 Ma for ~(60)Fe reported in a recent study. Model stellar nucleosynthesis yields suggest that both a high mass (5–6.5 M_⊙) Asymptotic Giant Branch (AGB) star or a supernova (SN) could be the source of ~(60)Fe and ~(26)Al present in the early Solar System. A high mass (~25M_⊙) SN appears more plausible because of the much higher probability of its close association with the protosolar molecular cloud than a high mass AGB star. Such a SN can also account for SSI abundance of ~(26)Al and its correlated presence with ~(60)Fe in chondrules.