R.K. Mishra and J.N. Goswami

Physical Research Laboratory, Navrangpura, Ahmedabad, 380009;India

The short-lived now-extinct nuclide ⁶⁰Fe, 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) ⁶⁰Fe/⁵⁶Fe is yet to be established. A combined study of ²⁶Al-²⁶Mg and ⁶⁰Fe-⁶⁰Ni 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 ⁶⁰Fe/⁵⁶Fe. Seven of the analyzed chondrules host resolved radiogenic excess in both⁶⁰Ni and ²⁶Mg resulting from in situ decay of the short-lived nuclides ⁶⁰Fe and ²⁶Al, respectively. The initial²⁶Al/²⁷Al 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 to 0.6 Ma after CAIs. The initial ⁶⁰Fe/⁵⁶Fe 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 ²⁶Al/²⁷Al values suggesting co-injection of the two short-lived nuclides, ⁶⁰Fe and ²⁶Al, into the protosolar cloud from the same stellar source. Considering ²⁶Al as a reliable early Solar System chronometer, this data set yield a SSI⁶⁰Fe/⁵⁶Fe value of (7.0±1.2) ×10^-7, if we adopt a half-life value of 2.6 Ma for ⁶⁰Fe 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 ⁶⁰Fe and ²⁶Al 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 of²⁶Al and its correlated presence with ⁶⁰Fe in chondrules.

Reference
Mishra RK and Goswami JN (in press) Fe-Ni and Al-Mg isotope records in UOC chondrules: Plausible stellar source of 60Fe and other short-lived nuclides in the early Solar System. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.011]
Copyright Elsevier

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E. G. Rivera-Valentin^1,2 and A. C. Barr^1,2

¹Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, RI 02912, USA
²Center for Lunar Origin and Evolution, Southwest Research Institute, Boulder, CO 80302, USA

Late accretion of a “veneer” of compositionally diverse planetesimals may introduce chemical heterogeneity in the mantles of the terrestrial planets. The size of the late veneer objects is an important control on the angular momenta, eccentricities, and inclinations of the terrestrial planets, but current estimates range from meter-scale bodies to objects with diameters of thousands of kilometers. We use a three-dimensional global Monte Carlo model of impact cratering, excavation, and ejecta blanket formation to show that evidence of mantle heterogeneity can be preserved within ejecta blankets of mantle-exhuming impacts on terrestrial planets. Compositionally distinct provinces implanted at the time of the late veneer are most likely to be preserved in bodies whose subsequent geodynamical evolution is limited. Mercury may have avoided intensive mixing by solid-state convection during much of its history. Its subsequent bombardment may have then excavated evidence of primordial mantle heterogeneity introduced by the late veneer. Simple geometric arguments can predict the amount of mantle material in the ejecta blanket of mantle-exhuming impacts, and deviations in composition relative to geometric predictions can constrain the length-scale of chemical heterogeneities in the subsurface. A marked change in the relationship between mantle and ejecta composition occurs when chemically distinct provinces are ~250 km in diameter; thus, evidence of bombardment by thousand-kilometer-sized objects should be readily apparent from the variation in compositions of ejecta blankets in Mercury’s ancient cratered terrains.

Reference
Rivera-Valentin EG and Barr AC (2014) Estimating the Size of Late Veneer Impactors from Impact-induced Mixing on Mercury. The Astrophysical Journal – Letters 782:L8.
[doi:10.1088/2041-8205/782/1/L8]

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