60Fe-60Ni chronology of core formation in Mars

Abstract

The timescales of accretion, core formation, and magmatic differentiation in planetary bodies can be constrained using extinct radionuclide systems. Experiments have shown that Ni becomes more siderophile with decreasing pressure, which is reflected in the progressively higher Fe/Ni ratios in the mantles of Earth, Mars and Vesta. Mars formed rapidly and its mantle has a high Fe/Ni ratio, so the ⁶⁰Fe-⁶⁰Ni decay system (<inf>t1/2</inf> = 2.62Myr) is well suited to establish the timescale of core formation in this object. We report new measurements of ⁶⁰Ni/⁵⁸Ni ratios in bulk SNC/martian (Shergotty-Nakhla-Chassigny) meteorites and chondrites. The difference in εNi60 values between SNC meteorites and the building blocks of Mars assumed to be chondritic (55% ordinary chondrites + 45% enstatite chondrites) is +0.028 ± 0.023 (95% confidence interval). Using a model of growth of planetary embryo, this translates into a time for Mars to have reached ~44% of its present size of 1.9-0.8+1.7Myr with a strict lower limit of 1.2 Myr after solar system formation, which agrees with a previous estimate based on ¹⁸²Hf-¹⁸²W systematics. The presence of Mars when planetesimals were still being formed may have influenced the formation of chondrules through bow shocks or by inducing collisions between dynamically excited planetesimals. © 2014.