Olivine grain growth in partially molten Fe–Ni–S: A proxy for the genesis of pallasite meteorites

1Giulio F.D.Solferino, 2Gregor J.Golabek
Earth and Planetary Science Letters 504, 38-52 Link to Article [https://doi.org/10.1016/j.epsl.2018.09.027]
1Department of Earth Sciences, Royal Holloway University of London, TW20 0EX Egham, United Kingdom
2Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
Copyright Elsevier

The origin of pallasites has been the focus of a number of recent studies. Yet, their formation process remains elusive, while the mechanism leading to the genesis of the sub-group termed ‘mixed type’ pallasites (containing polygonal, rounded, and fragmental olivines simultaneously) is unclear. Here we test the hypothesis of mixing of olivine fragments with Fe–Ni–S after a non-destructive impact followed by annealing employing both experimental analogues and numerical models.
The experimental series evidenced that the addition of sulfur to olivine + Fe–Ni accelerates olivine grain growth, though the growth rate is reduced when Fe–Ni–S is not fully molten. This is shown to be the consequence of competing growth of olivine and Fe–Ni grains.
Numerical models satisfying available formation constraints from natural samples indicate that planetesimals with radii ≥200 km are favorable for the genesis of rounded olivine-bearing pallasites by annealing of fragments in partially molten Fe–Ni–S. Moreover, early mixing in the planetesimal can form regions containing olivine grains with different grain sizes that could explain the formation of mixed-type pallasites.

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