¹Tim Lichtenberg, ²Tobias Keller,³Richard F.Katz,⁴Gregor J.Golabek,¹Taras V.Gerya
Earth and Planetary Science Letters 507, 154-165 Link to Article [https://doi.org/10.1016/j.epsl.2018.11.034]
¹Institute of Geophysics, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland
²Department of Geophysics, Stanford University, Stanford, CA 94305, United States
³Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, United Kingdom
⁴Bayerisches Geoinstitut, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany
Copyright Elsevier

Rocky planetesimals in the early solar system melted internally and evolved chemically due to radiogenic heating from ²⁶Al. Here we quantify the parametric controls on magma genesis and transport using a coupled petrological and fluid mechanical model of reactive two-phase flow. We find the mean grain size of silicate minerals to be a key control on magma ascent. For grain sizes ≳1 mm, melt segregation produces distinct radial structure and chemical stratification. This stratification is most pronounced for bodies formed at around 1 Myr after formation of Ca, Al-rich inclusions. These findings suggest a link between the time and orbital location of planetesimal formation and their subsequent structural and chemical evolution. According to our models, the evolution of partially molten planetesimal interiors falls into two categories. In the magma ocean scenario, the whole interior of a planetesimal experiences nearly complete melting, which would result in turbulent convection and core–mantle differentiation by the rainfall mechanism. In the magma sill scenario, segregating melts gradually deplete the deep interior of the radiogenic heat source. In this case, magma may form melt-rich layers beneath a cool and stable lid, while core formation would proceed by percolation. Our findings suggest that grain sizes prevalent during the internal heating stage governed magma ascent in planetesimals. Regardless of whether evolution progresses toward a magma ocean or magma sill structure, our models predict that temperature inversions due to rapid ²⁶Al redistribution are limited to bodies formed earlier than ≈1 Myr after CAIs. We find that if grain size was ≲1 mm during peak internal melting, only elevated solid–melt density contrasts (such as found for the reducing conditions in enstatite chondrite compositions) would allow substantial melt segregation to occur.

¹Yann-Aurélien Brugier, ¹Jean Alix Barrat,²Bleuenn Gueguen, ¹Arnaud Agranier, ^3,4Akira Yamaguchi, ⁵Addi Bischoff
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.12.009]
¹Laboratoire Géosciences Océan (UMR CNRS 6538), Université de Bretagne Occidentale et Institut Universitaire Européen de la Mer, Place Nicolas Copernic, 29280 Plouzané, France
²UMS CNRS 3113, Université de Bretagne Occidentale et Institut Universitaire Européen de la Mer, Place Nicolas Copernic, 29280 Plouzané, France
³National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
⁴Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (the Graduate University for Advanced Studies), Tokyo 190-8518, Japan
⁵Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

The Ureilite Parent Body (UPB) was a C-rich planetary embryo disrupted by impact. Ureilites are fragments of the UPB mantle and among the most numerous achondrites. Zinc isotopic data are presented for 26 unbrecciated ureilites and a trachyandesite (ALM-A) from the same parent body. The δ66Zn values of ureilites range from 0.40 to 2.71 ‰ including literature results. Zinc isotopic compositions do not correlate with the compositions of olivine cores, with C and O isotopic compositions, with Zn abundances, nor with shock grades. The wide range of δ66Zn displayed by the ureilites is chiefly explained by evaporation processes that took place during the catastrophic breakup of the UPB. During breakup, the high temperatures of the UPB mantle allowed Zn to evaporate, regardless of the intensity the shock suffered by the ureilitic debris. For the most shocked of them, post-shock heating permitted greater evaporation, and heavier Zn isotopic compositions. The surface of the UPB was certainly much colder than the mantle before the breakup. Therefore, crustal rocks were probably less prone to Zn evaporation. ALM-A, the sole crustal rock analyzed at present, has a δ66Zn value (0.67 ‰) significantly higher than those of regular chondrites. This result indicates that its mantle source displayed already non-chondritic Zn isotopic compositions before the breakup of the UPB.