Alkali magmatism on a carbonaceous chondrite planetesimal

1Jérôme Aléon,2Alice Aléon-Toppani,3Bernard Platevoet,3Jacques-Marie Bardintzeff,4Kevin D. McKeegan, 5François Brisset
Proceedings of the National Academy of Sciences of the United States of Americs (PNAS) (in Press) Link to Article [DOI:
https://doi.org/10.1073/pnas.1919550117]
1Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR 7590, Sorbonne Université, Museum National d’Histoire Naturelle, CNRS, Université Pierre et Marie Curie, Institut de Recherche pour le Développement, 75005 Paris, France;
2Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France;
3Université Paris-Saclay, Sciences de la Terre, Volcanologie-Planétologie, UMR CNRS 8148 Geosciences Paris Sud, F-91405 Orsay, France;
4Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095-1567;
5Université Paris-Saclay, CNRS, Institut de Chimie Moléculaire et des Matériaux d’Orsay, 91405 Orsay Cedex, France

Recent isotopic and paleomagnetic data point to a possible connection between carbonaceous chondrites and differentiated planetary materials, suggesting the existence, perhaps ephemeral, of transitional objects with a layered structure whereby a metal-rich core is enclosed by a silicate mantle, which is itself overlain by a crust containing an outermost layer of primitive solar nebula materials. This idea has not received broad support, mostly because of a lack of samples in the meteoritic record that document incipient melting at the onset of planetary differentiation. Here, we report the discovery and the petrologic–isotopic characterization of UH154-11, a ferroan trachybasalt fragment enclosed in a Renazzo-type carbonaceous chondrite (CR). Its chemical and oxygen isotopic compositions are consistent with very-low-degree partial melting of a Vigarano-type carbonaceous chondrite (CV) from the oxidized subgroup at a depth where fluid-assisted metamorphism enhanced the Na content. Its microdoleritic texture indicates crystallization at an increasing cooling rate, such as would occur during magma ascent through a chondritic crust. This represents direct evidence of magmatic activity in a carbonaceous asteroid on the verge of differentiating and demonstrates that some primitive outer Solar System objects related to icy asteroids and comets underwent a phase of magmatic activity early in the Solar System. With its peculiar petrology, UH154-11 can be considered the long-sought first melt produced during partial differentiation of a carbonaceous chondritic planetary body, bridging a previously persistent gap in differentiation processes from icy cometary bodies to fully melted iron meteorites with isotopic affinities to carbonaceous chondrites.

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