Collisional and alteration history of the CM parent body

1Lionel G.Vacher, 1Yves Marrocchi, 1Johan Villeneuve, 2Maximilien J.Verdier-Paoletti, 3,4Matthieu Gounelle
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.08.006]
1CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-les-Nancy, F-54501, France
2Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015, USA
3IMPMC, MNHN, Sorbonne Universités, UMR CNRS 7590, 57 rue Cuvier, 75005 Paris, France
4Institut Universitaire de France, Maison des Universités, 103 boulevard Saint-Michel, 75005 Paris, France
Copyright Elsevier

Boriskino is a little studied CM2 chondrite composed of millimeter-sized clasts of different lithologies and degrees of alteration. Boriskino thus offers a good opportunity to better understand the preaccretionary alteration history and collisional evolution that took place on the CM parent body. The least altered lithology displays 16O-poor Type 1a calcite and aragonite grains (δ18O ≈ 30-37‰, δ17O ≈ 15-18‰ and Δ17O ≈ -2 to 0‰, SMOW) that precipitated early, before the establishment of the petrofabric, from a fluid whose isotopic composition was established by isotopic exchange between a 16O-poor water and 16O-rich anhydrous silicates. In contrast, the more altered lithologies exhibit 16O-rich Type 2a and veins of calcite (δ18O ≈ 17-23‰, δ17O ≈ 6-9‰ and Δ17O ≈ -4 to -1‰, SMOW) that precipitated after establishment of the deformation, from transported 16O-rich fluid in preexisting fractures. From our petrographic and X-ray tomographic results, we propose that the more altered lithologies of Boriskino were subjected to high intensity impact(s) (10-30 GPa) that produced a petrofabric, fractures and chondrule flattening. Taking all our results together, we propose a scenario for the deformation and alteration history of Boriskino, in which the petrographic and isotopic differences between the lithologies are explained by their separate locations into a single CM parent body. Based on the δ13C-δ18O values of the Boriskino Type 2a calcite (δ13C ≈ 30-71‰, PDB), we propose an alternative δ13C-δ18O model where the precipitation of Type 2a calcite can occurred in an open system environment with the escape of 13C-depleted CH4 produced from the reduction of C-bearing species by H2 released during serpentinization or kamacite corrosion. Assuming a mean precipitation temperature of 110°C, the observed δ13C variability in T2a calcite can be reproduced by the escape of ≈ 15-50% of dissolved carbon into CH4 by Rayleigh distillation.

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