Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
8910573 | Geochimica et Cosmochimica Acta | 2018 | 70 Pages |
Abstract
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 (δ18Oâ¯ââ¯30-37â°, δ17Oâ¯ââ¯15-18â° and Î17Oâ¯ââ¯â2 to 0â°, 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 (δ18Oâ¯ââ¯17-23â°, δ17Oâ¯ââ¯6-9â° and Î17Oâ¯ââ¯â4 to â1â°, 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) (10-30â¯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 (δ13Câ¯ââ¯30-71â°, 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 110â¯Â°C, the observed δ13C variability in T2a calcite can be reproduced by the escape of â15-50% of dissolved carbon into CH4 by Rayleigh distillation.
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geochemistry and Petrology
Authors
Lionel G. Vacher, Yves Marrocchi, Johan Villeneuve, Maximilien J. Verdier-Paoletti, Matthieu Gounelle,