Collisional and alteration history of the CM parent body

摘要

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 displays16O-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 a16O-poor water and16O-rich anhydrous silicates. In contrast, the more altered lithologies exhibit16O-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 transported16O-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 of13C-depleted CH4produced from the reduction of C-bearing species by H2released 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 CH4by Rayleigh distillation.