Shock degree and graphite geothermometry in ureilites NWA 6871 and NWA 3140

Carbon aggregates from two differently shocked ureilites were analyzed to gain insight into the shock transformation of graphite to diamond in ureilites, which happened when the ureilite parent body (UPB) was most likely destroyed by massive impact events. We present data for carbon aggregates from the highly shocked (U-S6) Northwest Africa (NWA) 6871 and the medium shocked (U-S3) NWA 3140. Both samples contain abundant carbon aggregates which were analyzed by X-ray diffraction and micro-Raman spectroscopy revealing the presence of close associations of (compressed) nanographite, micro- and nanodiamond, as well as Fe-rich phases. Graphite and diamond in NWA 6871 show shock indicators that are absent in NWA 3140. Based on Raman geothermometry on graphite, we calculated mean temperatures of 1368 +/- 120 degrees C and 1370 +/- 120 degrees C for NWA 3140 and NWA 6871, respectively. For comparison, a geothermometer based on the partitioning of Cr between olivine and low-Ca pyroxene was applied on NWA 3140, which yielded a temperature of only 1215 +/- 16 degrees C. The graphite-based temperatures are the highest reported for graphite in ureilites so far and exceed calculated magmatic temperatures for ureilites from silicate- and chromite-based geothermometers. Graphite temperatures fall into the temperature field of catalytic diamond synthesis, which supports the hypothesis of direct transformation from graphite to diamond upon shock. Although the temperatures estimated seem to be independent of the shock degree, they can be ascribed to the shock event that destroyed the UPB.