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

Abstract

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 AE 120 °C and 1370 AE 120 °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 AE 16 °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.

Formation of diamond and lonsdaleite in ureilites by impact shock processing of graphite

“…(2022) calculated 1314°C for Yamato‐74123, and Christ et al. (2022) calculated 1368 and 1370°C for NWA 3140 and NWA 6871. Based on the equation given by Cody et al.…”

“…(2008) to graphite in ureilites. Since then, this thermometric expression has been used in several publications to calculate a maximum temperature based on graphite in ureilitic samples (e.g., Barbaro et al., 2020, 2022; Christ et al., 2022). Ross et al.…”

“…Ross et al (2011) were the first to apply the equation of Cody et al (2008) to graphite in ureilites. Since then, this thermometric expression has been used in several publications to calculate a maximum temperature based on graphite in ureilitic samples (e.g., Barbaro et al, 2020Barbaro et al, , 2022Christ et al, 2022). Ross et al (2011) calculated a maximum parent body temperature of 990 AE 120°C for one AHS sample, whereas Barbaro et al (2020) obtained temperatures in the range of 1103-1361°C for three AHS samples, Barbaro et al (2022) calculated 1314°C for Yamato-74123, and Christ et al (2022 calculated 1368 and 1370°C for NWA 3140 and NWA 6871.…”

“…However, it should be noted that the equation given by Ross et al (2011) deviates from the one originally published by Cody et al (2008). The publications by Barbaro et al (2020), Barbaro et al (2022), andChrist et al (2022) all use the modified equation given by Ross et al (2011). A detailed elaboration on this aspect is given in the appendix (Table S1).…”

Canyon Diablo lonsdaleite is a nanocomposite containing c/h stacking disordered diamond and diaphite

“…Since its first description in 1967, natural and synthetic materials with diffraction data matching the Canyon Diablo lonsdaleite have been widely reported. For example, from meteorites [6][7][8][9][10]57], impact structures [11][12][13] and terrestrial sediments [14]. Structural features used for lonsdaleite identification are known from diamonds occurring in various terrestrial geological settings including ultrahigh-pressure metamorphic rocks [15] and cratonic lamproite colluviums [58], in highly strained, mechanically twinned pink diamonds [59] as well as in nanodiamonds found in primitive meteorites [25,29] and synthetic materials unrelated to shock [2,20,22,23].…”