Graphite-Based Geothermometry on Almahata Sitta Ureilitic Meteorites

Abstract

The thermal history of carbon phases, including graphite and diamond, in the ureilite meteorites has implications for the formation, igneous evolution, and impact disruption of their parent body early in the history of the Solar System. Geothermometry data were obtained by micro-Raman spectroscopy on graphite in Almahata Sitta (AhS) ureilites AhS 72, AhS 209b and AhS A135A from the University of Khartoum collection. In these samples, graphite shows G-band peak centers between 1578 and 1585 cm−1 and the full width at half maximum values correspond to a crystallization temperature of 1266 °C for graphite for AhS 209b, 1242 °C for AhS 72, and 1332 °C for AhS A135A. Recent work on AhS 72 and AhS 209b has shown graphite associated with nanodiamonds and argued that this assemblage formed due to an impact-event. Our samples show disordered graphite with a crystalline domain size ranging between about 70 and 140 nm. The nanometric grain-size of the recrystallized graphite indicates that it records a shock event and thus argues that the temperatures we obtained are related to such an event, rather than the primary igneous processing of the ureilite parent body.

Characterization of carbon phases in Yamato 74123 ureilite to constrain the meteorite shock history

“…T max values range between 1265 and 1334 (± 120) °C. These temperatures are slightly higher than those obtained by Ross et al (2011) on graphite in AhS #7 ureilitic fragment (T max of 990 ± 120 °C), whereas they are very similar to those obtained by Barbaro et al (2020b) Table 3. Table 2.…”

“…Such nano-graphite cannot be the pristine graphite of the UPB, which should have been micrometer-sized, due to the long residence time spent in the UPB deep interior. Therefore, as reported by Barbaro et al (2020b) for three AhS ureilitic fragments, nano-graphite formed by shock.…”

“…We applied the geothermometric approach by Cody et al (2008) and Ross et al (2011), following the same procedure as reported in Barbaro et al (2020aBarbaro et al ( , 2020b for Almahata Sitta samples (AhS 209b, AhS 72, and AhS A135A), to determine the T max recorded by graphite.…”

“…In order to compare our Γ G data with those published by Ross et al (2011) and Barbaro et al (2020b), we corrected our data for the instrumental peak broadening using a high-This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change.…”

“…* See Cody et al (2008) and Ross et al (2011) for a detailed description of the applied geothermometry. The temperature values recorded by graphite in AhS #7 sample are after Ross et al (2011) and those recorded for AhS 209, AhS 72, and AhS A135A are from Barbaro et al (2020b).…”