Graphite-based geothermometry of Almahata Sitta ureilites

Almahata Sitta (AhS) is the first meteorite produced from impact of a known asteroid. The 2008TC3 asteroid was discovered on 6th October 2008 and tracked a few hours before the impact in Sudan [1]. AhS is defined as a "polymict breccias" and its fragments show a great lithological diversity of ureilitic rocks and enstatite, ordinary, carbonaceous and Rumuruti chondrites. We have investigated three ureilitic stones (AhS 209B, AhS 72 and AhS A135A) from the AhS meteorite by Micro-Raman Spectroscopy (MRS) to retrieve geothermometric information using the relatively recent approach adopted by [2]. This approach is based on the measurement of the Full Width at Half Maximum (FWHM) of the graphite G band that, by comparison to previous geothermometric approaches, can provide temperatures of high temperature environments (i.e._900- 1000 °C). The Raman spectra of graphite from the samples were collected in order to retrieve the graphite crystallization temperature hereafter Tmax [2], and in turn to apply it to the ureilite parent body. In addition, the peak position of the graphite Raman spectra and the I(D)/I(G) ratio (I=intensity; D=D band; G=G band) from MRS allowed us to interpret the investigated graphite as nanographite [3]. Our calculated temperatures are slightly higher than those reported by [2] on the ureilitic fragment #7 of AhS [e.g. 990 (±120) °C]. However, if we account for the temperature uncertainties of this approach (i.e. ±120 °C), our data do agree with those by [2] to better than one e.s.d. This range of temperatures agrees with the range of peak equilibration temperatures of ureilites recorded by pyroxene thermometry [4;5;6]. References: [1] Shaddad et al. (2010) - Meteorit Planet Sci. [2] Ross et al. (2011) - Meteorit Planet Sci. [3] Ferrari & Roberson (2000) - Phis. Rev. B. [4] Warren et al. (2012) - Meteorit and Planet Sci. [5] Goodrich et al. (2013) - Meteorit Planet Sci. [6] Herrin et al. (2010) - Meteorit Planet Sci.