Investigation of Carbon Phases in Frontier Mountain Ureilites Using Raman Spectroscopy and X-Ray Diffraction to Unveil Shock Metamorphism Features

The evolutionary history of ureilites has been debated for years. In this context, the characterization of carbon phases in these meteorites can be of great scientific importance, providing information on the formation of graphite and diamond and revealing the history and origin of this group of achondrites. In this work, a multi-analytical approach was adopted, integrating optical microscopy, micro-Raman spectroscopy, and X-ray diffraction (XRD), using instruments of the Department of Geosciences at the University of Padova. Optical microscopy made it possible to confirm and determine the shock degree of the samples, assigning FRO 01030 (S1), FRO 01147 (S3–S4), FRO 90233 (S4), and FRO 90036 (S5) based on the classification of Stöffler et al. (1991; 2018). Micro-Raman analyses were carried out on the carbonaceous aggregates of all four Antarctic samples. The Raman data show the G and D bands at 1580 and 1350 cm⁻¹, respectively, and the diamond peak at 1332 cm⁻¹ in the medium–high shock sample FRO 01147 (S3–S4). At this shock level, the D′ band centered at 1620 cm⁻¹ is also evident as a shoulder of the main G peak. XRD analyses performed on the samples at the lowest and highest shock degrees, namely FRO 01030 (S1) and FRO 90036 (S5), show the presence of graphite only in the former, whereas in the latter they highlight the coexistence of nanodiamond and nanographite. In addition, the petrographic characterization of silicates (mosaicism and planar fractures) suggests that diamonds formed as a result of a major shock event (≥ 15 GPa) on the ureilitic parent body (UPB). The results obtained are consistent with those observed in the ureilites Y-74123 (Barbaro et al., 2022) and NWA 7983 (Nestola et al., 2020) and in the Frontier Mountain (Barbaro et al., 2023b). Graphite geothermometry from Raman data indicates Tmax ranges of 1406–1503 °C (±120 °C) for FRO 01030, 1086–1503 °C (±120 °C) for FRO 01147, 1068–1585 °C (±120 °C) for FRO 90233, and 1172–1380 °C (±120 °C) for FRO 90036. The data obtained in this work can enrich our understanding of carbon evolution in the primordial Solar System.