Carbon polymorphs in Frontier Mountain ureilitic meteorites: A correlation with increasing the degree of shock?

Ureilites meteorite fragments present different levels of shock classified on the basis of optical observations of shock features in silicates. We have investigated by scanning electron microscopy (SEM), micro X-Ray Diffraction (XRD) and Micro-Raman Spectroscopy (MRS) five ureilitic fragments (FRO 95028, FRO 01089, FRO 97013, FRO 01088 and FRO 01012) with increasing degree of shock (from S2 to S6) with the purpose to observe if there is any correlation between the level of shock recorded by silicates and the presence of the different carbon polymorphs formed by shock. XRD patterns showed that all investigated samples contain nano-graphite. Besides this phase, sample FRO 95028, with S2 degree of shock, contains nanodiamond, while samples from level of shock S3 to S6 contain both nano- and micro-diamond. XRD results support the shock formation of micrometer-diamonds found in FRO 01089, FRO 97013, FRO 01088 and FRO 01012 with the assistance of (Fe, Ni)-alloys as catalysts at pressure >10 GPa (S3 shock level recorded by silicates). The formation of polycrystalline diamond is already allowed at pressure between 5-10 GPa. Temperature estimated by a graphite-thermometer based on MRS data, provided values in the range of 1291 to 1398 °C ±120 °C, revealing that there is not a considerable variation of the graphite temperature with the increasing degree of shock. • The silicates shock features and the carbon polymorphs present some correlation. • Polycrystalline diamonds are already allowed to form at 5–10 GPa. • Micro diamonds are only present in samples with shock levels from S3 to S6. • The graphite temperature and shock features in silicates are not correlated. • The shock event(s) is the formation process of ureilitic nanographite and diamond. [ABSTRACT FROM AUTHOR]