Shock‐heated graphite in three IAB iron meteorites—Implications on the formation of diamond

Iron meteorites, originating from the deepest parts of their parent bodies and separated during major break-up events, surprisingly rarely contain diamonds despite experiencing similar pressure–temperature conditions as diamond-bearing ureilites. In this study, graphite from three non-magmatic IAB iron meteorites Canyon Diablo, Campo del Cielo, and Yardymly was analyzed using micro-Raman spectroscopy, revealing the presence of the graphite G-band, the disorder-induced D-band, and occasionally the D′-band. Temperature estimates based on the G-band full width at half maximum (ranging from 1155 to 1339°C) are consistent with those found in ureilites. However, unlike in ureilites, no diamond bands were detected, as confirmed by μ-X-ray diffraction. The absence of diamonds is interpreted to be related to the thermal and mechanical properties of the iron meteorite matrix. Its high thermal diffusivity results in similar temperatures to ureilites, but its ductility dissipates shock-wave energy through plastic deformation, unlike the brittle ureilite matrix, which more effectively transmits the energy. Consequently, graphite in iron meteorites was heated but did not experience the high-pressure conditions required for diamond formation. Thus, we propose that impacts must either involve substantial energy or that graphite must be located close to the impact site, where it can experience high energies before these dissipate.