Origin of diamonds in the Canyon Diablo and Novo Urei meteorites

Detailed optical studies of the textural features of several hundred diamonds and associated phases in three specimens of the Canyon Diablo meteorite and one specimen of the Novo Urei achondrite were made. Diamonds in the Canyon Diablo are associated exclusively with troilite, graphite, and schreibersite, and they occur both in shock-heated and unreheated specimens. Diamonds in the Novo Urei occur in pods and lenses interstitial to slightly deformed olivine and pyroxene crystals. The diamonds, once formed, were shattered and partially graphitized. X-ray and electron microscope studies show that the diamond bodies, which range in size up to 2 mm in Canyon Diablo, are now composed of 0.5-μ subindividuals with nearly random orientations. The evidence obtained from these studies indicates that the diamonds probably precipitated as a stable phase from a late-freezing residuum composed primarily of iron sulfide, iron phosphide, and carbon, under high gravitational pressures. The diamonds were subsequently shattered, distorted, and partially graphitized, possibly from a pressure drop due to the disruption of the large parent body. The physical evidence against the theory that the diamonds originated by high temperature and pressures due to shock is convincing.