Scientists have offered new insights into the origin of diamonds in a group of stony meteorites called ureilites. The ureilites are fragments of a larger asteroidal parent body that was smashed to pieces through at least one major collision with another dwarf planet or asteroid. Ureilites commonly contain large quantities of carbon in the form of graphite and also diamonds in lesser abundance. Previously, researchers have proposed that the diamonds in ureilites formed like those in Earth’s deep interior, where the high pressures needed to form a diamond are provided by the weight of the overlying rock. This would imply long growth times at high static pressures within a planet-sized body. However, new research conducted by Fabrizio Nestola (University of Padova, Italy), Cyrena Goodrich (Universities Space Research Association at the Lunar and Planetary Institute), and their colleagues shows there is no evidence requiring formation under the high static pressures and long-growth-time conditions of a planet’s deep interior.

The research team investigated fragments of three diamond-bearing ureilites and, in one of these samples, found the largest single-crystal diamonds ever discovered in a ureilite. In addition to diamonds of up to 100 micrometers in size, numerous nests of nanodiamonds, as well as nanographite, were found in the ureilites. A multi-pronged approach using scanning electron microscopy, micro-X-ray diffraction, transmission electron microscopy, and micro-Raman spectroscopy suggests that the most likely process by which both microdiamonds and nanodiamonds in ureilites formed is in a shock event. The shock level recorded by silicates in the ureilite samples is characterized by a peak pressure possibly as low as 15 GPa. Micrometer-sized diamonds can form from crystalline graphite in shock events when catalyzed by metallic Fe-Ni-C liquid, which was demonstrably present during the major shock events that occurred on the ureilite parent body, and do not require high static pressures and long growth times. The origin of diamonds in ureilites has important implications for models of planetary formation in the early solar system. The high static pressure model suggested that the ureilite parent body might have been a large planetary embryo; however, the shock origin is consistent with a much smaller, asteroid-sized parent body. READ MORE