25 November 2020
A shock origin for meteorite diamonds
Published online 30 September 2020
Analyses of samples from rare meteorites demonstrate how their diamonds likely formed during sudden impacts.
Ureilites are rare meteorites that differ in mineral composition from other rocky meteorites, incorporating a significant percentage of carbon in the form of graphite and tiny diamonds. Scientists have long debated the origin of these diamonds.
An international team of scientists, including researcher at the Saudi Aramco R&D Center and another at the University of Khartoum, Sudan, analysed samples from two ureilites, one that fell in the Nubian Desert in 2008 and another found in Morocco in 2013. Their findings refute a commonly held hypothesis that the diamonds originate from the high-pressure mantle of the planet-sized bodies the meteorites came from. Instead, they resulted when a parent body experienced a major impact causing smaller rock bodies to split from it.
Multiple microscopy and spectroscopy examinations showed that silicates in the samples had a high degree of shock metamorphism: their texture had been transformed by the power of an impact. Further, the diamonds shared a striped crystal texture and blade shape similar to the graphite in the samples.
“This suggests the diamonds were formed from the graphite under the pressure of a sudden impact, leaving no time for the diamonds to assume their common crystal structure,” says Cyrena Goodrich at the Lunar and Planetary Institute in Houston, US, who co-led the study.
The team suggests that metals within the ureilites would have melted on impact, hastening the transformation of graphite into diamond. This is supported by evidence from the industrial production of diamonds, which uses metallic iron to catalyse diamond formation under lower pressures and higher reaction rates than mantle-formed diamonds.
Nestola, F. et al. Impact shock origin of diamonds in ureilite meteorites. PNAS http://dx.doi.org/10.1073/pnas.1919067117 (2020).