Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 117, Issue 41, Pages 25310-25318Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1919067117
Keywords
diamond; impact shock; ureilitic diamonds; ureilites; carbon phases
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Funding
- Programma Nazionale di Ricerche in Antartide (PNRA) [PNRA18 00247-A]
- NASA [80NSSC17K0165, NNX17AH09GS03, 80NSSC19K0507]
- IMPACt project [R164WEJAHH]
- PNRA
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The origin of diamonds in ureilite meteorites is a timely topic in planetary geology as recent studies have proposed their formation at static pressures >20 GPa in a large planetary body, like diamonds formed deep within Earth's mantle. We investigated fragments of three diamond-bearing ureilites (two from the Almahata Sitta polymict ureilite and one from the NWA 7983 main group ureilite). In NWA 7983 we found an intimate association of large monocrystalline diamonds (up to at least 100 mu m), nanodiamonds, nanographite, and nanometric grains of metallic iron, cohenite, troilite, and likely schreibersite. The diamonds show a striking texture pseudomorphing inferred original graphite laths. The silicates in NWA 7983 record a high degree of shock metamorphism. The coexistence of large monocrystalline diamonds and nanodiamonds in a highly shocked ureilite can be explained by catalyzed transformation from graphite during an impact shock event characterized by peak pressures possibly as low as 15 GPa for relatively long duration (on the order of 4 to 5 s). The formation of large (as opposed to nano) diamond crystals could have been enhanced by the catalytic effect of metallic Fe-Ni-C liquid coexisting with graphite during this shock event. We found no evidence that formation of micrometer(s)-sized diamonds or associated Fe-S-P phases in ureilites require high static pressures and long growth times, which makes it unlikely that any of the diamonds in ureilites formed in bodies as large as Mars or Mercury.
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