Impact shock origin of diamonds in ureilite meteorites

Abstract

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 µm), 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.

Characterization of carbon phases in Yamato 74123 ureilite to constrain the meteorite shock history

“…Our study provides further evidence in support of the diamond formation mechanism in ureilites proposed for NWA 7983 ureilite by Nestola et al (2020). According to this mechanism, the formation of micrometer-sized diamond crystals from graphite observed in Y-74123 is likely due to the combined effect of highly heterogeneous P-T-conditions due to shock wave propagation and immediate penetration of Fe-Ni melt into carbon aggregates, This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.…”

“…i.e., on the basis of the presence of spots and rings, we can state that nano-graphite coexists with micro-and nano-diamonds in Y-74123, as also observed by Goodrich et al (2020) and Nestola et al (2020) in the NWA 7983 ureilite.…”

“…Indeed, these authors proposed that diamonds in ureilites could have formed at high-pressure (above 12 GPa) by spontaneous shock transformation from graphite, and at low pressure (6-10 GPa) by a solid-state catalytic transformation from graphite in presence of a Fe-Ni melt. Additional support to the shock hypothesis is provided in a recent work by Nestola et al (2020) on Almahata Sitta samples (AhS 72 and AhS 209b), as well as on NWA 7983. In their study, graphite associated with nano-and (in NWA 7983) microdiamonds was reported, suggesting that the conversion from graphite to diamond was triggered by an impact event and was favored by the catalytic effect of Fe-Ni melts.…”

“…http://www.minsocam.org/ whereas the formation of nano-diamonds resulted from direct transformation from graphite (i.e., even without the catalytic Fe-Ni melt). The occurrence of Fe compounds, as observed in Y-74123, could explain the formation of diamonds at pressures ≥15-20 GPa (Nestola et al 2020), which is lower than the pressure of 30-60 GPa estimated for diamonds formed in impact cratering processes on Earth (see, e.g., Koeberl et al 1997, and references therein). In Nestola et al (2020) it is clearly reported how the catalyzed formation of diamonds by metallic melts during a shock event can also account for simultaneous formation of microand nanodiamonds in ureilites.…”

“…The occurrence of Fe compounds, as observed in Y-74123, could explain the formation of diamonds at pressures ≥15-20 GPa (Nestola et al 2020), which is lower than the pressure of 30-60 GPa estimated for diamonds formed in impact cratering processes on Earth (see, e.g., Koeberl et al 1997, and references therein). In Nestola et al (2020) it is clearly reported how the catalyzed formation of diamonds by metallic melts during a shock event can also account for simultaneous formation of microand nanodiamonds in ureilites. These authors, with the aim to explain this process, reported an example of a pulsed heating experiment performed on a graphite-metal charge in a static high-pressure apparatus (Varfolomeeva 1971).…”