Diamonds are rare and precious, known to form from carbon under the extreme temperatures and pressures within the Earth’s mantle over the course of billions of years. They are also found in some meteorites. Fabrizio Nestola, University of Padova, Italy, and Goethe University Frankfurt, Germany, Cyrena A. Goodric, Lunar and Planetary Institute, Universities Space Research Association, Houston, TX, USA, and colleagues have pinned down a convincing explanation for the presence of microdiamonds and nanodiamonds in ureilite meteorites. The genesis of diamonds in these meteorites has been a controversial topic for many years.

Diamonds that Fell to Earth

A meteor hit the Earth in the village of Novy Urey in the Mordovia Republic of Russia in the autumn of 1886. Ureilites are named after this village. The find represented a rare stony type of meteorite with an intriguing mineral composition, containing a relatively high proportion of carbon. Another well-known ureilite landed in Goalpara in Assam, India, and was found in 1868. In 2008, the tiny asteroid “2008 TC₃” entered the Earth’s atmosphere and exploded above Sudan’s Nubian Desert, scattering fragments of meteorite that were retrieved in December 2008.

Technically, the ureilite meteorites represent the second largest group of achondrites, stony meteorites that do not contain round grains formed from molten droplets. They are ultramafic rocks mainly composed of olivine (magnesium iron silicate) and pyroxene (inosilicates that can contain various combinations of metals such as calcium, iron, magnesium, and several other transition metals in minor amounts). The meteorites also contain interstitial carbon, metallic iron, and sulfide phases. The team explains that the meteorite minerals are known to have experienced igneous processing at temperatures up to 1,200–1,300 °C

The researchers have examined fragments of three ureilites known to contain diamonds—two from the Almahata Sitta polymict ureilite and one from the NWA 7983 main group ureilite. The team’s detailed examination using X-ray diffraction, Raman spectroscopy, and electron microscopy techniques revealed what they refer to as “an intimate association of large monocrystalline diamonds (up to at least 0.1 mm, the largest monocrystal of extraterrestrial diamond reported so far), nanodiamonds, nanographite particles, as well as nanoscopic grains of metallic iron, cohenite, troilite, and likely schreibersite” in NWA 7983.

Impact Shock instead of Slow Growth

The main inference from this study concerns the origins of the meteorites themselves and how this sits with our theories on how the Solar System itself formed. “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,” the team explains. This would have been characterized by peak pressures possibly as low as 15  GPa for several seconds. The deepest diamonds that form in Earth’s mantle have been generated by pressures above 20 GPs that persist for eons rather than mere seconds.

The team adds that the formation of diamonds could have been enhanced by the catalytic effect of a metallic iron phase containing iron and nickel in the presence of carbon during the shock event. Fundamentally, it is the shock event that created the diamonds. “We found no evidence that formation of micrometer-sized diamonds or associated iron-sulfur-phosphorus phases in ureilites require high static pressures and long growth times,” the team explains. “This makes it unlikely that any of the diamonds in ureilites formed in bodies as large as [the planets] Mars or Mercury,” they add. “We find no compelling evidence that diamonds in ureilites formed in large planetary bodies or planetary embryos.”

“The next step is to understand whether what we see in ureilitic meteorites is a common feature of all carbon-bearing meteorites which have undergone an important impact event,” Nestola told ChemistryViews. This should reveal the origin of diamonds that fall to earth.

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• Impact shock origin of diamonds in ureilite meteorites,
  Fabrizio Nestola, Cyrena A. Goodrich, Marta Morana, Anna Barbaro, Ryan S. Jakubek, Oliver Christ, Frank E. Brenker, M. Chiara Domeneghetti, M. Chiara Dalconi, Matteo Alvaro, Anna M. Fioretti, Konstantin D. Litasov, Marc D. Fries, Matteo Leoni, Nicola P. M. Casati, Peter Jenniskens, Muawia H. Shaddad,
  Proc. Natl. Acad. Sci. USA 2020.
  https://doi.org/10.1073/pnas.1919067117