A large planetary body inferred from diamond inclusions in a ureilite meteoriteOPEN ACCESS 

Farhang Nabiei, James Badro, Teresa Dennenwaldt, Emad Oveisi, Marco Cantoni, Cécile Hébert, Ahmed El Goresy, Jean-Alix Barrat & Philippe Gillet

Nature Communications, Volume 9, Article number: 1327 (2018)


“Ureilites are a type of meteorite that are believed to be derived from a parent body that was impacted in the early solar system. Here, the authors analyse inclusions within diamonds from a ureilite meteorite and find that they must have formed at above 20 GPa suggesting the parent body was Mercury- to Mars-sized.”

“Asteroid 2008 TC3 fell in 2008 in the Nubian desert in Sudan1, and the recovered meteorites, called Almahata Sitta, are mostly dominated by ureilites along with various chondrites2. Ureilite fragments are coarse grained rocks mainly consisting of olivine and pyroxene, originating from the mantle of the ureilite parent body (UPB)3 that has been disrupted following an impact in the first 10 Myr of the solar system3. High concentrations of carbon distinguishes ureilites from all other achondrite meteorites3, with graphite and diamond expressed between silicate grains.

There are three mechanisms suggested for diamond formation in ureilites: (i) shock-driven transformation of graphite to diamond during a high-energy impact4, (ii) growth by chemical vapor deposition (CVD) of a carbon-rich gas in the solar nebula5, and (iii) growth under static high-pressure inside the UPB6. Recent observation7 of a fragment of the Almahata Sitta ureilite (MS-170) revealed clusters of diamond single crystals that have almost identical crystallographic orientation, and separated by graphite bands. It was thus suggested that individual diamond single crystals as large as 100 μm existed in the sample, which have been later segmented through graphitization7. The formation of such large single-crystal diamond grains along with δ15N sector zoning observed in diamond segments7 is impossible during a dynamic event8,9 due to its short duration (up to a few seconds10), and even more so by CVD mechanisms11, leaving static high-pressure growth as the only possibility for the origin of the single-crystal diamonds.

Owing to their stability, mechanical strength and melting temperature, diamonds very often encapsulate and trap minerals and melts present in their formation environment, in the form of inclusions. In terrestrial diamonds, this has allowed to estimate the depth of diamond formation, and to identify the composition and petrology of phases sampled at that depth. Therefore, diamonds formed inside the UPB can potentially hold invaluable information about its size and composition.

In this study, we investigated the Almahata Sitta MS-170 section using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). We studied the diamond–graphite relation and discovered different types of inclusions that were chemically characterized by energy dispersive X-ray (EDX) spectroscopy, crystallographically by electron diffraction, and morphologically by TEM imaging. The composition and mineralogy of these inclusions points to pressures in excess of 20 GPa inside the UPB, which in turn implies a planetary body ranging in size between Mercury and Mars.”