![]() ![]() In order to avoid any sampling bias in such multicomponent inclusions, the composition was measured only on those grains that were completely embedded inside the diamond host determined by electron tomography, leaving aside those that had been partially cut during focused ion beam (FIB) preparation. This is further confirmed by the constant and stoichiometric bulk chemical composition of these inclusions. It is noteworthy that troilite, kamacite, and schreibersite are never found as isolated mono-mineralic inclusions in the diamonds, but always together inside a very sharply defined polyhedral arrangement two arguments promoting the idea that these inclusions crystalized as a single-Fe–Ni–S–P phase during diamond formation, that later decomposed into different phases. 4), or concentrates at grain boundaries in smaller inclusions (Supplementary Fig. The latter either dissociates to a separately detectable phosphide phase in larger inclusions (Fig. 2c): FeS-troilite, (Fe,Ni)-kamacite, and minor amounts of (Fe,Ni) 3P-schreibersite. ![]() 5) show that the sulfide inclusions have dissociated to three phases (Fig. Both chemical and crystallographic analysis (Supplementary Table 1 and Supplementary Fig. However, they show evidence of transformation to low-pressure phases during decompression, similarly to those found in deep terrestrial diamond inclusions 15. All the inclusions are faceted indicating that they were trapped as solid crystalline phases rather than melts. ![]() The overwhelming majority of inclusions are iron-rich sulfides, found either as isolated grains with sizes up to a few 100 nanometers, or as trails of small particles ranging from 50 nm down to a few nanometers (Fig. 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.įull size image Iron–sulfur type inclusions in diamond 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. In this study, we investigated the Almahata Sitta MS-170 section using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). Therefore, diamonds formed inside the UPB can potentially hold invaluable information about its size and composition. 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. 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. The formation of such large single-crystal diamond grains along with δ 15N sector zoning observed in diamond segments 7 is impossible during a dynamic event 8, 9 due to its short duration (up to a few seconds 10), and even more so by CVD mechanisms 11, leaving static high-pressure growth as the only possibility for the origin of the single-crystal diamonds. It was thus suggested that individual diamond single crystals as large as 100 μm existed in the sample, which have been later segmented through graphitization 7. Recent observation 7 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. There are three mechanisms suggested for diamond formation in ureilites: (i) shock-driven transformation of graphite to diamond during a high-energy impact 4, (ii) growth by chemical vapor deposition (CVD) of a carbon-rich gas in the solar nebula 5, and (iii) growth under static high-pressure inside the UPB 6. High concentrations of carbon distinguishes ureilites from all other achondrite meteorites 3, with graphite and diamond expressed between silicate grains. ![]() 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 system 3. Asteroid 2008 TC 3 fell in 2008 in the Nubian desert in Sudan 1, and the recovered meteorites, called Almahata Sitta, are mostly dominated by ureilites along with various chondrites 2. ![]()
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