Diamonds in ureilites: how did they form?

Ureilites represent the second largest group of achondritic meteorites. Most ureilite fragments contain significant amounts of interstitial carbon-rich material, which is present as diamond, graphite, other graphitic compounds and hydrocarbons. In this study we investigated diamonds in three ureilitic fragments (AhS 209b, AhS 72 and NWA 7983) by scanning electron microscopy, X-ray diffraction and transmission electron microscopy with the aim to shed light on their origin. Almahata Sitta (AhS) fragments show a mixture of nanodiamond (with the presence of stacking disorder of diamond) and nanographite, while in NWA 7983 the simultaneous presence of micro- and nanodiamonds associated with nanographite was detected. Laboratory experiments (Davydov et al. 2004, 2006, 2011, 2014, 2015) demonstrated that graphite, nano-diamonds and micro-diamonds can be produced together from carbon precursors in even less than a few seconds at high pressure and high temperature like those simulating natural impact shock events. Such processes are consistent with the diamond/graphite textures observed, particularly the micro-diamond + nano-diamond assemblage in NWA 7983. Furthermore, this assemblage cannot be a product of high static pressures, which should produce micro-diamonds alone (no nano-diamonds). The results obtained in our study suggested that the origin of ureilitic diamonds is consistent with a shock event through the conversion of precursor carbon materials and not with a formation under high static pressure condition in a large planetary body (Nabiei et al. 2018). The impact event hypothesis is also supported by presence of stacking disorder of diamond, which could be formed during an asteroidal impacts event (Németh et al. 2014).