THE ORIGIN OF DIAMONDS IN UREILITES.

Recent studies have proposed that diamonds in ureilites formed at static pressures >20 GPa [1,2]. This would imply either that the ureilite parent asteroid was a Mercury- to Mars-sized embryo [2], or that diamonds in ureilites are exogenous [3]. Ureilites are ultramafic achondrites containing ~2-8 wt.% carbon [4]. The carbon is dominantly graphite, occurring in lath-shaped areas along silicate grain boundaries. In "unshocked" ureilites these areas consist of mm-sized graphite crystals [5,6]. In most ureilites (shocked to various degrees) they are polycrystalline. Diamonds are a minor component in graphite areas. Hypotheses for the origin of the diamonds include: (1) high static pressure in a planetary-sized body [1,2,7]; (2) transformation from graphite by impact shock [8-13]; (3) chemical vapor deposition [14,15]. Most authors favor shock origin, but [1,2] argued that a ureilite (MS-170) from Almahata Sitta (AhS) contains large diamonds formed at high static pressures. Here we investigate the origin of diamonds in ureilites Northwest Africa (NWA) 7983 and MS-170.