Conditions of origin of natural diamonds of peridotite affinity

Abstract

Studies of mineral inclusions in natural diamonds and rare diamondiferous xenoliths from kimberlites show that most diamonds are associated with a dunite or harzburgite paragenesis. The diamondiferous peridotites and dunites have predominantly coarse or tabular textures that suggest low‐temperature (<1100°C) equilibration. Application of the KD Fe/Mg(Ga/Ol) geothermometer of O'Neill and Wood to analytical data for the minerals in these rocks shows that most have equilibrated below 1100°C. Application of this thermometer to pairs of olivine and garnet crystals included in individual diamonds indicates that the diamonds have crystallized in the range 900°–1300°C, with a majority of estimated equilibration temperatures falling in the range below 1150°C. Comparison of these estimates of equilibration temperature with the zone of invariant vapor composition solidus for kimberlite and garnet lherzolite determined by Eggler and Wendlandt suggests that many diamonds may have formed in subsolidus events.