Peridotitic and websteritic diamondites provide new information regarding mantle melting and metasomatism induced through the subduction of crustal volatiles

Diamondites are mantle xenoliths comprised of polycrystalline diamond intergrown with garnet and minor clinopyroxene. Diamondites have some geochemical characteristics distinct from monocrystalline diamonds. Examples include 13C-depletion with modal δ13C value at −18‰ containing a high abundance of websteritic garnets (73%). This is in contrast to monocrystalline diamonds that show a strong mean δ13C value at −5‰ and low abundance of websteritic inclusions (2%). At present, geochemical studies focusing on diamondites are lacking, relative to coated and monocrystalline diamond. As a consequence, there exists a substantial volume of abundant mantle material that has been largely overlooked. We have determined the coupled δ13C–δ15N values and N-concentrations for 20 samples of mantle diamondite. Although their provenance is uncertain, these diamondites are thought to originate from Southern Africa because the major and rare Earth element (REE) compositions for the garnets are consistent with other Southern African diamondites.The coupled δ13C–δ15N values, N-concentrations in the diamond and REE patterns for the garnets we conclude that the source of the 13C-depleted carbon and 15N-enriched nitrogen is crustal in origin. This is by way of recycling subducted oceanic lithosphere beneath a stable craton, possibly the Kaapvaal craton in southern Africa. The peridotitic and websteritic garnet intergrowths have REE patterns similar to eclogitic garnets and we propose their petrogenesis due to mixing between a volatile saturated eclogitic melt and mantle peridotite. We propose that diamondites represent distinct diamond-forming event(s) related to mantle melting in the sub-cratonic mantle. Diamondite-formation events are proposed to be unrelated to most monocrystalline and coated diamonds formed by metasomatic processes involving little to no mantle melting.