Diamond growth from C-H-N-O recycled fluids in the lithosphere: Evidence from CH4 micro-inclusions and δ13C-δ15N-N content in Marange mixed-habit diamonds

Mixed-habit ( octahedral+ cuboid) diamonds from the Marange alluvial deposits in the eastern Zimbabwe craton have high nitrogen and hydrogen contents that provide an opportunity to evaluate diamond growth mechanisms and C-N-H-O bearing fluids in the lithospheric keel. Light grey cuboid sectors with hydrogen-containing defects trap abundant dispersed CH4 inclusions (Raman peaks at 2917 cm−1) associated with graphite (Raman peaks at 1580 cm−1). Clear octahedral sectors are richer in nitrogen and free of any such inclusions. Core to rim co-variations of δ13C-δ15N and N content can be explained by a mixing trend between earlier fluids that are CH4-rich and later fluids that are more CO3- or CO2-rich. Marange diamonds have limited overall δ13C variation, but do show fractionation during growth towards higher δ13C values. This trend can be explained by diamond precipitation from mixed CH4 and CO2 fluids, where isotopic fractionation occurs as the amount of fluid wanes. Calculated δ15N values for diamond source fluids evolving in this manner are between + 2.3 and + 6.4‰. These N isotopic compositions require CH4-rich and CO3-/CO2-rich 'end-member' fluids to have a recycled metasedimentary component perhaps introduced with subduction of eclogite.