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 delta C-13-delta N-15 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 delta C-13 variation, but do show fractionation during growth towards higher delta C-13 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 delta N-15 values for diamond source fluids evolving in this manner are between +2.3 and +6.4 parts per thousand. 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. (C) 2016 Elsevier B.V. All rights reserved.