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.

Ureilite meteorites are partially melted asteroidal-peridotite residues, or more rarely, cumulates that can contain greater than three weight percent carbon. Here we describe an exceptional C-rich lithology, composed of 34 modal % large (up to 0.8 mm long) crystalline graphite grains, in the Antarctic ureilite meteorite Miller Range (MIL) 091004. This C-rich lithology is embedded within a silicate region composed dominantly of granular olivine with lesser quantities of low-Ca pyroxene, and minor FeNi metal, high-Ca pyroxene, spinel, schreibersite and troilite. Petrological evidence indicates that the graphite was added after formation of the silicate region and melt depletion. Associated with graphite is localized reduction of host olivine (Fo(88-89)) to nearly pure forsterite (Fo(99)), which is associated with FeNi metal grains containing up to 11 wt.% Si. The main silicate region is typical of ureilite composition, with highly siderophile element (HSE) abundances similar to 0.3 x chondrite, Os-187/Os-188 of 0.1260-0.1262 and Delta O-17 of -0.81 +/- 0.16 parts per thousand. Mineral trace-element analyses reveal that the rare earth elements (REE) and the HSE are controlled by pyroxene and FeNi metal phases in the meteorite, respectively. Modeling of bulk-rock REE and HSE abundances indicates that the main silicate region experienced similar to 6% silicate and >50% sulfide melt extraction, which is at the lower end of partial melt removal estimated for ureilites. Miller Range 091004 demonstrates heterogeneous distribution of carbon at centimeter scales and a limited range in Mg/(Mg + Fe) compositions of silicate grain cores, despite significant quantities of carbon. These observations demonstrate that silicate rim reduction was a rapid disequilibrium process, and came after silicate and sulfide melt removal in MIL 091004. The petrography and mineral chemistry of MIL 091004 is permissive of the graphite representing late-stage C-rich melt that pervaded silicates, or carbon that acted as a lubricant during anatexis and impact disruption in the parent body. Positive correlation of Pt/Os ratios with olivine core compositions, but a wide range of oxygen isotope compositions, indicates that ureilites formed from a compositionally heterogeneous parent body that experienced variable sulfide and metal melt-loss that is most pronounced in relatively oxidized ureilites with Delta O-17 between similar to 1.5 and similar to 0 parts per thousand. (C) 2016 Elsevier Ltd. All rights reserved.