Search

Structure, defects and inclusions in natural polycrystalline ballas-type diamonds are studied by complementary techniques: Scanning Electron microscopy, cathodo- and photoluminescence, X-ray diffraction, radiography and fluorescence, infra-red spectroscopy and carbon isotope analyses. Peculiarities of internal structure of some of the samples are explained by growth of the diamonds as volume-filling dendrites. Similarities of structure and mineralogy of trapped inclusions between ballas-type diamonds with fibrous and cuboid diamonds are established. However, significant differences between growth mechanisms of all these varieties exist and are possibly explained by variations in carbon supersaturation and diamond wettability by growth medium. [ABSTRACT FROM AUTHOR]

Data on the carbon and oxygen isotopic composition of carbonates from the rocks of the Giyani greenstone belt, Kaapvaal craton, South Africa are presented. This belt is immediately adjacent to the Hout River Shear Zone, along which the Southern Marginal Zone (SMZ) of the Limpopo granulite complex is overthrusted onto the Kaapvaal craton. The δ13C values are –2.1 and –2.2‰ for carbonates from two metapelite samples, –5.1‰ for a meta ultrabasite sample, and –7.1 and –7.7‰ for two metabasite samples, respectively. The δ18О values are 17.1 and 17.5‰ for carbonates from the metapelites, 14.3‰ for carbonates from the metaultrabasite, and 12.9 and 13.0‰ for carbonates from the metabasites. The comparison of the data obtained to the published data on the isotopic composition of carbonates from the rocks of the different greenstone belts indicates both hydrothermal (in the metabasites and the metaultrabasite) and sedimentary (in the metapelites) origins of the carbonate material in the rocks studied. Based on the comparison of these data to δ13C of magnesite that formed in ultrabasic granulites during the interaction of the Limpopo complex with the craton, the graphite and fluid inclusions in leucocratic garnet-bearing granitoids that transported fluids, and the carbon isotopic composition of graphite from metapelites of the SMZ, it is concluded that the carbon source was located outside the SMZ and represented a heterogeneous carbonate-bearing lithologies. It could be carbonate-bearing volcanogenic–sedimentary sequences of the greenstone belts of the Kaapvaal craton, which underwent prograde metamorphism during the interaction with the SMZ granulites. [ABSTRACT FROM AUTHOR]

Abstract: Data on the carbon isotope composition of graphite and CO2 from inclusions in quartz of granitoids of the Southern Marginal Zone (SMZ) of the Limpopo granulite belt, South Africa, are presented. The average values of δ13C for graphite and СО2 from fluid inclusions in quartz are found within the range that is typical of deep (mantle) carbon sources. The data are evidence of an external source of granitoid magmas. They are likely to be hybrid rocks that appeared during assimilation of host metapelites by trondhjemite magmas, while graphite in trondhjemites and granodiorites is a product of high-temperature (~900-1000°С) evolution of the CO2-rich fluid phase of granitoid magmas when they intrude the rocks of the granulite complex and was formed during assimilation that led to reduction of CO2 when sulfide-bearing metapelite material was contaminated by the magma. [ABSTRACT FROM AUTHOR]

The 35 paired diamond intergrowths of rounded colorless transparent and gray opaque crystals from the placers of northeastern Siberian Platform were investigated. Mineral inclusions (KFsp, Coe, E-Grt, Po) detected in studied samples belong to eclogitic paragenesis. The majority of studied samples have uniform ranges of nitrogen content (1126–1982 at. ppm) and carbon isotope composition (− 16.8 to − 23.2 ‰). These characteristics pointing towards subducted material are possible sources for their genesis. Two samples consist of a gray opaque crystal with the subduction-related characteristics (δ13C ca. − 21‰ and N ca. 1300 at. ppm) and a transparent crystal with low nitrogen content (412 and 29 at. ppm) and a heavy carbon isotopic composition (δ13C − 4.2 and − 4.6‰) common for primary mantle range. The higher degree of nitrogen aggregation in the crystals with mantle-like characteristics testifies their longer storage in the mantle conditions. These samples reflect multistage diamond growth history and directly indicate the mixing of mantle and subduction carbon sources at the basement of subcontinental lithospheric mantle of northeastern Siberian Platform. [ABSTRACT FROM AUTHOR]