Abstract
Primary carbonates in kimberlites are the main CO2 carriers in kimberlites and thus can be used to constrain the original carbon and oxygen-isotope composition of kimberlite melts and their deep mantle sources. However, the contribution of syn- and post-emplacement processes to the modification of the C–O-isotope composition of kimberlites is yet to be fully constrained. This study aims to shed new light on this topic through a detailed textural, compositional (major and trace elements), and in situ C–O–Sr isotopic characterisation of carbonates in the Benfontein kimberlite sills (Kimberley, South Africa). Our multi-technique approach not only reveals the petrographic and geochemical complexity of carbonates in kimberlites in unprecedented detail, but also allows identification of the processes that led to their formation, including: (1) magmatic crystallisation of Sr-rich calcite laths and groundmass; (2) crystallisation of late groundmass calcite from hydrothermal fluids; and (3) variable degrees of crustal contamination in carbonate-rich diapirs and secondary veins. These diapirs most likely resulted from a residual C–O–H fluid or carbonate melt with contributions from methane-rich fluids from the Dwyka shale wall rock, leading to higher 87Sr/86Sr and δ18O, but lower δ13C values than in pristine magmatic calcite. Before coalescing into the diapiric segregations, these fluids/melts also variably entrained early formed calcite laths and groundmass phases. Comparison between in situ and bulk-carbonate analyses confirms that O isotopic analyses of bulk carbonates from kimberlite rocks are not representative of the original isotopic signature of the kimberlite magma, whereas bulk C-isotope compositions are similar to those of the pristine magmatic carbonates. Calcite laths and most groundmass grains at Benfontein preserve isotopic values (δ18O = 6–8‰ and δ13C = − 4 to − 6‰), similar to those of unaltered carbonatites worldwide, which, therefore, probably correspond to those of their parental melts. This narrow range suggests kimberlite derivation from a mantle source with little contribution from recycled crustal material unless the recycled material had isotopic composition indistinguishable from typical mantle values.
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modified from Dawson and Hawthorne 1973), with an approximate location of the samples studied here (white stars). The inset includes a map of southern Africa with the location of Kimberley, where the Benfontein complex is located
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Acknowledgements
We would like to thank Jock Robey for invaluable support during field work in the Kimberley area, the De Beers group for providing access to the Benfontein Farm, and Ashton Soltys and Manal Bebbington for their help with sample selection and preparation. The authors also wish to acknowledge Juan Diego Martín, Xavier Llovet, and Eva Prats for their assistance with the CL system, EMPA, and SEM analysis at the Serveis Científico-Tècnics (UB); as well as Yoann Gréau, Sarah Gain, Rosanna Murphy, Yi-Jen Lai, and Hadrien Henry for their help with the SEM, LA-ICP-MS, and LA-MC-ICP-MS analysis at Macquarie University GeoAnalytical (MQGA, formerly GAU). We would also like to thank Etienne Deloule, Johan Villeneuve, and David Madre for their help with the SIMS analysis at Centre de Recherches Pétrographiques et Géochimiques (CRPG) in Nancy (France); as well as Hongxia Ma, Xiaoxiao Ling, Jiao Li, Yu Liu, and Guoqiang Tang at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing (China). This research was supported by the Australian Research Council (Discovery Early Career Researcher Award to AG; Grant no. DE-150100009); the Swiss National Science Foundation (Ambizione fellowship to AG; Grant no. PZ00P2_180126/1); the European Science Foundation—Europlanet 2020 Consortium (Project no. 16-EPN2-017); as well as funds from the ARC Centre of Excellence for Core to Crust Fluid Systems (CE110001017). This study used instrumentation funded by ARC Linkage Infrastructure, Equipment and Facilities (LIEF) and Department of Education, Science and Training (DEST) Systemic Infrastructure Grants, Macquarie University, National Collaborative Research Infrastructure Scheme (NCRIS) AuScope and Industry. This is contribution 1421 from the ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and 1364 in the ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC). Careful editorial handling by Daniela Rubato, and comments by Sebastian Tappe and an anonymous reviewer greatly improved the contents of this manuscript.
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Castillo-Oliver, M., Giuliani, A., Griffin, W.L. et al. New constraints on the source, composition, and post-emplacement modification of kimberlites from in situ C–O–Sr-isotope analyses of carbonates from the Benfontein sills (South Africa). Contrib Mineral Petrol 175, 33 (2020). https://doi.org/10.1007/s00410-020-1662-7
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DOI: https://doi.org/10.1007/s00410-020-1662-7