Journal
CHEMICAL GEOLOGY
Volume 422, Issue -, Pages 46-59Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.chemgeo.2015.12.018
Keywords
Diamond; Garnet; Oxygen isotopes; Carbon isotopes; Eclogite; Mantle; Siberian craton
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Funding
- Russian Foundation for Basic Research [14-05-92107, 15-55-50033, 15-55-53029]
- Ministry of Education and Science of the Russian Federation [14.B25.31.0032]
- Australian Research Council [110101599]
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Diamonds of eclogitic assemblages are dominant in the placer diamond deposits of the northeastern Siberian platform. In this study we present new trace elements and stable isotopes (delta C-13 and delta O-18) data for alluvial diamonds and their garnet inclusions fromthis locality. Cr-rich garnets of peridotitic affinity in the studied diamonds have a narrow range of delta O-18 values from5.7% to 6.2%, which is largely overlapping with the accepted mantle range. This narrow range suggests that the garnet inclusions showing different REE patterns and little variations in oxygen isotopes may have formed by different processes involving fluid/melts that, however, were in oxygen isotopic equilibrium with the mantle. The trace element composition of the eclogitic garnet inclusions supports a crustal origin for at least the high-Ca garnets, which show flat HREE patterns and in some cases a positive Eu-anomaly. High-Ca eclogitic garnets generally show heavier oxygen isotope compositions (delta(18)O6.5-9.6%) than what is observed in low-Ca garnets (delta O-18 5.7-7.4%). The variability in oxygen isotopes and trace elements is suggested to be inherited from contrasting crustal protoliths. The relationship between the high delta O-18 values of inclusions and the low delta C-13 values of the host diamonds implies that the high-Ca garnet inclusions were derived from intensely hydrated (e.g., delta O-18 N7%) and typically oxidised basaltic rock close to the seawater interface, and that the carbon for diamonds was closely associated with this protolith. (C) 2016 Elsevier B.V. All rights reserved.
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