Eclogitic diamonds from variable crustal protoliths in the northeastern Siberian craton: Trace elements and coupled δ13C–δ18O signatures in diamonds and garnet inclusions.

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 (δ 13 C and δ 18 O) data for alluvial diamonds and their garnet inclusions from this locality. Cr-rich garnets of peridotitic affinity in the studied diamonds have a narrow range of δ 18 O values from 5.7‰ 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 (δ 18 O 6.5–9.6‰) than what is observed in low-Ca garnets (δ 18 O 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 δ 18 O values of inclusions and the low δ 13 C values of the host diamonds implies that the high-Ca garnet inclusions were derived from intensely hydrated (e.g., δ 18 O > 7‰) and typically oxidised basaltic rock close to the seawater interface, and that the carbon for diamonds was closely associated with this protolith. [ABSTRACT FROM AUTHOR]