Inclusions of carbonate melts in diamonds from placers of the northeastern Siberian craton
V.S. Shatsky1,2,3, A.L. Ragozin1, V.V. Kalinina1, I.N. Kupriyanov1,
1 V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Science, Novosibirsk, Russia
2 Novosibirsk State University, Novosibirsk, Russia
3 A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia
Keywords: inclusions in diamonds, carbonate melt, diamond formation, mantle
Abstract
The article presents the results of studying unusual polyphase inclusions in alluvial diamonds from the northeastern part of the Siberian Craton. Polyphase inclusions in alluvial diamonds usually exhibit cracks surrounded by haloes of carbonate microinclusions formed during decrepitation of the inclusions. These haloes indicate that during exhumation, decrepitation of the inclusions occurred, with part of their contents released into the forming cracks. This means that the contents of the polyphase inclusions were melt at the time of entrapment, which gives us grounds to interpret them as inclusions of carbonate melts. Two diamonds contain mineral inclusions of eclogitic (rutile, garnet, sulfides; sample ISTD-151) and peridotitic (olivine, sulfides; sample ISTD-180) parageneses along with polyphase inclusions of carbonate melt. In diamond XLS-131, along with polyphase inclusions similar to those in diamonds ISTD-180 and ISTD-151, a polyphase inclusion of a fluoride melt is present. The polyphase inclusions of carbonate melts consist of two distinct zones: one composed predominantly of calcite, and the other of magnetite and carbonates. In all studied inclusions, the carbonate zone is represented by calcite with low contents of iron and magnesium impurities, regardless of whether the diamond contains mineral inclusions of eclogitic or peridotitic paragenesis. The compositional features of the inclusions are not consistent with compositions formed during melting of carbonatized peridotites and eclogites and differ fundamentally from the composition of inclusions of the natrocarbonatite association in superdeep diamonds. Thus, the melts represented in the inclusions were not in equilibrium with the host mantle rocks. The lack of equilibrium between the carbonate melts and the mantle is explained by their rapid entrapment and preservation by the growing diamonds. This suggests that these melts served as a source of carbon during diamond formation.
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