EVIDENCE OF EXTREME REDOX VARIATIONS IN SUBDUCTION-ZONE DIAMOND
V.S. Shatsky1,2,3, A.L. Ragozin1, I.N. Kupriyanov1, V.V. Kalinina1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Science, Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
3Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia
Keywords: Siberian craton, inclusions in diamond, mantle, subducted crust, moissanite
Abstract
The article presents new data on multiphase inclusions in two diamonds sampled from placers in the northeastern Siberian craton. Diamond HLS-4 is a round variety V crystal containing a multiphase mineral inclusion with more or less strongly oxidized iron carbides and moissanite in one part and calcite and iron oxides in another part. Another sample, HI-180, is a partly dissolved yellow cuboid that hosts a multiphase inclusion and numerous submicrometer inclusions delineating the crystal zoning. Sample HI-180 is deformed, with cracks and cavities up to 200 µm in size exposed in a polished section. The submicrometer inclusions plot in the field of microinclusions in fibrous diamonds from the world database, mostly near the silicic corner. They must have a hydrous composition, judging by the water-carbonate ratio of H2O/(H2O + CO2) = 0.80-0.82 estimated from FTIR data. The multiphase inclusion consists of quartz, Fe-armalcolite, anatase, and diamond grains in an amorphous matrix, as well as moissanite and calcite detected by SEM-EDS, FTIR and Raman spectroscopy. One diamond grain in the inclusion, in turn, encloses moissanite. The coexistence of calcite and moissanite in multiphase inclusions is evidence that cracks were fully healed up under extreme redox variations, possibly, during diamond growth in a subduction setting.
|
