Evidence for the Involvement of Fluoride Melts in Diamond Formation Processes in the Mantle of the Siberian Craton
V.S. Shatsky1,2,3, A.L. Ragozin1, V.V. Kalinina1
1 1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Science, Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
3A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk
Keywords: Siberian Craton, inclusions in diamonds, fluoride melts, diamond formation, mantle, subduction zone
Abstract
This paper presents new evidence for fluorine enrichment in the diamond-forming mantle of the Siberian Craton. For the first time, we have discovered inclusions containing calcium and aluminum fluorides in diamonds from placers in the northeast of Siberia. In contrast to microinclusions in fibrous diamonds and coated diamonds, the inclusions we studied in diamonds from the northeastern Siberian Craton placers can be interpreted as fluoride melts. In diamond ISTD-119, inclusions are oriented parallel to the octahedron faces and can be considered syngenetic. At the same time, in diamond ISTD-124, an elongated inclusion is oriented parallel to slip lines. This provides grounds to believe that the formation of the inclusion was synchronous with the stage of diamond deformation. In variety V diamonds XLS-147 and ISTD-193, the morphology of inclusions located in the central parts of the crystals allows them to be interpreted as healed fractures. Within the inclusions, fluorine is present as compounds with calcium, having stoichiometry close to fluorite. The inclusions exhibit areas consisting of calcium fluorides and areas consisting of carbonates and/or iron oxides. In variety V diamond XLS-147, aluminosilicate inclusions and inclusions consisting of aluminosilicate and fluoride parts were identified. However, unlike inclusions in diamonds ISTD-119, ISTD-193, and ISTD-124, the fluoride part of the inclusions in diamond XLS-147 is represented by aluminum fluorides. There is a body of evidence for the growth of type V diamonds in a subduction zone. The compositional variations of the inclusions we studied indicate immiscibility of fluoride melts with aluminosilicate and carbonatite melts. The inclusions we have investigated testify to a fluorine-enriched diamond-forming mantle of the Siberian Craton at the time of diamond formation.
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