EVIDENCE FOR THE INVOLVEMENT OF FLUORIDE MELTS IN DIAMOND FORMATION PROCESSES IN THE MANTLE OF THE SIBERIAN CRATON

This paper presents new evidence for the fluorine enrichment of the diamond-bearing mantle of the Siberian craton. For the first time, we have discovered inclusions containing calcium and aluminum fluorides in placer diamonds from the northeastern Siberian craton. In contrast to microinclusions in fibrous diamonds and coated diamonds, the studied inclusions can be interpreted as fluoride melts. In diamond ISTD-119, inclusions are oriented parallel to the octahedron faces and can be considered syngenetic, whereas in diamond ISTD-124, an elongated inclusion is oriented parallel to slip lines. This suggests 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 cracks. Fluorine is present in the inclusions, as calcium compounds with stoichiometry close to fluorite. The inclusions comprise zones consisting of calcium fluorides and zones consisting of carbonates and/or iron oxides. In variety V diamond XLS-147, aluminosilicate and aluminosilicate–fluoride inclusions have been identified. In contrast to the inclusions in diamonds ISTD-119, ISTD-193, and ISTD-124, the fluoride component of these inclusions is aluminum fluoride. There is evidence for the growth of variety V diamonds in subduction zone. The compositional variations of the studied inclusions indicate immiscibility of fluoride, aluminosilicate, and carbonatite melts. These inclusions testify to the fluorine-enriched diamond-forming mantle of the Siberian craton at the time of the diamond formation.