Home – Home – Jornals – Russian Geology and Geophysics 2009 number 12

We discuss the chemistry of exceptionally rare phlogopite inclusions coexisting with ultramafic (peridotitic) and eclogitic minerals in kimberlite-hosted diamonds of Yakutia, Arkhangelsk, and Venezuela provinces. Phlogopite inclusions in diamonds are octahedral negative crystals following the diamond faceting in all 34 samples (including polymineralic inclusions). On this basis phlogopite inclusions have been interpreted as syngenetic and in equilibrium with the associated minerals. In ultramafic diamonds phlogopites coexist with subcalcic high-Cr₂O₃ pyrope and/or chromite, olivine, and enstatite (dunite/harzburgite (H) paragenesis) or with clinopyroxene, enstatite, and/or olivine, and pyrope (lherzolite (L) paragenesis). Ultramafic phlogopites have high Mg# [100 ?Mg/(Mg+Fe)] from 92.4 to 95.2 and Cr₂O₃ higher than TiO₂ in H-phlogopites (1.5-2.5 wt.% versus 0.1-0.4 wt.%, respectively) but lower in L-phlogopites (0.15-0.5 wt.% versus 1.3-3.5 wt.%, respectively). Eclogitic (E) phlogopites show Mg# from 47.4 to 85.3 inclusive, and very broad ranges of TiO₂ up to 12 wt.%. The primary syngenetic origin of phlogopite is indicated, besides other factors, by its compositional consistency with the associated minerals. The analyzed phlogopites are depleted in BaO (0.10-0.79 wt.%), and their F and Cl contents are highly variable, reaching 1.29 and 0.49 wt.%, respectively. The latter is in line with high Cl enrichment in some unaltered kimberlites and in nanometric fluid inclusions from diamonds. The presence of syngenetic phlogopite in kimberlite-hosted diamonds provides important evidence that volatiles participated in diamond formation and that at least part of diamonds may have been related to early stages of kimberlites formation.