Syngenetic phlogopite inclusions in kimberlite-hosted diamonds: implications for role of volatiles in diamond formation

Abstract

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 a part of diamonds may have been related to early stages of kimberlites formation.

The growth of fibrous, cloudy and polycrystalline diamonds

“…Domanik and Holloway, 2000). Although this specific mica is not found in diamonds, the presence of mica in diamond inclusions has been reported in the past (Guthrie et al, 1991;Walmsley and Lang, 1992;Sobolev et al, 2009). Si-rich micas were reported as trapped crystallized mineral phases in cloudy diamonds from Koffiefontein, South Africa ; or as secondary phases that would have precipitated from trapped fluids in fibrous diamonds from the Diavik Mine, Canada (Klein-BenDavid et al, 2006), where they are believed to reflect the presence of high density fluids rich in Si, K and water from which diamond grows.…”

Type I eclogites from Roberts Victor kimberlites: Products of extensive mantle metasomatism

“…This trinity seems to be ubiquitous in the Type I eclogites and strongly argues for a metasomatic process. It noteworthy that the classic metasomatic mineral phlogopite is commonly found associated with sulphides or carbonates in Roberts Victor eclogites, and is also found (though rarely) as inclusions in diamond (Sobolev et al, 2009). Such K-rich metasomatism (Griffin et al, 2003) indicating the chemical fingerprinting of the peridotitic wall-rock by metasomatism.…”

Mantle–slab interaction and redox mechanism of diamond formation

“…The main carbon-bearing minerals of the subducted materials are carbonates, which are thermodynamically stable up to P-T conditions of the lower mantle (10,11,18). As evidenced by the compositions of inclusions in diamond, which vary from strongly reduced, e.g., metallic iron and carbides (19)(20)(21)(22)(23), to oxidized, e.g., carbonates and CO 2 (6,20,(24)(25)(26)(27)(28), carbonates may be involved in the reactions with reduced deep-seated rocks, including Fe 0 -bearing species (29)(30)(31). A scale of these reactions is determined mainly by the capacity of subducted carbonate-bearing domains.…”