Composition of Secondary Melt Inclusions in Magnesiochromite of a Mantle Lherzolite Xenolith from the V. Grib Kimberlite Pipe (East European Craton) as an Indicator of Low H2O Content of the Kimberlite Melt

Abstract

This paper describes secondary crystallized melt inclusions trapped in magnesiochromite of lherzolite xenolith from the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province). It is shown that the inclusions are microportions of melt related to magmatism, which was further formed this pipe. Daughter minerals assemblage of inclusions in magnesiochromite contain Na‒K‒Ca-, Na‒Mg-, Ca‒Mg-, Mg-, and Ca-bearing carbonates; Na–Mg carbonates with additional PO\(_{4}^{{3 - }}\), Cl^–, and SO\(_{4}^{{2 - }}\) anions; chlorides; sulfate; phosphate; and silicate. The mineral assemblage of daughter phases, the amount of carbonates (77 vol %) and silicates (tetraferriphlogopite) (15 vol %) and Ca : Na : K ratios within the inclusions indicate that this melt was an alkali-enriched carbonate liquid with a low content of SiO₂ (≤6 wt %) and H₂O (≤0.6 wt %). As is known, serpentine in kimberlites is a major H₂O-bearing mineral, but the problem of water sources during serpentinization of kimberlites and an actual H₂O content in kimberlite melts is a controversial issue. The absence of serpentine and the low H₂O content (≤0.6 wt %) of the studied melt inclusions in comparison with those of kimberlites of the V. Grib kimberlite pipe (10‒14 wt %) indicate the key role of external fluids during serpentinization of these kimberlites.