Composition of secondary melt inclusions within magnesiochromite of mantle lherzolite xenolith from V. GRIB kimberlite (East European craton) as an indicator of low H₂O content in the kimberlite melt

This paper describes secondary crystallized melt inclusions entrapped in magnesiochromite of lherzolite xenolith from the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province). The inclusions represent snapshots of the melt, associated with magmatic processes that subsequently formed the V. Grib kimberlite pipe. Various Na-K-Ca-, Na-Mg-, Ca-Mg-, Mg-, Ca-carbonates, Na–Mg-carbonates with additional anions PO₄^3-–, Cl^–, SO₄^2–, chlorides, sulphate, phosphate and silicate were identified among the daughter phases. The mineral assemblage of daughter phases, carbonate (77 vol. %) and silicate (15 vol. %) content and Ca : Na : K ratios within the inclusions show that inclusions’ parental melt was an alkali-rich carbonate liquid with low amounts of SiO₂ (≤6 wt%) and H₂O (≤0.6 wt%). Serpentine is known to be the main water-rich mineral in kimberlites, however the water sources during serpentinization of kimberlites and the H₂O content in kimberlite melts remains controversial issue. The absence of serpentine and low water content in the studied melt inclusions in comparison with those in the kimberlites of the V. Grib pipe (10–14 wt. %) indicate predominance of external fluids in the serpentinization of these kimberlites.