Compositions of Kimberlite Melts: A Review of Melt Inclusions in Kimberlite Minerals

Abstract

The paper presents a comprehensive review of currently available data on melt inclusions entrapped in minerals of kimberlites of different age and different provenance in ancient cratons. The crystallized melt inclusions represent snapshots of kimberlite melts at different stages of their evolution. All of the inclusions are completely crystallized and consist of daughter minerals and shrinkage bubbles, which sometimes contain low-density CO₂, but no aqueous fluids and quenched silicate glasses have been found so far. Although more than 60 mineral species have been identified among the daughter phases in the inclusions, all inclusions hosted in various minerals from different kimberlites have closely similar or even identical composition. The daughter minerals are various Na–K–Ca, Na–Ca, Na–Mg, K–Ca, Ca–Mg, Ca, Mg, and Na carbonates; Na–Mg and Na carbonates with additional anions Cl^–, \({\text{SO}}_{4}^{{2 - }}\), and \(\text{PO}_{4}^{3 - }\); and alkali sulfates, chlorides, phosphates, sulfides, oxides, and silicates. Alkali carbonates, sulfates, and chlorides are usually absent from among the groundmass phases of most kimberlites sampled worldwide, except the Udachnaya-East kimberlite in Siberia. However, this mineral assemblage, in association with such widespread kimberlite minerals as olivine, micas, monticellite, spinel-group minerals, perovskite, rutile, ilmenite, calcite, and dolomite, is common in the crystallized melt inclusions in all studied kimberlites. Carbonates (~30 to 85 vol %) always dominate over silicates (no more than 18 vol %) in all inclusions. All inclusions also contain variable (2 to 55 vol %.) amounts of chlorides (halite and sylvite). In cases where the abundance of carbonates is relatively low (30–50 vol %), the other major phases within inclusions are chlorides (18–55 vol %) rather than daughter silicates, as could be expected based on the traditional paradigm of the silicate composition of kimberlite melts. Published data on melt inclusions in the kimberlite minerals strongly imply that parental kimberlite melts were generated and further evolved within the Na₂O–K₂O–CaO–MgO–CO₂–Cl system, that is, they were alkali-rich carbonate/carbonate–chloride liquids. According to various estimates, SiO₂ content in kimberlite melts could have varied during different stages of their evolution from a few to 19 wt %. Clearly, kimberlite bodies are altered in the crust via interaction with meteoric and/or connate waters, resulting in serpentinization of kimberlite olivine and dissolution of many bona fide magmatic minerals, such as alkali carbonates, sulfates, and chlorides. In the traditional approach to studying kimberlites, the role of such components as Na₂O, CO₂, Cl, and to a lesser extent K₂O, S, and F in the petrogenesis of kimberlite magmas and rocks have been largely underestimated, while the roles of olivine- and serpentine-forming components, such as of SiO₂, MgO, and H₂O are still exaggerated.