How unique is the Udachnaya-East kimberlite? Comparison with kimberlites from the Slave Craton (Canada) and SW Greenland

Abstract

The origin of alkali carbonates and chlorides in the groundmass of unaltered Udachnaya-East kimberlites in Siberia is still controversial. Contrary to existing dogma that the Udachnaya-East kimberlite was either contaminated by the crustal sediments or platform brines, magmatic origin of the groundmass assemblage has been proposed on the basis of melt immiscibility textures, melt inclusion studies, and strontium and neon isotope compositions. We further tested the idea of alkali- and chlorine enrichment of the kimberlite parental melt by studying olivine-hosted melt inclusions and secondary serpentine in kimberlites from the Slave Craton, Canada (Gahcho Kué, Jericho, Aaron and Leslie pipes) and southern West Greenland (Majuagaa dyke). Host olivine phenocrysts closely resemble groundmass olivine from the Udachnaya-East kimberlite in morphology, compositions (high-Fo, low-Ca), complex zoning with cores of varying shapes and compositions and rims of constant Fo. Melt inclusions in olivine consist of several translucent and opaque daughter phases and vapour bubble(s). The daughter crystals studied in unexposed inclusions by laser Raman spectroscopy and in carefully exposed inclusions by WDS-EDS are represented by Na-K chlorides, calcite, dolomite, magnesite, Ca-Na, Ca-Na-K and Ca-Mg-Ba carbonates, bradleyite Na 3 Mg(CO 3)(PO 4), K-bearing nahpoite Na 2(HPO 4), apatite, phlogopite and tetraferriphlogopite, unidentified sulphates, Fe sulphides, djerfisherite, pyrochlore (Na,Ca) 2Nb 2O 6(OH,F), monticellite, Cr-spinel and Fe-Ti oxides. High abundances of Na, K (e.g., (Na + K)/Ca = 0.15-0.85) and incompatible trace elements in the melt inclusions are confirmed by LA-ICPMS analysis of individual inclusions. Heating experiments show that melting of daughter minerals starts and completes at low temperatures (~ 100 °C and 600 °C, respectively), further reinforcing the similarity with the Udachnaya-East kimberlite. Serpentine minerals replacing olivine in some of the studied kimberlites demonstrate elevated abundances of chlorine (up to 3–4 wt.%), especially in the early generation. Despite heterogeneous distribution of chlorine such abundances are significantly higher than in the serpentine in abyssal and ophiolitic peridotites (< 0.5 wt.%). The groundmass of most kimberlites, including those studied here and altered kimberlites from the Udachnaya pipe, contain no alkali carbonates and chlorides and have low Na 2O (< 0.2 wt.%). We believe that alteration disturbs original melt compositions, with the alkaline elements and chlorine being mostly affected. However, the compositions of melt inclusions and serpentine are indicative of the chemical signature of a parental kimberlite melt. It appears that enrichment in alkalies and chlorine, as seen in unaltered Udachnaya-East kimberlites, is shared by other kimberlites, and thus can be assigned to deep mantle origin.