Chloride and carbonate immiscible liquids at the closure of the kimberlite magma evolution (Udachnaya-East kimberlite, Siberia)

Abstract

The compositions of parental magmas forming kimberlitic rocks remain largely unknown because of masking effects of syn-eruptive contamination and degassing, and post-magmatic alteration. Among most affected elements are volatiles (H 2O and CO 2) and alkalies (Na and K). This study attempts to overcome the problems related to the alteration of kimberlites by detailed petrographic and chemical analyses of exceptionally fresh, and thus essentially anhydrous (< 0.5 wt.% H 2O), kimberlite samples from the Udachnaya-East pipe (Daldyn–Alakit region, Siberia). The groundmass of these kimberlites contains abundant carbonate (calcite, shortite, zemkorite) and chloride (halite, sylvite) minerals, cementing olivine phenocrysts, and forming round segregations (“nodules”). The nodules, belonging to the chloride and chloride–carbonate types, show no evidence of thermometamorphic effects on the contacts with the host kimberlite. The chloride–carbonate nodules demonstrate liquid immiscibility textures that are remarkably similar to those observed in the olivine-hosted chloride–carbonate melt inclusions at ∼ 600 °C. The similarity of oxygen and carbon isotope values of carbonates from the groundmass and nodules ( δ 18O 12.5 to 13.9‰ VSMOW; δ 13C –3.7 to –2.7‰ VPDB) points to their common origin at similar temperatures. We argue for crystallisation of the chloride–carbonate nodules from residual kimberlite melts, pooled after exhaustion of the silicate melt component. The enrichment of the residual melt in alkali carbonate and chloride is partly reflected in the bulk groundmass compositions (10–11 wt.% CO 2, 2.3–3.2 wt.% Cl, 2.6–3.7 wt.% Na, and 1.6–2.0 wt.% K). We propose that this enrichment is inherited from the kimberlite parental magma, and it can be responsible for the kimberlite low liquidus temperatures, low viscosities, and rapid emplacement.