Ultrafresh salty kimberlite of the Udachnaya–East pipe (Yakutia, Russia): A petrological oddity or fortuitous discovery?

Abstract

An ultrabasic/ultramafic composition of kimberlite magmas is difficult to reconcile with existing models of the kimberlite mantle source and melting conditions, inferred magma temperatures and rheological properties, and the style of magma ascent and emplacement. The inconsistencies in current thinking indicate serious flaws in understanding kimberlite magma compositions. Much of the uncertainty over true kimberlite compositions may stem from almost ubiquitous hydration and leaching of kimberlite rocks. This study presents petrographic and geochemical data for kimberlite samples largely unaffected by postmagmatic modification, from the Devonian Udachnaya–East pipe in Siberia. These samples are unusually enriched in chlorine and sodium, yet they are essentially anhydrous. These features are consistent with the phase composition of the groundmass which is dominated by minerals such as Na-Ca carbonates, Na–K chlorides and sulphates which appear to be – in our samples – co-magmatic with common silicates and oxides, but are unknown in other kimberlites, or rarely found within magmatic assemblages. We suggest that a kimberlite parent melt of essentially non-silicate composition, with high concentrations of alkalis, CO2 and Cl may be a viable alternative to the currently favoured water-rich, high-Mg model primary melt. Entrainment of mantle silicates into such a melt en route to the surface, followed by gravitational accumulation of mantle olivine and liquidus oxides (perovskite, Cr-spinel) at the bottom of vertically extensive magma bodies after emplacement, would explain the observed properties of kimberlite magma/rock, notably enrichment in olivine and trace elements in the hypabyssal kimberlite facies. A carbonate melt composition would retain attributes of the standard model such as trace element enrichment via low degrees of partial melting, it would explain low temperatures of crystallisation and the exceptional rheological properties that enable kimberlite primary melts to segregate from the lithospheric source and buoyantly ascend at high speed, while mixing and reacting with country rocks.