Sr, Nd, and Pb isotope evidence for a mantle origin of alkali chlorides and carbonates in the Udachnaya kimberlite, Siberia

Abstract

The kimberlite rocks of the Udachnaya-East pipe (Siberia) are uniquely fresh and contain very high abundances of primary volatiles (Cl, CO₂, S). Alkali elements and chlorine are extremely abundant in the reconstructed kimberlite melt compositions, and this enrichment is very important for our understanding of deep-mantle melting and melt transport. Here we present new isotopic data that confirm a mantle origin for these kimberlitic chlorides and carbonates, and constrain the kimberlite emplacement age as ca. 347 Ma. The initial Nd and Pb isotope ratios in a large salt aggregate, in a Cl-S–enriched water leachate of the groundmass, and in the silicate fraction of the groundmass are very similar ( ε _Nd = +3 to +4, ²⁰⁶Pb/²⁰⁴Pb = 18.6, ²⁰⁷Pb/²⁰⁴Pb = 15.53), implying a comagmatic origin of the chlorides and carbonates and the silicates. Combined Sr, Nd, and Pb isotope data are used to rule out any significant contributions to the kimberlite chlorine budget from crustal sources, such as the Cambrian evaporite sequences of the Siberian platform. Our data support the interpretation that exsolved Na-K chloride and Na-K-Ca carbonate formed directly from original uncontaminated kimberlite magma. High Cl abundances in kimberlites suggest the presence of a Cl-rich reservoir in the deep sublithospheric mantle.