Magnesium and iron isotope signatures of Udachnaya-East kimberlites – Insights into the origin of kimberlite magmas

Kimberlite magmas are some of the deepest and most enigmatic on Earth. They are considered to originate at the base of lithospheric mantle and characterised by strong enrichment in incompatible elements and CO 2 . It is generally agreed that kimberlites form from low-degree melts of carbonated peridotites, later diluted by silicate components during ascent. Here we present the first Mg and Fe isotope data for variably serpentinised and non-serpentinised kimberlites from the Udachnaya-East pipe, Siberian craton. The mantle source of these kimberlites has been previously characterised as a PREMA-like, isotopically primitive convecting mantle reservoir. These kimberlites display exceptionally low Hf/Hf* (0.25–0.29) and Zr/Zr* (0.26–0.32), high CaO/Al 2 O 3 (3.48–6.89), and negative anomalies in K, Pb, Sr, Zr, Hf, and Ti, collectively indicating a significant carbonate component contribution. Elevated Ce/Pb (22–63) and low Ba/Nb (6–9) further suggest an absence of recycled sedimentary carbonates input. Whole-rock δ 26 Mg values (−0.32‰ to −0.22‰) contrast sharply with the markedly lighter δ 26 Mg of subduction-modified mantle melts, and instead resemble those of olivine melilitites (−0.30‰ to −0.26‰) metasomatised by mantle-derived carbonated melts. Based on the reconstructed composition of the primary kimberlite melt and previously published Sr, Nd and Hf radiogenic isotope data, the mantle-like Mg isotope signatures of whole-rock kimberlites are interpreted to support the previous models which reflect mixing between mantle derived carbonate-rich melts and lithospheric mantle material. The elevated whole-rock δ 56 Fe values (0.05‰ to 0.12‰), together with high V /V* ratios (15.2–26.5) and Fe isotope based partial melting modelling, indicate that the initial kimberlite melts were rich in CO 2 component and the formation of these melts is likely linked to the redox melting of carbon-bearing mantle where carbon is stored in its reduced form.