The chemical stratigraphy of the sub-cratonic lithospheric mantle and its sampling by kimberlites tracked by olivine trace element content

Abstract

The chemical composition of olivine, the main constituent of Earth's upper mantle, is increasingly used to explore metasomatic and igneous processes in Earth's interior, the evolution of alkaline‑carbonatitic and kimberlitic melts, and their associated diamond potential. Yet, the trace element-poor nature of olivine in mantle-derived peridotites, and especially the low concentration of incompatible trace elements, is analytically challenging. In most studies of mantle peridotites, the chemical characterization of olivine is limited to low-current major element analyses by electron probe. Fuller characterization of olivine trace element compositions allow tracing the T-P-fO₂ conditions, nature and metasomatic modification of the lithospheric mantle. In this study, we define the chemical variability of olivine in peridotites sampled through an entire sub-cratonic lithospheric mantle section (spanning 70–230 km), via detailed and comprehensive geochemical characterization of coarse granular to porphyroclastic/fluidal textured spinel, spinel-garnet and garnet peridotites from the Devonian Udachnaya-East kimberlite (Siberian craton). Combined with thermobarometric modelling, bulk-rock composition and major element composition of the coexisting minerals, this provides novel insights into the utility of olivine as a recorder of processes in the lithospheric mantle.

The high-resolution data and the integrated textural/chemical characterization of Udachnaya-East peridotites show that olivine composition is variably controlled by a combination of T-P-(fO₂) conditions, and melt-related enrichment, and faithfully discriminates between two groups of xenoliths. Refractory, coarse granular peridotites equilibrated at low-T (770–1000 °C) are generally composed of olivine depleted in all trace elements. In contrast, fertile to refertilized garnet-bearing rocks equilibrated at high-T (1180–1360 °C) and P = 5–6 GPa are enriched in all basaltic components, and record earlier significant addition of HFSE, Fe, Ca, Al and P at the base of the Siberian lithosphere. A stand-alone group is constituted by metasomatically-derived wehrlites and dunites, which equilibrated at low-T but preserve selective trace element enrichment in olivine. Olivine in fertile and strongly metasomatized peridotites can store surprisingly high amounts of all trace elements and especially Ti, Nb, Ta, Zr and Hf, with sometimes suprachondritic Nb amounts and Nb/Ta ratios. These results highlight the potential of olivine as a HFSE repository, and require its inclusion as a player in the terrestrial HFSE mass balance. Integrated to literature data, our results enable also to discuss the applicability of diagrams used for discrimination between spinel- and garnet-bearing peridotitic olivine, thus improving our knowledge of the provenance of the xenocrystic cargo in kimberlites.