Evolution of heterogeneous lithospheric mantle in a plume environment beneath the Kerguelen Archipelago

A combined petrographic, geochemical and Sr-Nd-Pb isotopic investigation of peridotite xenoliths from the Kerguelen Archipelago (southern Indian Ocean) provides new insights into melt migration mechanisms and the sources of heterogeneities in the mantle associated with the long-lived (~115 my) Kerguelen mantle plume. Large variations of trace element concentrations in clinopyroxenes and their isotopic compositions reflect the strong imprint of complex, multistage metasomatic episodes during evolution of the lithospheric mantle under the Kerguelen Archipelago. Two metasomatic agents have been identified that have interacted with the mantle peridotite matrix: (1) a basaltic melt, and (2) a carbonatitic melt that produced extremely high and variable incompatible element abundances in clinopyroxenes, which are attributed to chromatographic effects associated with metasomatic melt transport by porous flow through the mantle. Isotopic compositions of 12 peridotite xenoliths indicate that both types of metasomatic melts are related to the alkaline magmatism produced by the Kerguelen plume. In contrast, isotopic data from a single dunite xenolith indicate the strong influence of a continental lithospheric component, probably derived from Gondwanaland, that either forms part of the Kerguelen Plateau or was incorporated into the mantle beneath Kerguelen and mixed with plume-derived material. Our geochemical study of Kerguelen xenoliths testifies to the importance of plumes as mechanisms for producing metasomatic melts with highly variable compositions and for entraining different components that may act as contaminants for erupted lavas.