Evolution of Heterogeneous Lithospheric Mantle in a Plume Environment Beneath the Kerguelen Archipelago

Abstract

A combined petrographic, geochemical and Sr‐Nd‐Pb isotopic graphy; metasomatized xenoliths 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 INTRODUCTION isotopic compositions reflect the strong imprint of complex, multi- It is now well accepted that the Earth’s mantle behaves stage metasomatic episodes during evolution of the lithospheric mantle as a viscous fluid over geologic time-scales. The variation under the Kerguelen Archipelago. Two metasomatic agents have been and amplitude of mantle geochemical heterogeneities identified that have interacted with the mantle peridotite matrix: (1) reflect the eYciency of heat and mass transfer processes a basaltic melt, and (2) a carbonatitic melt that produced extremely as well as the composition of entrained components (e.g. high and variable incompatible element abundances in clinopyroxenes, Hauri et al., 1994). The heterogeneous nature of the which are attributed to chromatographic eVects associated with convecting mantle has been delineated through the geometasomatic melt transport by porous flow through the mantle. chemical study of (1) mid-ocean ridge basalts (MORB) Isotopic compositions of 12 peridotite xenoliths indicate that both and oceanic island basalts (OIB) (e.g. Zindler & Hart, types of metasomatic melts are related to the alkaline magmatism 1986; Hofmann, 1997), and (2) mantle xenoliths, typically produced by the Kerguelen plume. In contrast, isotopic data from a characterized by a much greater geochemical variability single dunite xenolith indicate the strong influence of a continental than their host and related basalts (e.g. Hauri et al., lithospheric component, probably derived from Gondwanaland, that 1993). Mantle plumes represent the main mechanism for either forms part of the Kerguelen Plateau or was incorporated into entrainment in the mantle (Hart et al., 1992; Hauri et al., the mantle beneath Kerguelen and mixed with plume-derived 1994). material. Our geochemical study of Kerguelen xenoliths testifies The study of mantle xenoliths associated with ocean to the importance of plumes as mechanisms for producing meta- island hotspot volcanism can provide important insomatic melts with highly variable compositions and for entraining formation on melt extraction processes and sources of diVerent components that may act as contaminants for erupted heterogeneities in the mantle. The main purpose of this paper is to address the formation of compositional