Hf Isotope Evidence for a Miocene Change in the Kerguelen Mantle Plume Composition

Abstract

We report high-precision multiple collector inductively coupled plasma INTRODUCTION mass spectrometry (MC-ICP-MS) analyses of Hf isotopic comSpatial and temporal isotope variations in basalts can positions for 39 Kerguelen Archipelago volcanic rocks that add a provide important constraints on the origin of the mantle new perspective to the temporal evolution of the Kerguelen mantle source of hotspots, plume dynamics and the interaction plume. Samples cover the entire range of chemical and Sr, Nd and of plumes with the oceanic lithosphere. The relationships Pb isotopic variations reported for the archipelago, and vary in age between ocean island basalts (OIB) and mantle geofrom 29 to 0·1 Ma. Their Hf isotopic compositions show the chemistry have been investigated in recent publications largest variation for an individual oceanic island and Hf correlates on both ridge-axis hotspots (Iceland; Hanan & Schilling, negatively with alkalinity. The Hf–Nd isotopic compositions of the 1997; Chauvel & Hemond, 2000; Kempton et al., 2000), pre-Miocene flood basalt group (>25 Ma) are distinct from those and within-plate hotspots (Hawaii; e.g. Lassiter et al., of the 25 Ma group 1986), and are therefore critical for delimiting the range of flood basalts require interaction between a shallow reservoir of of enriched isotopic variations for OIB, and (2) during asthenospheric mantle or oceanic lithosphere and the Kerguelen plume formation of the Kerguelen Archipelago the environment core, best represented by the Mt. Crozier basalts on the archipelago. of the hotspot changed from ridge-centred at >40 Ma This scenario is consistent with the relative proximity of the Kerguelen to the present intra-plate position. The archipelago has Archipelago to the Southeast Indian Ridge (>300–400 km) when been centred on or near the hotspot because the Antarctic the >25 Ma flood basalts erupted. The younger alkaline volcanic plate has remained essentially stationary with respect to rocks were generated by low-degree melting in the plume envelope, the hotspot reference frame since the Eocene (Duncan an area of enriched mantle material, different from the plume core, & Richards, 1991). The recent extensive and systematic and presumably formed by different combinations of old recycled sampling of volcanic stratigraphic sections from the Keroceanic crust and plateaux. The enriched geochemical character of guelen Archipelago provides a new framework within the Kerguelen plume core supports the hypothesis that the EM Iwhich to study the evolution of the Kerguelen plume. type plume source includes recycled oceanic plateaux. The Kerguelen Archipelago formed on the northernmost part of the Kerguelen Plateau on the Antarctic plate, far from continental margins. In contrast to the