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

We report high-precision multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) analyses of Hf isotopic compositions for 39 Kerguelen Archipelago volcanic rocks that add a new perspective to the temporal evolution of the Kerguelen mantle plume. Samples cover the entire range of chemical and Sr, Nd and Pb isotopic variations reported for the archipelago, and vary in age from 29 to 0·1 Ma. Their Hf isotopic compositions show the largest variation for an individual oceanic island and εHf correlates negatively with alkalinity. The Hf–Nd isotopic compositions of the pre-Miocene flood basalt group (>25 Ma) are distinct from those of the <10 Ma group of Upper Miocene and Quaternary Mt. Ross volcanic rocks and plot along separate en-echelon εHf–εNd arrays. The Lower Miocene Series mildly alkaline basalts from the Southeast Province are transitional between the two groups and indicate a major replacement of plume source components over a relatively short period of time. The geochemical systematics of the >25 Ma group of flood basalts require interaction between a shallow reservoir of asthenospheric mantle or oceanic lithosphere and the Kerguelen plume core, best represented by the Mt. Crozier basalts on the archipelago. This scenario is consistent with the relative proximity of the Kerguelen Archipelago to the Southeast Indian Ridge (∼300–400 km) when the >25 Ma flood basalts erupted. The younger alkaline volcanic rocks were generated by low-degree melting in the plume envelope, an area of enriched mantle material, different from the plume core, and presumably formed by different combinations of old recycled oceanic crust and plateaux. The enriched geochemical character of the Kerguelen plume core supports the hypothesis that the EM I-type plume source includes recycled oceanic plateaux. [ABSTRACT FROM PUBLISHER]