EVOLUTION OF THE MANTLE SOURCE OF THE KERGUELEN LIP: A PERSPECTIVE FROM HIGH-PRECISION ISOTOPIC DATA

The Kerguelen oceanic plateau in the southern Indian Ocean evolved from an initial setting in a continental break-up environment at ~120 Ma, to a position above the Southeast Indian Ridge (SEIR) at ~40 Ma, to a purely oceanic environment today. This represents a unique opportunity to study the evolution of the mantle source to the Kerguelen basalts with time. New high-precision Pb-Sr-Nd-Hf isotopic analyses (~200 samples) clearly identify differences in the origin of the components affecting the various stages of evolution of the Kerguelen Plateau and Kerguelen Archipelago. The initial setting of continental break-up facilitated interaction of mantle plume-derived magmas with continental-related material (<<10%) as shown in the Cretaceous record of volcanism on the Central and Southern Kerguelen Plateaus. Basalts from the Ninetyeast Ridge (82-38 Ma), the longest linear feature on Earth, lack any evidence for continental material in their source. Depleted mantle components (SEIR-type) participated in the formation of the submarine Northern Kerguelen Plateau (~34 Ma). The vast majority of the Kerguelen Archipelago (80%) is covered by flood basalts that erupted between 30 and 24 Ma. They evolved with decreasing age from tholeiitic basalts in the northwest, to transitional basalts in the central part of the archipelago, and to alkaline basalts in the Southeast Province. This transition from tholeiitic to mildly alkalic basalts reflects primarily changes in melting conditions (lower extents of partial melting at higher pressures), and is associated with an increase in crustal and lithosphere thickness as the distance from the SEIR increased. The new Pb-Sr-Nd-Hf isotopic data reveal that the flood basalts on the archipelago derive essentially from melting of an enriched component in the plume source. This enriched component dominates the chemistry of the alkalic basalts, whereas the older tholeiitic-transitional basalts contain a higher proportion of the depleted-SEIR component. Finally, only the rejuvenated stages of volcanic activity on the archipelago (<10 Ma) show the presence of another, even more enriched component that originates from interaction of these fractionated low-degree melts with the Cretaceous Kerguelen Plateau.