Pb and Hf isotope variations along the Southeast Indian Ridge and the dynamic distribution of MORB source domains in the upper mantle
Résumé
Lead and Hf isotope data for a suite of closely spaced (5-10 km) basalt glasses, sampled over a distance of ∼1140 km along the Southeast Indian Ridge (SEIR) between 88°E and 100°E, confirm the presence of ancient heterogeneous mantle beneath the Indian Ocean. The Pb isotopes show a non-Gaussian, "fat tail" distribution that is bimodal. The paired Hf and Pb isotope data, combined with previously published data, define two populations that reveal the presence of compositional streaks within the upper mantle. The number and density of the streaks follow a Poisson distribution where the characteristic streak thickness is ∼20 km. The mantle 'unit' giving rise to each MORB sample represents a 'mixture of mixtures' with a multi-stage mixing history. There is no correlation of K/Ti or Na8 with Pb or Hf isotope ratios, and both domains encompass both N- and E-MORB sources, ruling out two different regimes of melting in a homogeneous source. Apparently, mantle convection has folded together distinct composite reservoirs of heterogeneous mantle, and stretched them into streaks that remain coherent and discernable units. The Hf and Pb isotope data reveal the presence of two distinct DUPAL mantle components in the upper mantle. The isotope signature of component 1 resembles a continental lithosphere source having Pb isotope composition similar to Karoo picrites (erupted at ∼180 Ma). This component likely originated by mixing of Kaapvaal cratonic lithosphere and/or lower crust with the asthenosphere through mantle plume-lithosphere interaction during the early rifting stages of Gondwana. Component 2 likely originated from mixing of recycled oceanic lithosphere plus sediment with the asthenosphere (which had been previously contaminated by component 1) during the upwelling of Indian Ocean mantle plumes over the last 180 Ma. The variation in Pb and Hf isotopes along the SEIR reveals that, while the two DUPAL components are spatially distributed in a Poisson fashion, and therefore well-mixed, their relative proportions are spatially variable in the Indian Ocean upper mantle.