Beyond EM-1: Lavas from Afanasy-Nikitin Rise and the Crozet Archipelago, Indian Ocean

Abstract

Lavas from Afanasy-Nikitin Rise, possibly the Late Cretaceous product of the Crozet hotspot, cover a wide range of isotopic compositions that includes the lowest ( 206 Pb 204 Pb) t (to 16.77) and ϵ Nd ( t ) (to −8) values yet found among oceanic islands or spreading centers worldwide, as well as high ( 87 Sr/ 86 Sr) t (to 0.7066). In contrast, young basalts from the Crozet Archipelago exhibit a narrow range of variation around ϵ Nd ∼ +4, 87 Sr/ 86 Sr ∼ 0.7040, and 206 Pb/ 204 Pb ∼ 19.0, closely resembling that of shield lavas of the Reunion hotspot. The Afanasy-Nikitin rocks also have much higher Ba/Nb, Ba/Th, and Pb/Ce than modern oceanic island or ridge lavas, as well as high La/Nb. The data do not obviously support the Crozet plume model but, assuming the model to be plate tectonically correct, would indicate that the plume-source composition either changed dramatically or that Afanasy-Nikitin magmatism involved significant amounts of nonplume mantle. The low 206 Pb/ 204 Pb, low ϵ Nd lavas provide the best evidence to date of the sort of material that, by variably contaminating much of the Indian mid-ocean-ridge basalt (MORB) source asthenosphere, may be responsible for the isotopic difference between most Indian MORB and Pacific or North Atlantic MORB. The combined isotopic and trace element results suggest an ultimate origin in the continental crust or mantle lithosphere for this material, although whether it was cycled through the deep mantle or resided at shallow levels in the convecting mantle cannot currently be determined.