Role of recycled oceanic basalt and sediment in generating the Hf–Nd mantle array

Abstract

The isotopic composition of oceanic basalts suggests that they are composed of true recycled oceanic crust and sediments, which are mixed with the depleted mantle. Following its subduction, oceanic crust either contributes to the source of island-arc volcanic rocks or it is recycled into the mantle1. Most2,3, but not all authors4 believe that recycled crust is incorporated into the plume source of oceanic basalts. The hafnium (Hf) and neodymium (Nd) isotopic compositions of basalts from oceanic islands and mid-ocean ridges exhibit a linear relationship—the mantle array—which is thought to result from mixing between material from the depleted mantle and an enriched recycled component. Here, we model the Hf–Nd isotopic composition of oceanic basalts as a mixture of recycled oceanic crust and depleted mantle and find that recycling of basalt alone is not sufficient to reproduce the mantle array. We conclude that oceanic sediments, which have a relatively high 176Hf/177Hf ratio, must also be recycled. Combining oceanic sediments with recycled oceanic basalts and subsequent mixing with depleted mantle peridotite produces Hf and Nd isotopic compositions that coincide with the mantle array. The composition of bulk continental crust requires the existence of a complementary low 176Hf/177Hf reservoir, which we suggest is zircon-rich sediment.