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

Following its subduction, oceanic crust either contributes to the source of island-arc volcanic rocks or it is recycled into the mantle(1). Most(2,3), but not all authors(4) 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 Hf-176/Hf-177 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 Hf-176/Hf-177 reservoir, which we suggest is zircon-rich sediment.