Magmatism associated with the Izu-Bonin arc in the Pacific Ocean initiated around 50 million years (Myr) ago, when the Pacific plate began subducting beneath the Philippine Sea plate. The compositions of volcanic rocks that have formed in the arc since about 42 Myr ago are distinctly different from those that formed earlier(1,2). Here we use new and previously published lead isotope data to determine the sources of the Izu-Bonin magmas. Although typical Pacific Ocean crust now flanks the Izu-Bonin trench(3-5), our results suggest that since 42 Myr ago, arc magmas have incorporated lead from subducted ocean crust that is compositionally distinct from Pacific Ocean crust but similar to that found in the Indian Ocean region. Yet, no such Indian-type ocean crust has been found in the northwestern Pacific so far. On the basis of plate tectonic reconstructions, we infer that such crust-probably generated by the now-extinct Izanaghi-Pacific spreading centre-must have been subducting at the Izu-Bonin trench until a few million years ago. The upper mantle that melts to generate such crust is different from that thought to underlie much of the Pacific Ocean basin. Because the Izanaghi-Pacific spreading centre apparently tapped into Indian-type mantle, our results suggest that this mantle domain extends well beyond its currently accepted eastern boundary along northwest Pacific subduction zones.
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