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Most magmatism occurring on Earth is conventionally attributed to passive mantle upwelling at mid-ocean ridges, to slab devolatilization at subduction zones, or to mantle plumes. However, the widespread Cenozoic intraplate volcanism in northeast China1-3 and the young petit-spot volcanoes4-7 offshore of the Japan Trench cannot readily be associated with any of these mechanisms. In addition, the mantle beneath these types of volcanism is characterized by zones of anomalously low seismic velocity above and below the transition zone8-12 (a mantle level located at depths between 410 and 660 kilometres). A comprehensive interpretation of these phenomena is lacking. Here we show that most (or possibly all) of the intraplate and petit-spot volcanism and low-velocity zones around the Japanese subduction zone can be explained by the Cenozoic interaction of the subducting Pacific slab with a hydrous mantle transition zone. Numerical modelling indicates that 0.2 to 0.3 weight per cent of water dissolved in mantle minerals that are driven out from the transition zone in response to subduction and retreat of a tectonic plate is sufficient to reproduce the observations. This suggests that a critical amount of water may have accumulated in the transition zone around this subduction zone, as well as in others of the Tethyan tectonic belt13 that are characterized by intraplate or petit-spot volcanism and low-velocity zones in the underlying mantle.
The Cenozoic intraplate volcanism in northeast China is located more than 1,000 km westward of theJapan Trench1, while the young alkaline basalts (0-6 Ma) known as petit-spots outcrop up to 600 km eastward of the trench4 (Fig. 1). The formation mechanism of these types of onshore and offshore volcanism is still debated, as there is no geological and geophysical correlation with mantle plumes or arc volcanism6,14.
Seismic tomography models indicate that in this region the Pacific Plate is currently stagnant in the mantle transition zone (MTZ), extending continuously up to nearly 1,000 km to the inland of northeast China3,8,10. Thus, it has been proposed that the Cenozoic intraplate magmatism is related to the dehydration of the Pacific slab in the MTZ2,15.
The primary petit-spot magma has been determined to be volatilerich with extremely enriched mantle (EM1-like) isotopic compositions6,7. The lack of hotspot tracks in this region excludes a contribution from a mantle plume. It...