Japan Association of Mineralogical Sciences

The lithosphere-asthenosphere boundary (LAB) is located between the rigid lithosphere (the tectonic plate in the framework of plate tectonics) and the ductile asthenosphere, across which the heat and material transport mechanisms change drastically owing to dynamic processes such as conduction, convection, and magma transport.The constant supply of water-rich fluids released from the subducting slab significantly hydrate the LAB in subduction zone settings, affecting its origin and the dynamic processes operating in the LAB.This is because the water in the mantle significantly decreases its viscosity and melting temperature.Previously, our group revealed the petrologic structure across the LAB in subduction zone settings based on careful geothermobarometry of spinel peridotite xenoliths from the Ichinomegata maar on the back-arc side of the Northeast Japan Arc (Sato and Ozawa 2019).Accordingly, the mantle beneath Ichinomegata consists of two distinct layers.The lithospheric mantle (25-40 km) is granular, amphibole-bearing, and subsolidus, whereas the top of the asthenosphere (40-55 km) is porphyroclastic, amphibole-free, and partially molten.