81Kr in the Great Artesian Basin, Australia: a new method for dating very old groundwater

In central Japan near the triple junction of the Pacific, Philippine Sea and North American (or Ohotsuku) plates, the volcanic chain deflects towards backarc compared to the adjacent northeast Japan and Izu¯Bonin arcs, and lies ~200¯300 km above the Wadati¯Benioff zone. Numerical modeling shows that thermal recovery of the subducting Pacific plate is slow, due to the overlapping Philippine sea plate, which shifts the dehydration reactions to greater depths along the Pacific plate and causes the magmatism above the deep Wadati¯Benioff zone. The low geothermal gradient along the subducting Pacific plate also implies that a considerable amount of H2O is carried further down by phase A which is formed just above the subducting plate, without being released for magmatism. Central Japan can be regarded as an entrance for H2O into the deep mantle.