Water Cycling in Subduction Zones and the Mantle

Abstract

To understand the transportation and the circulation of water in subduction zones and the mantle, the maximum H2O content of rocks under variable pressure-temperature conditions was reviewed. The maximum H2O content is also important for assessing the potential storage of H2O in the mantle, which shows that the mantle can contain 4.6 to 12.5 times more H2O than the current ocean mass, assuming a standard geothermal gradient. Although the estimation has a large degree of uncertainty, the mantle is a significant storage, and plays an important role in global water circulation, especially due to the unexpectedly high capacities of nominally anhy-drous minerals (NAMs). Based on this information, water circulation in subduction zones (es-pecially in the Japan arcs) and the mantle has been discussed with the relevant numerical mod-eling and geophysical and geological observations. Consequently, the following points have been clarified: (1) breakdown depths of the major hydrous minerals within and around the subduct-ing slab depend on the thermal structure (deeper for colder environments); (2) when a hot slab, including a spreading mid-ocean ridge, subducts, both water and heat are supplied to the forearc region, contributing to the formation of regional metamorphic belts; (3) both high-pressure-type and high-temperature-type metamorphism occur within a limited time in a single forearc domain associated with ridge subduction ; (4) arc magmatism and regional metamorphism are regarded as a series of geological events corresponding to different stages of a temporal change in the thermal state of a subduction zone; (5) although major dehydration of subducting slabs occurs at depths shallower than 200 km, triggering arc magmatism or regional metamorphism, the subducting slab and the bottom portion of the overlying mantle wedge above the slab transport several 100 to several 1000 ppm of H2O with NAMs to the transition zone ; (6) the influx of H2O into the transition zone by this mechanism is comparable to or exceeds outgassing by magma-tism at mid-ocean ridges and hotspots; (7) the influx was suppressed in the past where the potential temperature was high (hot-dry regime), while it will be enhanced in the future as the Earth cools (cold-wet regime), resulting in stabilization and prolongation of mantle convection; and, (8) considering the presence of very cold subduction zones such as central Japan, where a significant amount of water is exceptionally transported to the deep mantle, the present-day Earth is probably in a transition from a hot-dry regime to a cold-wet regime. In any case, the maximum H2O content in NAMs in the upper mantle controls the influx , which needs to be constrained tightly for more accurate estimations of global water circulation.