The transport of water in subduction zones.

The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low (≤5°C/km), lawsonite may carry water into great depths of ≤300 km. In the hot subduction zone where the geothermal gradient is high (>25°C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of <80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low- T/low- P hydrous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of ≤60-80 km to release large amounts of water. In contrast, the low- T/low- P hydrous minerals are replaced by low- T/high- P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80-160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultrahigh-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately. [ABSTRACT FROM AUTHOR]