Abstract
Abstract Hydration of the subduction zone forearc mantle wedge influences the downdip distribution of seismicity, the availability of fluids for arc magmatism, and Earth's long term water cycle. Reconstructions of present‐day subduction zone thermal structures using time‐invariant geodynamic models indicate relatively minor hydration, in contrast to many geophysical and geologic observations. We pair a dynamic, time‐evolving thermal model of subduction with phase equilibria modeling to investigate how variations in slab and forearc temperatures from subduction infancy through to maturity contribute to mantle wedge hydration. We find that thermal state during the intermediate period of subduction, as the slab freely descends through the upper mantle, promotes extensive forearc wedge hydration. In contrast, during early subduction the forearc is too hot to stabilize hydrous minerals in the mantle wedge, while during mature subduction, slab dehydration dominantly occurs beyond forearc depths. In our models, maximum wedge hydration during the intermediate phase is 60%–70% and falls to 20%–40% as quasi‐steady state conditions are approached during maturity. Comparison to global forearc H 2 O capacities reveals that consideration of thermal evolution leads to an order of magnitude increase in estimates for current extents of wedge hydration and provides better agreement with geophysical observations. This suggests that hydration of the forearc mantle wedge represents a potential vast reservoir of H 2 O, on the order of 3.4–5.9 × 10 21 g globally. These results provide novel insights into the subduction zone water cycle, new constraints on the mantle wedge as a fluid reservoir and are useful to better understand geologic processes at plate margins.
Plain Language Summary Subduction is the process by which old, water‐rich rocks of the ocean floor are recycled into the Earth's mantle in a conveyor belt‐like system. As this material (the subducting slab) enters the mantle it undergoes heating, releasing its bound water while simultaneously cooling the surrounding mantle. The rate and magnitude of this heating is not constant through time but undergoes a dramatic change over the lifecycle of a subduction zone. Water release during subduction controls where earthquakes and volcanoes do or do not occur and has an impact on cycling between the earth, atmosphere, and oceans. In this study we model the time‐varying nature of water storage within the relatively cool uppermost mantle directly above the slab (the forearc mantle wedge). We find that, worldwide, the forearc mantle wedge contains ∼1 order of magnitude more water than previously thought, representing a mass equivalent to ∼0.4% of Earth's oceans. Temperature changes within the slab and mantle through time are the main cause of this increased hydration. These results are supported by seismic observations and provide insights into the maximum depth of damaging earthquakes, the fluid source for explosive volcanic eruptions, and the dynamic parameters that allow for subduction to progress.
Key Points Warm conditions in dynamic subduction models promote slab dehydration leading to long‐lasting water storage in the upper mantle Dynamic thermal‐petrologic models show maximum mantle wedge hydration during intermediate stages as the slab sinks through the upper mantle Conditions are too warm to stabilize hydrous minerals during subduction infancy and dehydration occurs too deep during mature subduction