A study by van Keken et al. (2011) investigates the depth-dependent flux of water (H₂O) from subducting slabs worldwide. Using global thermal models of subduction zones, the researchers predict the metamorphic facies and H₂O content of downgoing slabs. They find that mineralogically bound water can efficiently pass through old and fast subduction zones, such as in the western Pacific, but nearly complete dehydration occurs in hot subduction zones like Cascadia. The top of the slab is sufficiently hot in all subduction zones to cause significant dehydration of the upper crust, including sediments and volcanic rocks. The degree and depth of dehydration in the deeper crust and uppermost mantle vary widely, depending on composition (gabbro vs. peridotite) and local pressure and temperature conditions. The upper mantle dehydrates at intermediate depths in all but the coldest subduction zones. On average, about one-third of the bound H₂O subducted globally reaches 240 km depth, carried mainly in gabbro and peridotite sections. The predicted global flux of H₂O to the deep mantle is smaller than previous estimates but still amounts to about one ocean mass over the age of the Earth. This suggests that the mantle H₂O content increases by 0.037 wt% (370 ppm) over the age of the Earth, consistent with inferred H₂O concentrations in the mantle. The study highlights the importance of understanding the mechanism and location of fluid release from subducting slabs and the amount of water that can be retained and recycled to the deep mantle. The results show that dehydration in subduction zones follows a general pattern that depends strongly on position in the slab. The study also discusses the implications of these findings for the global water cycle and the role of subduction in recycling water to the deep mantle. The study concludes that the global flux of H₂O from subducting slabs is significant, and that the hydration state of the slab and mantle plays a crucial role in determining the amount of water that can be recycled to the deep mantle. The study also identifies several remaining questions for future research, including the hydration state of the incoming slab, the role of unusual bulk compositions produced by metasomatism, fluid flow paths, the role of "solid" diapirs in carrying fluid into the mantle wedge, and the effects of volatiles other than H₂O on stabilizing hydrous phases.A study by van Keken et al. (2011) investigates the depth-dependent flux of water (H₂O) from subducting slabs worldwide. Using global thermal models of subduction zones, the researchers predict the metamorphic facies and H₂O content of downgoing slabs. They find that mineralogically bound water can efficiently pass through old and fast subduction zones, such as in the western Pacific, but nearly complete dehydration occurs in hot subduction zones like Cascadia. The top of the slab is sufficiently hot in all subduction zones to cause significant dehydration of the upper crust, including sediments and volcanic rocks. The degree and depth of dehydration in the deeper crust and uppermost mantle vary widely, depending on composition (gabbro vs. peridotite) and local pressure and temperature conditions. The upper mantle dehydrates at intermediate depths in all but the coldest subduction zones. On average, about one-third of the bound H₂O subducted globally reaches 240 km depth, carried mainly in gabbro and peridotite sections. The predicted global flux of H₂O to the deep mantle is smaller than previous estimates but still amounts to about one ocean mass over the age of the Earth. This suggests that the mantle H₂O content increases by 0.037 wt% (370 ppm) over the age of the Earth, consistent with inferred H₂O concentrations in the mantle. The study highlights the importance of understanding the mechanism and location of fluid release from subducting slabs and the amount of water that can be retained and recycled to the deep mantle. The results show that dehydration in subduction zones follows a general pattern that depends strongly on position in the slab. The study also discusses the implications of these findings for the global water cycle and the role of subduction in recycling water to the deep mantle. The study concludes that the global flux of H₂O from subducting slabs is significant, and that the hydration state of the slab and mantle plays a crucial role in determining the amount of water that can be recycled to the deep mantle. The study also identifies several remaining questions for future research, including the hydration state of the incoming slab, the role of unusual bulk compositions produced by metasomatism, fluid flow paths, the role of "solid" diapirs in carrying fluid into the mantle wedge, and the effects of volatiles other than H₂O on stabilizing hydrous phases.