Subduction factory: 4. Depth-dependent flux of H₂O from subducting slabs worldwide

TLDR

The study uses a global compilation of subduction zone thermal structures to predict the metamorphic facies and water content of downgoing slabs. The authors model slab metamorphism and water content based on the compiled thermal data. Mineral‑bound water can traverse old and fast subduction zones but is largely lost in hot zones such as Cascadia, the upper crust dehydrates significantly, deeper dehydration depends on composition and P/T, and roughly one third of subducted water reaches 240 km depth, contributing about one ocean mass of H₂O to the deep mantle over Earth’s history, raising mantle water content by 0.037 wt % and aligning with inferred.

Abstract

[1] A recent global compilation of the thermal structure of subduction zones is used to predict the metamorphic facies and H2O content of downgoing slabs. Our calculations indicate that mineralogically bound water can pass efficiently through old and fast subduction zones (e.g., in the western Pacific), whereas hot subduction zones such as Cascadia see nearly complete dehydration of the subducting slab. The top of the slab is sufficiently hot in all subduction zones that the upper crust, including sediments and volcanic rocks, is predicted to dehydrate significantly. The degree and depth of dehydration in the deeper crust and uppermost mantle are highly diverse and depend strongly on composition (gabbro versus 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 in slabs reaches 240 km depth, carried principally and roughly equally in the 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. At this rate, the overall mantle H2O content increases by 0.037 wt % (370 ppm) over the age of the Earth. This is qualitatively consistent with inferred H2O concentrations in the Earth's mantle assuming that secular cooling of the Earth has increased the efficiency of volatile recycling over time.