Subduction zones are a locus of material transfer between the surface and the solid Earth and play a key role in global cycling of water and other volatiles over geologic time. Balancing the global water budget requires knowing the amount of water recycled to the mantle by subduction, but current estimates span an order of magnitude (Hacker, 2008;Rupke, 2004;Schmidt & Poli, 2003;van Keken et al., 2011). The largest source of uncertainty for subduction zone water fluxes is the amount of water carried in the subducting slab mantle. Hydrous minerals in the slab mantle are the reservoir most likely to carry water past the volcanic arc and into the deep mantle (Hacker, 2008), so this level of uncertainty represents roughly an order of magnitude difference in the rate at which water moves from surface to subsurface reservoirs, which has implications for eustatic sea level change (Parai & Mukhopadhyay, 2012).Oceanic lithosphere approaching deep ocean trenches tends to form faults due to plate bending, and these bend-related faults promote water circulation and hydration of the incoming plate crust and mantle (Faccenda, 2014;Faccenda et al., 2009;Hatakeyama et al., 2017;Korenaga, 2007). Plate bending can reactivate normal faults inherited from mid-ocean ridge tectonism or, if the incoming plate is subducting oblique to existing weak zones, new faults are broken parallel to the trench (Fujie et al., 2018;Zhou et al., 2015). Serpentinized fault zones may be up to several hundred meters wide, with the extent of serpentinization strongly controlled by permeability, which varies with depth (Hatakeyama et al., 2017). Water carried in the subducting slab can take different paths through the subduction zone: hydrous minerals in the crust tend to dehydrate into the mantle wedge (Jarrard, 2003), generating melts that feed arc volcanoes and potentially contributing to intermediate depth earthquakes (Boneh et al., 2019;Faccenda et al., 2012;Jung et al., 2004), while hydrous phases in deeper and colder parts of the slab can remain stable past the volcanic arc and subduct into the deeper mantle.Observational constraints on the amount of water carried by subducting oceanic lithosphere come primarily from geophysical data and rely on relationships between serpentinization, porosity, and geophysical observables
