Estimating excess bound water content due to serpentinisation in mature slow-spreading oceanic crust using Vp/Vs.

Mature oceanic crust carries chemically bound water which may be released in subduction zones or delivered to the deep mantle. Estimating water content in slow-spreading crust is challenging due to its complex lithology, requiring both P- and S-wave seismic velocity (Vp and Vs), the latter of which has been limited. Here we show 2D high-resolution Vp, Vs and excess bound water models due to serpentinisation of mature Atlantic crust near the Lesser Antilles. The ridge-parallel line crosses eight seafloor-spreading segments with equal numbers of magma-robust and magma-poor. Hydration is highly variable and mainly accommodated in strongly serpentinised peridotites, dominantly in magma-poor segments, which are not preferentially located near fracture zones. Serpentinised peridotites (17% of the crust) host four times more water than normal magmatic crust, increasing Atlantic subduction bound water budget by ~ 50%. This has implications back in geological time such as during supercontinent breakups when slow-spreading crust subduction was more common.