The wide range of H2The O contents recorded in the minerals of the exhumed mantle rocks have been difficult to interpret, as they often record a combination of melting, metasomatism and diffusion processes in spatially isolated samples. Here we determine the temporal variations of H2O content in pyroxenes from a 24 Ma time series of abyssal peridotites exposed along the Vema fracture zone (Atlantic Ocean). The H2O contents of pyroxenes correlate with crustal ages and pyroxene chemistry and increase towards younger, more refractory peridotites. These variations are inconsistent with the post-melting residuals and opposed to trends often observed in mantle xenoliths. Hydrogen enrichment after melting occurred by ion diffusion during cryptic metasomatism of peridotite residues by low-level volatile-rich melts and was particularly effective in the more depleted peridotites. The presence of hydric melting under the ridges results in a generalized incorporation of hydrogen into the oceanic lithosphere, probably reducing the viscosity of the mantle compared to dry models.
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