Hydrogen enrichment after fusion in the oceanic lithosphere

Abstract

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.