High H₂O Content in Pyroxenes of Residual Mantle Peridotites at a Mid Atlantic Ridge Segment

Global correlations of mid-ocean-ridges basalt chemistry, axial depth and crustal thickness have been ascribed to mantle temperature variations affecting degree of melting. However, mantle H₂O content and elemental composition may also play a role. How H₂O is distributed in the oceanic upper mantle remains poorly constrained. We tackled this problem by determining the H₂O content of orthopyroxenes (opx) and clinopyroxenes (cpx) of peridotites from a continuous lithospheric section created during 26 Ma at a 11°N Mid-Atlantic Ridge segment, and exposed along the Vema Transform. The H₂O content of opx ranges from 119 ppm to 383 ppm; that of cpx from 407 ppm to 1072 ppm. We found anomalous H₂O-enriched peridotites with their H₂O content not correlating inversely with their degree of melting, although H₂O is assumed to be incompatible during melting. Inverse correlation of H₂O with Ce, another highly incompatible component, suggests post-melting H₂O enrichment. We attribute a major role to post-melting temperature-dependent diffusion of hydrogen occurring above the melting region, where water-rich melt flows faster than residual peridotites through dunitic conduits cross-cutting the uprising mantle. Accordingly, estimates of the H₂O content of the MORB mantle source based on H₂O in abyssal peridotites can be affected by strong uncertainties.