High H(2)O 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 H(2)O content and elemental composition may also play a role. How H(2)O is distributed in the oceanic upper mantle remains poorly constrained. We tackled this problem by determining the H(2)O 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 H(2)O content of opx ranges from 119 ppm to 383 ppm; that of cpx from 407 ppm to 1072 ppm. We found anomalous H(2)O-enriched peridotites with their H(2)O content not correlating inversely with their degree of melting, although H(2)O is assumed to be incompatible during melting. Inverse correlation of H(2)O with Ce, another highly incompatible component, suggests post-melting H(2)O 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 H(2)O content of the MORB mantle source based on H(2)O in abyssal peridotites can be affected by strong uncertainties.