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