Serpentinization of mantle peridotites along an uplifted lithospheric section, Mid Atlantic Ridge at 11° N.

Mantle peridotites from an exposed lithospheric section (Vema Lithospheric Section, VLS), generated during ~26 Ma at a ~80 km long Mid Atlantic Ridge segment (11° N), have been sampled and studied to understand the evolution of the serpentinization process. The VLS was uplifted due to a 10 Ma transtensional event along the Vema transform. Before the uplift residual mantle rocks were lying beneath a 0.8-1.3 km thick basaltic crustal layer. The major and trace element compositions of the serpentinites, as well as their H, O, Sr, Cl and B isotopic compositions were interpreted based on thermal models of lithospheric spreading from ridge axis. The results suggest that serpentinization occurred mostly near the ridge axis. Serpentinization temperatures, estimated from stable isotopes, are consistent with resetting of the closure temperatures during the tectonic uplift of the lithospheric sliver, reflected by decreasing ?18O and increasing ?11B values. Modeling shows that the thermal influence of the transtensional event affected mainly the region close to the RTI (ridge-transform intersection). Petrological, elemental and isotopic data suggest that, when the ultramafic basal unit of the VLS was uplifted and exposed on the ocean floor, serpentinization became superseded by low temperature water-rock reactions, with Fe-Mn crust formation, which is still progressing, as recorded by ?D. Ultramafic mylonites, prevalent in a short stretch of the VLS, show only a partial serpentinization process, together with pervasive contamination by low-temperature Fe-Mn crust.