Serpentinization and deserpentinization reactions in the upper mantle beneath fuerteventura revealed by peridotite xenoliths with fibrous orthopyroxene and mottled olivine

Mantle xenoliths collected from Fuerteventura, one of the easternmost Canary Islands, exhibit a complex evolutionary history comprising events of depletion, serpentinization, dehydration and melt metasomatism. Each of these events left imprints on both the texture and chemistry of the xenoliths. Extensive partial melting is shown by complete lack of primary clinopyroxene, the ultra-refractory trace element composition of orthopyroxene porphyroclasts, and low heavy rare earth element contents as compared with abyssal peridotites sampled along mid-ocean ridges and oceanic fracture zones, in the xenoliths least affected by later metasomatism. In many xenoliths the original orthopyroxene porphyroclasts and some olivines are replaced by fibrous aggregates of orthopyroxene and/or large, deformed olivine porphyroclasts with mottled rims with stringy glass and fluid inclusions. Such features are very rare in ocean island xenoliths. Unusually high H2O and Cl concentrations, together with very high H2O/Ce and Cl/K ratios in interstitial glasses, suggest that the fibrous orthopyroxene formed by local serpentinization by hot seawater. The volume increase accompanying the serpentinization caused extensive fracturing of adjacent olivine porphyroclasts. The most likely scenario for local mantle invasion by hydrous fluids is along deep faults and fractures caused by tectonic movements along the continent-ocean transition during the early phases of the opening of the Atlantic Ocean. The peridotites were later (probably during the Canary Islands magmatism) dehydrated, causing the serpentine minerals to be replaced by porous domains of fibrous orthopyroxene. Hydrous fluids released by the deserpentinization escaped into neighbouring and overlying rocks leaving trails of fluid inclusions along fractures and grain boundaries causing mottled rims and zones in olivine porphyroclasts. During the Canary Islands magmatism the upper mantle beneath Fuerteventura was also infiltrated by enriched silicate magmas that caused different degrees of Fe-Ti-metasomatism. A higher degree of melt metasomatism in rocks with fibrous orthopyroxene and mottled olivine than in the massive harzburgites strongly suggests that the sublithospheric Canarian magmas reused serpentinized extensional faults during their rise to the surface. The strongest degree of melt metasomatism appears to have resulted in the formation of lherzolites, wehrlites, and dunites.