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.