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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. [ABSTRACT FROM AUTHOR]

Selective enrichment or depletion in either Zr and Hf (HFSE4+) or Nb and Ta (HFSE5+) is a feature commonly observed in many mantle-derived melts and amphiboles occurring as either disseminated minerals in mantle xenoliths and peridotite massifs or in vein assemblages cutting these rocks. The fractionation of Nb from Zr seen in natural mantle amphiboles suggests that their incorporation is governed by different crystal-chemical mechanisms. An extensive set of new partitioning experiments between pargasite–kaersutite and melt under upper-mantle conditions shows that HFSE incorporation and fractionation depends on amphibole major-element composition and the presence or absence of dehydrogenation. Multiple regression analysis shows that Amph/LDNb/Zr is strongly dependent on the mg-number of the amphibole as a result of a combination of amphibole and melt structure effects, so that the following generalizations apply: (1) high-mg-number amphiboles crystallized from unmodified mantle melts more easily incorporate Zr relative to Nb leading to an increase of the Nb/Zr ratio in the residual melt; (2) low-mg-number amphiboles, such as those found in veins cutting peridotites, may strongly deplete the residual melt in Nb and cause very low Nb/Zr in residual melts. Implications and applications to mantle environments are discussed. [ABSTRACT FROM PUBLISHER]

Titanite/liquid partition coefficients for most of the trace elements relevant in petrogenetic studies are provided for titanite-saturated liquids equilibrated at 1.5 GPa and 850 °C starting from lamproitic compositions. The high compatibility observed for REE, HFSE, Sr, V and Sc, and the strong incompatibility observed for actinides, large ion lithophile and light elements are discussed in terms of available crystal-chemical mechanisms for incorporation and crystal-structure control.The exchange vectors Na1Ca−1 and Al1Ti−1 allow local charge balance to be achieved after incorporation of REE3+ and R5+ at the Ca and Ti site, respectively. The significant amounts of H measured are also relevant in this regard [the exchange mechanism being (OH)1O−1 at the O1 site]. The incorporation of U4+ and Th4+ at the Ca site is scarce, and is likely balanced by that of Mg2+ at the Ti site; both these substitutions are responsible for strong structural strain.Titanite can thus be considered an important repository for REE and HFSE in metamorphic and igneous rocks, and its role must be accounted for when modelling trace-element residence during metamorphic reactions and late magmatic crystallisation history. Due to the measured differences in compatibility, titanite crystallisation increases the values of Nb/Ta and LREE/HREE ratios in residual liquids. The similar compatibility of U and Pb makes titanite suitable for U–Pb geochronology of igneous rocks only after common Pb correction. Finally, this study confirms that the titanite end member is not suitable for radioactive waste disposal due to the discussed crystal-chemical constraints, and that titanite-based waste forms should contain high amounts of Na+ and Mg2+. [Copyright &y& Elsevier]