Petrogenesis of spinel harzburgite and dunite suite xenoliths from Lanzarote, eastern Canary Islands: Implications for the upper mantle

We present data on petrography, mineral and whole rock major element relations and fluid inclusions on ultramafic xenoliths from Quaternary to Recent alkaline basalts in Lanzarote, eastern Canary Islands. The xenoliths have been divided into two main suites: the spinel harzburgite suite (harzburgites and rare lherzolites) and the spinel dunite suite (spinel dunites and rare spinel-plagioclase dunites). The spinel-harzburgite suite xenoliths from Lanzarote represent fragments of highly refractory, old suboceanic lithospheric mantle similar to that found beneath Hierro in the western part of the Canary Island chain. This mantle has been somewhat modified through a combination of melt extraction and metasomatism caused by infiltration of Fe-Ti-rich silicate melts and CO₂ fluids, probably in association with the formation of the Canary Islands. Also the spinel-dunite suite xenoliths show oceanic affinities, but are not directly related to the harzburgites through partial melting. Temperature estimates combined with isochores representing the densest CO₂ inclusions (T_h of - 12 ° C) in these nodules indicate a high geothermal gradient in the upper mantle under Lanzarote, 1100 ° C at ≥ 26 km depth, and a correspondingly thinned lithosphere ( ≥ 27 km). This implies much hotter conditions than those expected in "normal" suboceanic lithospheric mantle of an age corresponding to that off West Africa, and hotter conditions under Lanzarote than under the western Canary Islands. A possible explanation for this is the presence of a mantle plume under the Canary Islands, which causes thermal erosion at the base of the lithosphere, whereas ascending plume melts are responsible for heating, partial melting and metasomatism in the overlying mantle. Edge effects such as small-scale convection caused by interaction between hot plume material flowing eastwards underneath the lithosphere and the continental margin of West Africa, may account for enhanced thermal erosion under the easternmost Canary Islands and recurrent volcanism.