The Evolution of the Upper Mantle beneath the Canary Islands: Information from Trace Elements and Sr isotope Ratios in Minerals in Mantle Xenoliths

Abstract

AbstractLaser ablation microprobe data are presented for olivine, orthopyroxene and clinopyroxene in spinel harzburgite and lherzolite xenoliths from La Palma, Hierro, and Lanzarote, and new whole-rock trace-element data for xenoliths from Hierro and Lanzarote. The xenoliths show evidence of strong major, trace element and Sr isotope depletion (87Sr/86Sr ≤ 0·7027 in clinopyroxene in the most refractory harzburgites) overprinted by metasomatism. The low Sr isotope ratios are not compatible with the former suggestion of a mantle plume in the area during opening of the Atlantic Ocean. Estimates suggest that the composition of the original oceanic lithospheric mantle beneath the Canary Islands corresponds to the residues after 25–30% fractional melting of primordial mantle material; it is thus significantly more refractory than ‘normal’ mid-ocean ridge basalt (MORB) mantle. The trace element compositions and Sr isotopic ratios of the minerals least affected by metasomatization indicate that the upper mantle beneath the Canary Islands originally formed as highly refractory oceanic lithosphere during the opening of the Atlantic Ocean in the area. During the Canarian intraplate event the upper mantle was metasomatized; the metasomatic processes include cryptic metasomatism, resetting of the Sr–Nd isotopic ratios to values within the range of Canary Islands basalts, formation of minor amounts of phlogopite, and melt–wall-rock reactions. The upper mantle beneath Tenerife and La Palma is strongly metasomatized by carbonatitic or carbonaceous melts highly enriched in light rare earth elements (REE) relative to heavy REE, and depleted in Zr–Hf and Ti relative to REE. In the lithospheric mantle beneath Hierro and Lanzarote, metasomatism has been relatively weak, and appears to be caused by high-Si melts producing concave-upwards trace element patterns in clinopyroxene with weak negative Zr and Ti anomalies. Ti–Al–Fe-rich harzburgites/lherzolites, dunites, wehrlites and clinopyroxenites formed from mildly alkaline basaltic melts (similar to those that dominate the exposed parts of the islands), and appear to be mainly restricted to magma conduits; the alkali basalt melts have caused only local metasomatism in the mantle wall-rocks of such conduits. The various metasomatic fluids formed as the results of immiscible separations, melt–wall-rock reactions and chromatographic fractionation either from a CO2-rich basaltic primary melt, or, alternatively, from a basaltic and a siliceous carbonatite or carbonaceous silicate melt.