Oxygen isotope heterogeneity of the mantle beneath the Canary Islands: insights from olivine phenocrysts

A relatively narrow range of oxygen isotopic ratios (delta O-18 = 5.0-5.4 parts per thousand) is preserved in olivine of mantle xenoliths, mid-ocean ridge (MORB), and most ocean island basalts (OIB). The values in excess of this range are generally attributed either to the presence of a recycled component in the Earth's mantle or to shallow level contamination processes. A viable way forward to trace source heterogeneity is to find a link between chemical (elemental and isotopic) composition of the earlier crystallized mineral phases (olivine) and the composition of their parental magmas, then using them to reconstruct the composition of source region. The Canary hotspot is one of a few that contains similar to 1- to 2-Ga-old recycled ocean crust that can be traced to the core-mantle boundary using seismic tomography and whose origin is attributed to the mixing of at least three main isotopically distinct mantle components i.e. HIMU, DMM, and EM. This work reports ion microprobe and single crystal laser fluorination oxygen isotope data of 148 olivine grains also analyzed for major and minor elements in the same spot. The olivines are from 20 samples resembling the most primitive shield stage picrite through alkali basalt to basanite series erupted on Gran Canaria, Tenerife, La Gomera, La Palma and El Hierro, Canary Islands, for which shallow level contamination processes were not recognized. A broad range of delta O-18(olivine) values from 4.6 to 6.1 parts per thousand was obtained and explained by stable, long-term oxygen isotope heterogeneity of crystal cumulates present under different volcanoes. These cumulates are thought to have crystallized from mantle-derived magmas uncontaminated at crustal depth, representing oxygen isotope heterogeneity of source region. A relationship between Ni x FeO/MgO and delta O-18(olivine) values found in one basanitic lava erupted on El Hierro, the westernmost island of the Canary Archipelago, was used to estimate oxygen isotope compositions of partial melts presumably originated from peridotite (HIMU-type component inherited its radiogenic isotope composition from ancient, similar to 1 to 2 Ga, recycled ocean crust) and pyroxenite (young, <1 Ga, recycled oceanic crust preserved as eclogite with depleted MORB-type isotopic signature) components of the Canary plume. The model calculations yield 5.2 and 5.9 +/- 0.3 parts per thousand for peridotite- and pyroxenite-derived melts, respectively, which appeared to correspond closely to the worldwide HIMU-type OIB and upper limit N-MORB delta O-18 values. This difference together with the broad range of delta O-18 variations found in the Canarian olivines cannot be explained by thermodynamic effects of oxygen isotopic fractionation and are believed to represent true variations in the mantle, due to oceanic crust and continental lithosphere recycling.