Os isotope systematics in the Canary Islands and Madeira: Lithospheric contamination and mantle plume signatures

Osmium concentrations and isotopic signatures were measured ill 28 primaril y Holocene basalts (22 of which which have been analyzed for Sr-Nd-Pb isoto pe composition), two carbonatites and two mantle xenoliths from the Canary Islands, Selvagen Grande and Madeira in the eastern North Atlantic. Os-187/ Os-188 ratios in the basalts range from 0.129 to 0.183. nle Os isotope syst ematics indicate that the basalts fall into three petrogenetic groups: (1) a 'radiogenic' group with high Os-187/Os-188 from 0.152 to 0.1831 (2) an 'u nradiogenic' group with low Os-187/Os-188 from 0.129 to 0.138; (3) an 'inte rmediate' group with Os-187/Os-188 between 0.139 and 0.151. The Os isotope systematics of the radiogenic group samples are consistent with minor conta mination of the basalts by marine sediment. All samples in the unradiogenic group contain mantle xenoliths, and the unradiogenic Os carl be explained by bulk assimilation of less than or equal to 5% mantle peridotite in the f orm of disaggregated xenoliths. The radiogenic and unradiogenic groups are also characterized by higher Sr-87/Sr-86 and Pb-208/Pb-204 but lower Nd-143 /Nd-144 than samples with similar. Pb-206/Pb-204 from the intermediate grou p, which is interpreted to reflect interaction of plume magmas With rite li thospheric mantle. The intermediate group samples are believed to represent the isotopic signature of the the mantle plume. The Os isotopic compositio n of the Canary plume is among rite most radiogenic found in ocean island b asalts, comparable with the endmember HIMU islands Mangaia and Tubuaii, but at significantly lower Pb-206/Pb-204. The radiogenic Os and moderate Pb-20 6/Pb-204 signature of the Canary plume is consistent with a plume which con tains 25-35% of relatively young (similar to 1.2 Ga) recycled oceanic dust. Variable degree of mixing of the Canary Island plume source with shallow d epleted asthenosphere containing a component of Paleozoic oceanic crust pro duces the limited range in Os isotopic signatures observed in the Madeira a nd Canary Island basalts despite a large range in Pb-206/Pb-204 isotopic co mposition.