The 72 Ma geochemical evolution of the Madeira hotspot (eastern North Atlantic): recycling of Paleozoic (≤500 Ma) oceanic lithosphere

Abstract

Major element, trace element and Sr–Nd–Pb isotopic data of volcanic rocks from the Madeira Archipelago (eastern North Atlantic) and seamounts of the Madeira hotspot track (Ampère, Coral Patch and Ormonde) are presented in this study. Although the Sr and Nd isotopic ratios are similar to those in normal mid ocean ridge basalt, the incompatible element signatures and Pb isotopic compositions ( 206Pb/ 204Pb=18.7–19.8) show similarities to the high time integrated 238U/ 204Pb mantle component. On the 206Pb/ 204Pb versus 207Pb/ 204Pb isotope diagram, all samples plot below the Northern Hemisphere reference line (Δ 7/4=−1.0 to −7.2) and form a 430 Ma isochron. The Pb, as well as Sm–Nd, isotope data are consistent with the presence of recycled Paleozoic (≤500 Ma) oceanic crust in the Madeira source. Variations in major element and isotopic geochemistry (e.g. positive correlation between SiO 2 and FeO T with Pb isotope ratios in primitive samples) point to a heterogeneous plume source containing upper (primarily hydrothermally altered basaltic) and lower (primarily unaltered gabbroic) ocean crust and lithospheric mantle. The more fertile basaltic crustal component is preferentially sampled during the shield stage of volcanism, whereas the more depleted lower crust and lithospheric mantle components are preferentially sampled during the post-erosional stage. We propose that plume material becomes progressively depleted through melt extraction as it spreads out along the base of the lithosphere in the direction of plate motion. A systematic decrease in 143Nd/ 144Nd and increase in 207Pb/ 204Pb isotopic ratios with increasing age along the hotspot track and proximity to the Iberian peninsula are attributed to increased contamination by continental lithosphere.