Molybdenum isotope constraints on the genesis of alkaline ocean island basalts from Madeira hotspot

Plate subduction transports crustal materials (e.g., oceanic crust) into the deep mantle, generating mantle compositional heterogeneities. However, the transformation and long-term evolution of these materials under deep mantle conditions remain poorly understood. In this study, we present molybdenum (Mo) isotope data of well-characterized alkaline basalts from Madeira Island in the eastern North Atlantic. Based on their stratigraphic relationships and eruption ages, the samples are divided into shield stage (5–0.7 Ma) and post-erosional stage (<0.7 Ma) groups. The Mo isotopic compositions (δ 98/95 Mo = -0.50‰ to -0.22‰, relative to NIST SRM3134) of the shield stage lavas are lighter than those of mid-ocean ridge basalts (MORB; δ 98/95 Mo = -0.19 ± 0.01‰), and the post-erosional stage lavas have δ 98/95 Mo values (-0.31‰ to -0.17‰) similar to those of MORB. The Mo isotopes in both lavas show good correlation with radiogenic Sr-Pb isotope ratios. Besides, the samples also exhibit higher Ce/Mo ratios (average ∼45.42) compared to MORB (average ∼37.18). Notably, δ 98/95 Mo values correlate with eruption ages: the shield stage lavas have low δ 98/95 Mo values, whereas the post-erosional stage lavas show relatively higher values. These geochemical variations are interpreted to reflect differing contributions from distinct portions of recycled oceanic lithosphere, with a minor sedimentary input. Specifically, the shield stage magmas were affected by a recycled hydrothermally altered upper basaltic section (eclogitized, with high 87 Sr/ 86 Sr and U/Pb values, and low δ 98/95 Mo values), whereas the post-erosional stage magmas reflect lower ultramafic components (gabbro) with mantle-like Sr-Mo isotopes and U/Pb values. The light Mo isotope signatures of the Madeira hotspot alkaline basalts thus confirm the importance of recycled oceanic crust in the genesis of alkaline ocean island basalts (OIBs), highlighting that Mo isotopes can be used to trace subduction processes.