Ahmad, QasidQasidAhmad0000-0001-8366-9081Wille, Martin HermannMartin HermannWille0000-0003-1083-4730Rosca, CCRoscaLabidi, JJLabidiSchmid, Timothy ChrisTimothy ChrisSchmid0000-0002-9477-9654Mezger, KlausKlausMezger0000-0002-2443-8539König, SSKönig2024-10-112024-10-112022-10-12https://boris-portal.unibe.ch/handle/20.500.12422/88484Under modern oxidising Earth surface conditions, dehydrated subducted slabs show Mo isotope compositions as low as δ98/95Mo = −1.5 ‰, compared to the depleted mantle δ98/95Mo = −0.2 ‰. Such light Mo isotope compositions reflect the redox-dependent aqueous mobility of isotopically heavy Mo associated with slab dehydration. Here we analysed basaltic glasses from the South-Mid Atlantic Ridge, whose parental melts are influenced by the enriched Discovery and Shona mantle plumes. We report increasingly higher δ98/95Mo of up to −0.1 ‰ from the most depleted samples towards those tapping more enriched mantle sources. δ98/95Mo values correlate with radiogenic Sr and Nd isotopes, which indicates the recycling of Proterozoic sediments with a Mo isotopic composition that was not affected by subduction-related, oxic dehydration. We propose that the Mo isotope signatures were retained during subduction and reflect anoxic conditions during deep sea sedimentation in the mid-Proterozoic. Finally, Mo isotope fractionation between different terrestrial reservoirs likely depends on the slab redox budget, and therefore on the timing of subduction with regard to Earth’s surface oxygenation.enmolybdenum isotopessubduction zonesMORBsedimentsmantle plumeredox500 - Science::550 - Earth sciences & geologyarticle10.48350/17428310.7185/geochemlet.2236