Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments

Under modern oxidising Earth surface conditions, dehydrated subducted slabs show Mo isotope compositions as low as δ 98/95 Mo = − 1.5 ‰ , compared to the depleted mantle δ 98/95 Mo = − 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 δ 98/95 Mo of up to − 0.1 ‰ from the most depleted samples towards those tapping more enriched mantle sources. δ 98/95 Mo 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.