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 898/95Mo = -1.5 degrees/00, compared to the depleted mantle 898/95Mo = -0.2 degrees/00. Such light Mo isotope compositions reflect the redox-dependent aqueous mobility of isotopically heavy Mo associated with slab dehydra-tion. 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 898/95Mo of up to -0.1 degrees/00 from the most depleted sam-ples towards those tapping more enriched mantle sources. 898/95Mo values correlate with radiogenic Sr and Nd isotopes, which indicates the recycling of Proterozoic sedi-ments 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.