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.

Automated summary

Under modern oxidising Earth surface conditions, dehydrated subducted slabs show light Mo isotope compositions reflecting redox-dependent aqueous mobility of isotopically heavy Mo associated with slab dehydration. The correlation between Mo isotope values and radiogenic Sr and Nd isotopes suggests the recycling of Proterozoic sediments during anoxic deep sea sedimentation.