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

Ahmad, Q; Wille, M; Rosca, C; Labidi, J; Schmid, T; Mezger, K; König, S (2022). Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments. Geochemical perspectives letters, 23, pp. 43-48. European Association of Geochemistry 10.7185/geochemlet.2236

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Under 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.

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