Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments
Résumé
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 positively with radiogenic Sr and Nd isotopes, which indicates recycling of Proterozoic sediments with a Mo isotopic composition that was not affected by subductionrelated, 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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