Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments
Qasid Ahmad, Martin Wille, Carolina Rosca, Jabrane Labidi, Timothy Schmid, K. Mezger, S. König
Abstract
Under modern oxidising Earth surface conditions, dehydrated subducted slabs show Mo isotope compositions as low as 98/95 Mo = -1.5 , compared to the depleted mantle 98/95 Mo = -0.2 . Such light Mo isotope compositions reflect the redoxdependent 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 98/95 Mo of up to -0.1 from the most depleted samples towards those tapping more enriched mantle sources. 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.