The impact of primary processes and secondary alteration on the stable isotope composition of ocean island basalts

Molybdenum and uranium are redox sensitive elements that experience isotope fractionation in the Earth-surface environment. Recent studies have explored transfer of these fractionated signals subduction zones to arc magmas, but less is known about full variation isotopic tracers mantle, particularly domains mantle may feed hot spots. Here, we present Mo U data for lava samples from Pitcairn, Samoan Islands, St. Helena, representing classic localities EM-1 (enriched 1), EM-2 2) HIMU (high ?, where ? 238U/204Pb) endmembers. compositions highly variable each location, range largely overlaps between three localities. However, some this be attributable chemical weathering low-temperature alteration effects, as revealed by 234U/238U disequilibrium, also reported here; many samples, especially a 234U deficit, while few others excess. Samples with greatest deficit tend highest Ce/Mo given locality lowest ?98Mo typically show considerable suggesting was leached during preferential loss heavy isotopes. Of 238U activity equilibrium localities, most values lower than published average MORB, they ?238U within uncertainty MORB. In contrast, two similar MORB isotopically light ?238U. While low seen could incorporation subducted sediments into plume sources, such sediment would needed deposited under oxic oceanographic conditions were not prevalent until Neoproterozoic; radiogenic point towards much greater age recycled components sources. Instead, consider likely cause source region ancient ocean crust experienced devolatilization, which favored isotopes, has been proposed several recent studies. This line models origin source, along portion source. The U, MORB-like ?98Mo, reflect influence distinct pool mid-Proterozoic crust.