Using Barium Isotopes to Investigate the Origin of Fluids in Subduction Zones

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Subduction zones are locations on Earth where oceanic crust and sediments (the subducted slab) are introduced back into the deep Earth resulting in the prominent arc volcanoes at the surface that are associated with substantial volcanic and earthquake hazards. Subduction zones are also the main places of mass transfer between the surface and deep Earth, which controls long-term climate and plays a critical role in the evolution of Earth's heat budget. It is well-known that material released from the subducted slab imparts distinct chemical signatures to arc volcanism and geochemical evidence suggests that both subducted sediment and hydrothermally altered oceanic crust (AOC) play significant roles in arc lava generation. However, the exact physical processes responsible for transporting slab material into the arc is the subject of significant recent debate. In essence, one model poses that all the subducted components are mixed at the top of the slab, forming a 'melange' layer, which subsequently is the main source region for arc lavas. The second model invokes that the general structure of the subducted slab is intact through a large portion of the subduction process and only when sediments become hot enough to melt and AOC dehydrates are these components released to the mantle wedge and induce melting. In this project, researchers use a novel isotopic tool to investigate which of these two models is correct.This team has identified the element barium (Ba) and its isotopes as a geochemical tracer that can be used to investigate the origin of material released from the subducting slab and, thereby, distinguish between models that invoke sediment melting and ocean crust dehydration versus melange melting as the primary source of slab material in arc lavas. Physically, the two end-member models of slab material transport are very different and have different consequences for the thermal structure, distribution of volcanoes, and chemical budgets of crustal recycling in subduction zones. However, both models equally predict most of the unique chemical and isotopic characteristica of arc lavas. It is, therefore, critical to develop scientific tests that are capable of distinguishing the two different models. In both models, fluids are important vectors of slab material transport. However, the ultimate source of these fluids are different in that fluids are sourced from melange layers in one model and primarily extracted from AOC in the other. Barium is a highly fluid mobile element that displays characteristic enrichment over similarly incompatible elements like lanthanum and thorium in arc lavas. These researchers argue that Ba isotopes likely displays different values in melange, sedimentary and AOC sources of fluids. The isotope composition of the excess Ba is, therefore, likely to constrain the ultimate source of the fluids that carry the Ba. They hypothesize that evidence based on Ba isotopes can provide new constraints on the slab material transport mechanism in the subduction zones they have selected for study. In terms of Broader Impacts, this project will support graduate students who will incorporate this work into their theses, and will train them in cutting-edge geochemical tools. The results of this research will also be of broad interest to other fields that investigate the physical parameters that govern subduction zone magmatism, like seismologists and magnetotelluricists who use their tools to locate fluid flow in subduction zones.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.