Collaborative Research: Fluid-mobile element cycling (halogens, boron, lithium) through the forearc of Costa Rica

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A subduction zone is an area where two tectonic plates collide and the denser plate (the subducting slab, consisting of seafloor sediments, oceanic crust, and underlying mantle) sinks into the mantle beneath the more buoyant plate. Minerals in the slab containing water break down when exposed to the high temperatures encountered at depth beneath the seafloor, releasing fluids which ultimately lead to the production of magmas and explosive volcanism. In addition, these fluids are thought to influence the occurrence of earthquakes in the subduction zone. Fluids released by the subducting slab contain fluid mobile elements, such as lithium, chlorine and boron, and the composition of the fluids is diagnostic of the source in the subducting slab (sediments, oceanic crust, mantle) from which they derived. Cold and thermal springs located above the subducting slab provide a window into the fluids released at depth in a subduction zone. This project will investigate the geochemistry of spring fluids located off of Costa Rica between the subduction trench and the volcanoes produced by magmatism to identify the sources and amount of slab-derived fluids released at depth within a subduction zone. This information is necessary to understand volcano and earthquake behavior along active plate margins. This work will also provide scientific training for graduate and undergraduate students, as well as outreach presentations on subduction zones and associated hazards to the public in both the United States and Costa Rica.Subduction zones are crucial for understanding the chemistry deep and shallow reservoirs of Earth, yet very little is known about volatile behavior in forearc regimes. Here we propose to quantify the Li, B, and halogen budget throughout the inner subaerial forearc of Costa Rica through a geochemical investigation of forearc cold and thermal spring fluids. Lithium, boron, and the halogens (Cl, F, Br, I) are all highly fluid-mobile elements. Their incompatibility in minerals limits modification by fluid-rock interaction, thereby making them reliable tracers of fluid source. Costa Rica is an ideal locality for study because it is one of the few subduction zones in which forearc springs are subaerial due to the presence of several peninsulas and therefore easily accessible to sampling. Two of the peninsulas (Nicoya and Osa) are directly over the seismogenic zone and both are sites of large megathrust earthquakes. Slow slip events have also been observed in the Nicoya Peninsula. Elevated fluid pressures, commonly thought to be derived from metamorphic dehydration reactions, lower the effective stress and are therefore commonly invoked as a mechanism for aseismic slip. Fluids from forearc springs provide us with a direct window to fluid sources at seismogenic depths in subduction zones. Through the combined use of Li, B, and Cl stable isotopes, elemental concentrations, and elemental ratios (e.g., Br/Cl, I/Cl, B/Cl, Li/B), the following will be completed: 1) Identify the change(s) in slab-derived source along and across the forearc and how prograde metamorphism of the subducting slab controls Li, B, and halogen concentrations and isotope compositions across the forearc; 2) Provide an integrated view of loss of fluid-mobile elements during shallow subduction processes contributing to an improved evaluation of cycling through subduction zones; and 3) Evaluate correlation, or lack thereof, between varying fluid sources and seismic behavior.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.