Fluid-mobile and volatile element (Cl, B, and Li) cycling through the forearc: Case study of cold and thermal spring geochemistries from the Hikurangi accretionary prism, New Zeala

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A subduction zone is an area where two tectonic plates collide and the denser plate (also called a subducting slab, consisting of seafloor sediments, oceanic crust, and underlying mantle) is forced to sink into the mantle beneath the more buoyant plate. Hydrous minerals in the slab break down when exposed to the high temperatures encountered at depth, releasing fluids ultimately involved in the creation of arc magmas and explosive arc volcanoes. In addition, these fluids are also thought to influence seismic slip behavior and the manifestation of earthquakes. Therefore, determining the sources and amount of fluids released at depth within a subduction is critical for understanding volcanic and earthquake behavior along plate margins.Particular elements (for example, lithium, chlorine, and boron) are highly fluid-mobile, thereby making them excellent tracers of fluid source. These fluids impart diagnostic geochemical signatures to overlying material that can be used to trace mass transfer through subduction zones. This work will geochemically characterize cold and thermal spring waters from the Hikurangi (New Zealand subduction zone) accretionary prism to trace fluid sources along the fore-arc and quantify volatile flux through the fore-arc. Fluid sources to the fore-arc will be determined by examining variations in elemental concentrations (and ratios) and isotopic compositions. The data will be used to evaluate causal links between fluid sources and earthquake mechanics, such as shallow slow slip events. The Hikurangi margin is the ideal locality for this study due to the numerous exposed fore-arc springs, which allow a rare glimpse into the shallow part of the subduction zone, and the previously documented along arc variability in subduction parameters (e.g., amount of subducting sediment) and seismic slip behavior. Geochemical evidence for increased (or more shallow) dehydration reactions in the northern portion of the margin compared to the southern portion would support a link between dehydration reactions and shallow slow slip events.