Collaborative Research: He-CO2-N2 Isotopes and Dissolved Gases in Groundwaters of the Costa Rica Fore-arc Margin

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Fluid flow through subduction zones is a fundamental process on Earth. As oceanic plates get subducted into Earths interior, water and other volatiles (i.e. CO2, N2) are released from the subducting plate into the overlying mantle wedge. These fluids are the primary cause for the melting of the mantle and the production of water-rich magmas that cause explosive volcanic eruptions. In addition, volatiles are either efficiently recycled back to the atmosphere through subduction-related volcanoes (as is the case for N2) or are potentially carried into the deep mantle (as may be the case for CO2). The extent of volatile recycling in subduction zones, therefore, has significant bearing on the overall volatile composition of the mantle and the atmosphere. In this proposal, the research team will target localities within the Costa Rica subduction zone complex where gas-rich cold springs release volatiles to the atmosphere. In addition, several springs will be monitored continuously for gas composition in order to investigate temporal changes related to earthquake activity occurring in the region. The research plan of this project involves producing a volatile dataset - comprising the isotope systematics and abundance characteristics of He, CO2, N2, CH4, O2, and Ar together with major/trace element, stable isotope (D/H and 18O/16O) and carbon-14 signatures of aqueous fluids (groundwaters), with the aim of understanding details of volatile provenance, flux and temporal variability. These springs are located trench-ward of the volcanic front and therefore sample volatiles that are potentially released from the mantle lithosphere and/or crust of the subducting slab before it reaches a depth corresponding to the volcanic front. In this way, this work aims at investigating what proportions of the volatiles that are subducted actually reach the volcanic front. Additionally, as various volatile species are expected to have different release/retention characteristics on the down-going plate, we can gauge how volatiles fractionate from one another by sampling profiles parallel to the direction of plate convergence. Moreover, by taking advantage of well-mapped structural controls on the hydrogeology, we will also determine the influence of faulting on the loss of volatiles along three targeted sections of the Costa Rica fore-arc (Nicoya, Central Pacific Coast and Oso Peninsula). Finally, because the fore-arc region of Costa Rica is the location of numerous large earthquakes occurring on a large number of active faults, it is proposed to investigate the relationships between fault localities, the occurrence of cold springs, gas compositions, and earthquake activity through (a) a widespread sampling campaign targeting ~ 100 sampling locations at the three sections of the forearc, and (b) continuously monitoring the gas composition of several selected springs using specifically-developed instrumentation (Spartah device). In the case of a local earthquake, the team anticipates being able to capture such an event by continuous monitoring of gas chemistry and documenting any changes in the gas composition of the springs. This part of the project has the potential to better understand earthquake processes in this region. To further these aims and to provide research opportunities for local scientists, the PIs will contribute to the M.Sc. Hydrogeology program at the University of Costa Rica through lectures, practical sessions and field-based training modules covering various aspects of hydrogeology, hydrochemistry, volcanology and structural geology. Monitoring efforts at Poas volcano in Costa Rica will also continue to investigate the potential role of volatiles in signaling the onset of impending hazardous conditions at the volcano.