Constraining hidden mantle volatile fluxes in volcanically quiescent regions.

Earth is a habitable planet as a result of its unique atmospheric composition and the presence of liquid water at its surface, both of which help regulate long term climate. The amount volatiles (e.g., H2O, O2, CO2 and Ar) is controlled by a careful balance between the amount going into the solid Earth (for example during subduction) and that coming out (for example during volcanism). The majority of outgassing is thought to occur in volcanically and tectonically active regions. However, recent work has identified that volatiles may be passively degassed in regions not directly related to volcanism. The diffuse nature of this passive outgassing makes it difficult to identify, accurately quantify and determine its extent, meaning it has not previously been fully constrained. Determining these fluxes becomes increasingly difficult when trying to measure volatiles such as carbon, which are critical to understand due to their role in climate regulation, but take part in chemical and biological reactions. Whether this hidden diffuse flux is globally significant and whether it is an important component of the global volatile cycle over geological timescales, therefore remains unknown.
Groundwater is ubiquitous across the globe. Volatiles can accumulate within and interact with groundwater as they are exchanged between the deep interior and surface making it an excellent archive of these processes. Noble gases are powerful tracers of fluid provenance and physical processes as they are inert and different terrestrial reservoirs have distinct noble gas compositions. Noble gas measurements in groundwater therefore provide the ideal tool for understanding volatile fluxes from Earth’s interior to its surface. Typically helium has been used to investigate fluxes but in isolation, it cannot fully distinguish between different mantle sources. However our recent analytical advances allow for argon measurements at sufficient precision to determine excesses in groundwater and for...