Séminaire David Bekaert – Analysis of volatile elements in volcanic and hydrothermal systems: a reliable window into the solid Earth’s composition?

The geochemical composition of volatile elements (e.g., carbon, nitrogen, hydrogen) in mantle-derived gases holds important clues about the origin of Earth’s volatiles and their cycling between terrestrial reservoirs (i.e., atmosphere, mantle, crust). CO2-rich hydrothermal systems (e.g., bubbling hot springs) are ubiquitous throughout the world, in principle offering an opportunity to explore volatile element systematics within various mantle sources. Being inert and comprising a large number of primordial (i.e., from Earth’s formation) and secondary (e.g., radiogenic and fissiogenic) isotopes, heavy noble gases (Ar, Kr, Xe) are powerful tracers of the source(s) and processe(s) related to terrestrial volatile evolution. In practice, however, measurements of Ar-Kr-Xe isotopes in hydrothermal systems often resemble air at the analytical precision (several per mil level) of traditional noble gas mass spectrometers. In this talk, I will present recent progress on the development of a new analytical technique for the collection, purification, and ultrahigh precision (0.01 per mil level) analysis of volcanic Ar-Kr-Xe isotopes by dynamic isotope-ratio mass spectrometry, at the Woods Hole Oceanographic Institution (USA). Applying this new analytical approach to multiple volcanic systems worldwide (https://www.science.org/doi/10.1126/sciadv.adg2566), we discovered coherent physical fractionation of groundwater-derived Ar, Kr, and Xe isotopes, which mix with deep (mantle-derived and crustal) noble gases before reaching the surface. I will present several case studies demonstrating how this new analytical technique opens the door to exploring the spatial variability of both geological noble gas source signals and physical transport processes in the subsurface. Applying robust corrections for physical fractionation has emerged as a key step towards robustly detecting small, yet potentially significant, mantle heavy noble gas anomalies bearing information about the evolution of terrestrial volatiles within hydrothermal systems.