Distinguishing source effects from physical processes using the full suite of noble gas systematics across geothermal systems in Afar (Ethiopia)

As our society shifts toward sustainable resources to meet growing energy demands, geothermal systems represent a promising natural resource distributed globally across active magmatic segments or plate margins. These systems refer to any localized geological setting (volcanic or non-volcanic) where a portion of the Earth's thermal energy is extracted from a circulating fluid and transported to a point of use (Williams et al., 2011). While they constitute important sources of heat, strategic volatiles, and ore-forming elements, the storage, migration pathways, and release mechanisms of hot circulating fluids carrying strategic volatiles remain poorly understood. In detail, these fluids typically form through the interaction between convective groundwaters and shallow heat sources, but a wide range of additional physico-geochemical processes may occur and alter both the composition and release of volatiles.

In this study, we focus on Afar Region (Ethiopia), with the aim of evaluating and distinguishing source effects from physical processes by coupling high precision light (He-Ne) and heavy (Ar-Kr-Xe) noble gas isotope systematics in key hydrothermal systems. Free-gas samples were collected from natural bubbling pools in Central Afar (Dubti area) and North Afar (Dallol area) following sampling protocols adapted from Giggenbach and Goguel (1989). Light noble gas isotopes were analysed by conventional static mass spectrometry at the Centre de Recherches Pétrographiques et Géochimiques (CRPG). Ultrahigh precision heavy noble gas isotope analyses were conducted by state-of-the-art dynamic mass spectrometry at the Seltzer Laboratory (WHOI, MA, USA).

While He-Ne analyses can be combined to distinguish deep contributions from crustal and various mantle endmembers (source effects), stable Ar-Kr-Xe isotope systematics provide key information on the dynamics of water-gas interactions in the subsurface (physical processes). The resulting dataset is evaluated using the diffusive transport fractionation (DTF) model of Bekaert et al. (2023) to test how this globally significant process operates in the Afar Region. We also explore the possibility for heavy noble gas isotope systematics to provide complementary insights into source effects once corrected for the isotopic effect of subsurface fractionation. To better constrain volatile distributions within hydrothermal systems, we compare our dataset with a comprehensive literature compilation of volatile data from the East African Rift, shedding light on the influence of crustal age (i.e., cratonic vs. juvenile crust) on He isotopic and elemental compositions.

Our data yield He isotopic ratios ranging from 7.3 to 12.2 ± 0.20 R_A, consistent with contributions from deep mantle sources. ²⁰Ne/²²Ne ratios indicate a significant influence of an air-like component, with minor deviations from air attributable to source and/or fractionation processes. Xenon systematic are consistent with subsurface fractionation, with small additions of mantle-derived radiogenic and fissiogenic components.

The entire suite of noble gas isotopes provides a powerful means to identify how physical fractionation processes and deep vs. shallow sources of volatiles contribute to producing the complex geochemical signatures observed in surface emissions. The goal of this project will be to ultimately improve our understanding of whether subsurface water–gas interactions can trigger processes capable of modifying and potentially concentrating strategic volatiles along magmatic segments of the Afar Region.