The Syabru‐Bensi hydrothermal system in central Nepal: 2. Modeling and significance of the radon signature

Abstract

The Syabru-Bensi hydrothermal system (SBHS), located in the Nepal Himalayas, is characterized by numerous hot (>30°C) springs and the release of dry, cold (<35°C) CO 2 associated with radon-222, detailed in the companion paper. In the SBHS, CO 2 and radon fluxes on the ground vary over 5-6 orders of magnitude, reaching exceptional mean values of 100 kg m À2 d À1 and 12 Bq m À2 s À1 , respectively. This paper extends the companion paper by developing three quantitative models for the radon signature of CO 2 based on measurements of radon and radium concentrations in the spring waters and effective radium concentration of rocks and soils. The first model considers near-surface radon and CO 2 degassing from water, considered unlikely unless there exist currently unidentified large discharges of hydrothermal water. The second model considers CO 2 , arising from deeper hydrothermal sources, incorporating radon from shallow radium sources as it percolates upward toward the surface, considered more likely as a percolation depth of 100 m is sufficient to account for the observed radon discharge. The third model considers the observed peak radon concentrations in the gas zones and assumes that gaseous CO 2 can be transported from kilometer-scale depths through a fault network connected to the zones. This latter model affords the possibility that variations of physical parameters at depths associated with earthquake nucleation might be detectable at the surface. Gas-dominated transport might operate in other locations in Himalayas and elsewhere and may be an important aspect of the coupled mechanisms associated with seismically active orogens.

The Syabru‐Bensi hydrothermal system in central Nepal: 1. Characterization of carbon dioxide and radon fluxes

“…In summary, radon concentration appears similar in GZ1 and GZ2 (circa 40 × 10 3 Bq m −3 ) located close to the main hot springs, but significantly higher in GZ3 and GZ4 (circa 70 × 10 3 Bq m −3 ) located 500 m from the main hot springs. This feature of the SBHS is interpreted in the companion paper [ Girault and Perrier , ].…”

“…We also outline the methods, with fuller descriptions presented in the supporting information, and then present and discuss the results as compared with other sites worldwide. A detailed discussion of the significance and modeling of the radon signature is presented in the companion paper [ Girault and Perrier , ].…”

Large-scale organization of carbon dioxide discharge in the Nepal Himalayas

“…The Chilime site appears here intermediate, marginally compatible with simple degassing of radon‐bearing water. This analysis indicates that group I is anomalous and signals the presence of another mechanism, with radon being incorporated at a depth after CO 2 is degassed from water or perhaps suggesting that CO 2 is transported and released independently from hydrothermal circulations [ Girault and Perrier , ]. Differences in the radon flux can be accounted for by different depths of CO 2 degassing or by variations in the effective radium concentration of rocks (i.e., the radon source term) [ Girault et al ., ; Girault and Perrier , , Figure 14].…”

“…Radon concentration, expressed in Bq L −1 , was systematically measured on site in hot and cold water springs by radon emanometry in air [ Girault and Perrier , ]. Experimental uncertainties were 5–30%.…”

“…Carbon isotopic ratios of CO 2 (δ 13 C, defined relative to the standard values of Pee Dee belemnite, PDB) were obtained by mass spectrometry. Radon concentration, expressed in Bq L À1 , was systematically measured on site in hot and cold water springs by radon emanometry in air [Girault and Perrier, 2014]. Experimental uncertainties were 5-30%.…”