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

Frédéric Girault (1, 2) , Frédéric Perrier (1) , Robin Crockett (3) , Mukunda Bhattarai (4) , Bharat Prasad Koirala (4) , Christian France-Lanord (5) , Pierre Agrinier (1) , Magali Ader (1) , Frédéric Fluteau (1) , Claire Gréau (1) , Manuel Moreira (1)

1 IPGP - Institut de Physique du Globe de Paris
2 LGENS - Laboratoire de géologie de l'ENS
3 Department of Environmental and Geographical Sciences [Northampton]
4 National Seismic Centre, Nepal
5 CRPG - Centre de Recherches Pétrographiques et Géochimiques

The Syabru-Bensi hydrothermal system (SBHS), located at the Main Central Thrust zone in central Nepal, is characterized by hot (30–62°C) water springs and cold (<35°C) carbon dioxide (CO 2) degassing areas. From 2007 to 2011, five gas zones (GZ1–GZ5) were studied, with more than 1600 CO 2 and 850 radon flux measurements, with complementary self-potential data, thermal infrared imaging, and effective radium concentration of soils. Measurement uncertainties were evaluated in the field. CO 2 and radon fluxes vary over 5 to 6 orders of magnitude, reaching exceptional maximum values of 236 ± 50 kg m À2 d À1 and 38.5 ± 8.0 Bq m À2 s À1 , with estimated integrated discharges over all gas zones of 5.9 ± 1.6 t d À1 and 140 ± 30 MBq d À1 , respectively. Soil-gas radon concentration is 40 × 10 3 Bq m À3 in GZ1–GZ2 and 70 × 10 3 Bq m À3 in GZ3–GZ4. Strong relationships between CO 2 and radon fluxes in all gas zones (correlation coefficient R = 0.86 ± 0.02) indicate related gas transport mechanisms and demonstrate that radon can be considered as a relevant proxy for CO 2. CO 2 carbon isotopic ratios (δ 13 C from À1.7 ± 0.1 to À0.5 ± 0.1‰), with the absence of mantle signature (helium isotopic ratios R/R A < 0.05), suggest metamorphic decarbonation at depth. Thus, the SBHS emerges as a unique geosystem with significant deep origin CO 2 discharge located in a seismically active region, where we can test methodological issues and our understanding of transport properties and fluid circulations in the subsurface.

Domaines: Planète et Univers [physics]

Frédéric Girault, Frédéric Perrier, Robin Crockett, Mukunda Bhattarai, Bharat Prasad Koirala, et al.. The Syabru-Bensi hydrothermal system in central Nepal: 1. Characterization of carbon dioxide and radon fluxes. Journal of Geophysical Research : Solid Earth, 2014, ⟨10.1002/2013JB010301⟩. ⟨insu-01295841⟩