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

The Syabru-Bensi hydrothermal system (SBHS), located at the Main Central Thrust zone in central Nepal, is characterized by hot (30-62 degrees C) water springs and cold (<35 degrees C) carbon dioxide (CO2) degassing areas. From 2007 to 2011, five gas zones (GZ1-GZ5) were studied, with more than 1600 CO2 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. CO2 and radon fluxes vary over 5 to 6 orders of magnitude, reaching exceptional maximum values of 236 +/- 50 kg m(-2) d(-1) and 38.5 +/- 8.0 Bq m(-2) s(-1), with estimated integrated discharges over all gas zones of 5.9 +/- 1.6 t d(-1) and 140 +/- 30 MBq d(-1), respectively. Soil-gas radon concentration is 40 x 10(3) Bq m(-3) in GZ1-GZ2 and 70 x 10(3) Bq m(-3) in GZ3-GZ4. Strong relationships between CO2 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 CO2. CO2 carbon isotopic ratios (delta C-13 from -1.7 +/- 0.1 to -0.5 +/- 0.1 parts per thousand), 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 CO2 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.