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

Frédéric Girault, Frédéric Perrier, Robin G M Crockett, Mukunda Bhattarai, Bharat Prasad Koirala, Christian France-Lanord, Pierre Agrinier, Magali Ader, Frédéric Fluteau, Claire Gréau, Manuel Moreira

Research output: Contribution to JournalArticlepeer-review

Abstract

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 (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 × 103 Bq m−3 in GZ1–GZ2 and 70 × 103 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 (δ13C from −1.7 ± 0.1 to −0.5 ± 0.1‰), with the absence of mantle signature (helium isotopic ratios R/RA < 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.
Original languageEnglish
Article number5
Pages (from-to)4017-4055
Number of pages39
JournalJournal of Geophysical Research. Solid Earth
Volume119
Issue number5
Early online date12 May 2014
DOIs
Publication statusE-pub ahead of print - 12 May 2014

Keywords

  • CO2
  • radon
  • gas fluxes
  • accumulation chamber
  • hydrothermal system
  • Nepal Himalayas

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