The Syabru‐Bensi hydrothermal system in central Nepal: 1. Characterization of carbon dioxide and radon fluxes. Issue 5 (12th May 2014)

• Record Type: Journal Article
• Authors: Girault, Frédéric; Perrier, Frédéric; Crockett, Robin; Bhattarai, Mukunda; Koirala, Bharat Prasad; France‐Lanord, Christian; Agrinier, Pierre; Ader, Magali; Fluteau, Frédéric; Gréau, Claire; Moreira, Manuel
• Is Part Of: Journal of geophysical research. Volume 119:Issue 5(2014:May)
• Page Start: 4017
• Page End: 4055
• Publication Date: 2014-05-12
• Subjects: Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1002/2013JB010301
• Languages: English
• ISSNs: 2169-9313

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>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<sub>2</sub>) degassing areas. From 2007 to 2011, five gas zones (GZ1–GZ5) were studied, with more than 1600 CO<sub>2</sub> 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<sub>2</sub> and radon fluxes vary over 5 to 6 orders of magnitude, reaching exceptional maximum values of 236 ± 50 kg m<sup>−2</sup> d<sup>−1</sup> and 38.5 ± 8.0 Bq m<sup>−2</sup> s<sup>−1</sup>, with estimated integrated discharges over all gas zones of 5.9 ± 1.6 t d<sup>−1</sup> and 140 ± 30 MBq d<sup>−1</sup>, respectively. Soil‐gas radon concentration is 40 × 10<sup>3</sup> Bq m<sup>−3</sup> in GZ1–GZ2 and 70 × 10<sup>3</sup> Bq m<sup>−3</sup> in GZ3–GZ4. Strong relationships between CO<sub>2</sub> and radon fluxes in all gas zones (correlation coefficient <italic>R</italic> = 0.86 ± 0.02) indicate related gas transport mechanisms and demonstrate that radon can be considered as a relevant proxy for CO<sub>2</sub>. CO<sub>2</sub> carbon isotopic ratios (<italic>δ</italic><sup>13</sup>C from −1.7 ± 0.1 to −0.5 ± 0.1‰), with the absence of mantle signature (helium isotopic ratios <italic>R</italic>/<italic>R<sub>A</sub></italic> < 0.05), suggest metamorphic decarbonation at depth. Thus, the SBHS emerges as a unique geosystem with significant deep origin CO<sub>2</sub> 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.</p> </abstract>