Publication Date:
2014-04-05
Description:
The Syabru-Bensi hydrothermal system (SBHS), located at the Main Central Thrust (MCT) 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 (GZ1-5) 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 ( EC Ra ) measurements of soil samples. Measurement techniques and their uncertainties were evaluated in the field. CO 2 and radon flux values 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 the whole SBHS area of 5.9 ± 1.6 t d -1 and 140 ± 30 MBq d -1 , respectively. In gas zones GZ1-2, radon concentrations are 40 × 10 3 Bq m -3 , and higher in gas zones GZ3-4, ca . 70 × 10 3 Bq m -3 . 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 support the concept that radon can be considered as a relevant proxy for CO 2 . CO 2 carbon isotopic ratios (δ 13 C from –1.7 ± 0.1 to –0.5 ± 0.1 ‰), with 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 of medium spatial scale 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 both the shallow and deep subsurface.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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