Chemical and isotopic study of thermal springs and gas discharges from Sierra de Chiapas, Mexico
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Abstract
Thermal water and gas discharges south-east of El Chichón volcano, Mexico are associated mainly with NW-SE oriented fault systems. Spring discharges include i) waters with Na-Cl composition and TDS>3000 mg/L; ii) waters with Ca- composition and TDS values between 1400 and 2300 mg/L; iii) waters with Na-Cl composition and TDS of 800 to 2400 mg/L and sulphate content up to 650 mg/L and iv) waters with Ca-
composition and low salinity (TDS <250mg/L). Most of these waters are associated with free-gas discharges of
(up to 93 % by vol.),
(2.4 to 31.2 % by vol.) and Ar (up to 1.25 % by vol.) with a predominant meteoric origin.
S is present only in gas samples collected at El Azufre (up to 1.1 % by vol.). The δ13C
values are always below -9.7% (PDB) and suggest a partially biogenic origin for . Chemical and isotopic features of spring discharges indicate that fluid circulation in the Sierra de Chiapas is mainly regulated by meteoric waters that tend to infiltrate the upper and middle-Cretaceous carbonate units up to the lower Cretaceous-upper Jurassic evaporitic formations (by López-Ramos, 1982). The latter provide the main source of the species in solution. No evidence for high-to-medium enthalpy systems at depth beneath the Sierra de Chiapas has been found.
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