Chemical and isotopic compositions of thermal springs, fumaroles and bubbling gases at Tacana Volcano (Mexico-Guatemala): implications for volcanic surveillance

This study presents baseline data for future geochemical monitoring of the active Tacana volcano-hydrothermal system (Mexico-Guatemala). Seven groups of thermal springs, related to a NW/SE-oriented fault scarp cutting the summit area (4,100m a.s.l.), discharge at the northwest foot of the volcano (1,500-2,000m a.s.l.); another one on the southern ends of Tacana (La Calera). The near-neutral (pH from 5.8 to 6.9) thermal (T from 25.7A degrees C to 63.0A degrees C) HCO3-SO4 waters are thought to have formed by the absorption of a H2S/SO2-CO2-enriched steam into a Cl-rich geothermal aquifer, afterwards mixed by Na/HCO3-enriched meteoric waters originating from the higher elevations of the volcano as stated by the isotopic composition (delta D and delta O-18) of meteoric and spring waters. Boiling temperature fumaroles (89A degrees C at similar to 3,600m a.s.l. NW of the summit), formed after the May 1986 phreatic explosion, emit isotopically light vapour (delta D and delta O-18 as low as -128 and -19.9aEuro degrees, respectively) resulting from steam separation from the summit aquifer. Fumarolic as well as bubbling gases at five springs are CO2-dominated. The delta C-13(CO2) for all gases show typical magmatic values of -3.6 +/- 1.3aEuro degrees vs V-PDB. The large range in He-3/He-4 ratios for bubbling, dissolved and fumarolic gases [from 1.3 to 6.9 atmospheric He-3/He-4 ratio (R (A))] is ascribed to a different degree of near-surface boiling processes inside a heterogeneous aquifer at the contact between the volcanic edifice and the crystalline basement (He-4 source). Tacana volcano offers a unique opportunity to give insight into shallow hydrothermal and deep magmatic processes affecting the CO2/He-3 ratio of gases: bubbling springs with lower gas/water ratios show higher He-3/He-4 ratios and consequently lower CO2/He-3 ratios (e.g. Zarco spring). Typical Central American CO2/He-3 and He-3/He-4 ratios are found for the fumarolic Agua Caliente and Zarco gases (3.1 +/- 1.6 x 10(10) and 6.0 +/- 0.9 R (A), respectively). The L/S (5.9 +/- 0.5) and (L + S)/M ratios (9.2 +/- 0.7) for the same gases are almost identical to the ones calculated for gases in El Salvador, suggesting an enhanced slab contribution as far as the northern extreme of the Central American Volcanic Arc, Tacana.