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|Authors: ||Rouwet, D.*|
Santiago, J. A.*
|Title: ||Chemical and isotopic compositions of thermal springs, fumaroles and bubbling gases at Tacaná Volcano (Mexico–Guatemala): implications for volcanic surveillance|
|Title of journal: ||Bullettin of Volcanology|
|Issue Date: ||1-Jun-2008|
|Keywords: ||Tacaná volcano|
|Abstract: ||This study presents baseline data for future
geochemical monitoring of the active Tacaná 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 Tacaná (La Calera).
The near-neutral (pH from 5.8 to 6.9) thermal (T from 25.7°C to 63.0°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 (δD and δ18O) of meteoric and spring waters.
Boiling temperature fumaroles (89°C at~3,600m a.s.l. NW of the summit), formed after the May 1986 phreatic explosion, emit isotopically light vapour (δD and δ18O as
low as −128 and −19.9‰, respectively) resulting from steam separation from the summit aquifer. Fumarolic as well as bubbling gases at five springs are CO2-dominated.
The δ13CCO2 for all gases show typical magmatic values of −3.6 ± 1.3‰ vs V-PDB. The large range in 3He/4He ratios for bubbling, dissolved and fumarolic gases [from 1.3 to
6.9 atmospheric 3He/4He ratio (RA)] 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 (4He source). Tacaná volcano offers a unique opportunity to give insight into
shallow hydrothermal and deep magmatic processes affecting the CO2/3He ratio of gases: bubbling springs with lower
gas/water ratios show higher 3He/4He ratios and consequently lower CO2/3He ratios (e.g. Zarco spring). Typical Central American CO2/3He and 3He/4He ratios are found for the fumarolic Agua Caliente and Zarco gases (3.1 ± 1.6 × 1010 and 6.0 ± 0.9 RA, 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|
|Appears in Collections:||Papers Published / Papers in press|
05.02.01. Geochemical data
04.08.06. Volcano monitoring
04.04.12. Fluid Geochemistry
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