Fluid geochemistry of Tacaná volcano-hydrothermal system

Abstract

Tacaná hosts an active volcano-hydrothermal system, characterized by boiling temperature fumaroles, near the summit (3,600–3,800 m asl), and bubbling degassing thermal springs near its base (1,000–2,000 m asl). The magmatic signature of gases rising to the surface is attested by their high CO2 contents (δ13CCO2 = −3.6 ± 1.3 ‰), and relatively high 3He/4He ratios (6.0 ± 0.9 RA), with a CO2/3He ratio typical for the Central American Arc (2.3 × 1010–6.9 × 1011). Such magmatic signature is practically identical for the near-summit fumaroles, and the bubbling gases at the base of Tacaná edifice. Besides the HCO3-enrichment in thermal spring waters, the springs (pH 5.8–6.7) show a SO4-and minor Cl-enrichment: a CO2 and H2S + SO2-rich magmatic steam condenses into a deeper geothermal aquifer, and the resulting hydrothermal fluid mixes with meteoric waters near the surface. The recharge area for the thermal springs is located at higher elevations (>400 m higher than spring outlet elevation), as inferred from the δD-δ18O data for rivers, thermal and cold springs. These general insights of the Tacaná volcano-hydrothermal system serve as the baseline for future volcanic surveillance, and geothermal prospection. The main locus of hydrothermal activity is located inside the Tacaná horseshoe-shaped crater in the northwestern sector of the volcanic edifice. In terms of volcanic hazard, this sector can be considered the most probable site for future phreatic activity.