The hydrothermal-volcanic system of Rincon de la Vieja volcano (Costa Rica): a combined (inorganic and organic) geochemical approach to undertanding the origin of the fluid discharges and its possible application to volcanic surveillance.

In the period 1998–2002 thermal spring discharges of Rincon de la Vieja volcano (NW Costa Rica) have been sampled and analyzed for major, trace and isotopic (18O/16O and D/H in waters and 3He/4He and 13C/12C in CO2 in gases) composition. The boiling pools hosted inside the summit crater (Active Crater) are characterized by high contents of magmatic-related compounds (SO2, HCl and HF) that strongly affect the chemistry of the crater lake. These chemical features are not shown by the thermal discharges seeping out in the surrounding area of the volcano. Here, the shallow aquifer apparently masks any possible clues related to the magmatic system. This suggests that the fluid vents located inside the Active Crater are likely to represent the most appropriate sampling sites for geochemical surveillance purposes, although the high gas discharge rate from the lake may occasionally prevent any fluid sampling. Alternatively, as already suggested by recent studies on the behavior of light hydrocarbons in different volcanic environments, the compositional features of the organic gas fraction in the more accessible outer flank thermal discharges could usefully be utilized, at least, to assess the thermodynamic conditions of the volcanic-hydrothermal system, since these compounds are affected by secondary processes only at limited extent.