Fumarolic gases at Mombacho volcano (Nicaragua): presence of magmatic gas species and implications for volcanic surveillance

Mombacho is a deeply dissected volcano belonging to the Quaternary volcanic chain of Nicaragua. The southern, historic collapse crater (El Crater) currently hosts a fumarolic field with a maximum temperature of 121°C. Chemical and isotopic data from five gas-sampling field campaigns carried out in 2002, 2003 and 2005 highlight the presence of high-temperature gas components (e.g. SO₂, HCl and HF), which indicate a significant contribution of juvenile magmatic fluids to the hydrothermal system feeding the gas discharges. This is strongly supported by the mantle-derived helium and carbon isotopic signatures, although the latter is partly masked by either a sedimentary subduction-related or a shallow carbonate component. The observed chemical and isotopic composition of the Mombacho fluids seems to indicate that this volcanic system, although it has not experienced eruptive events during the last centuries, can be considered active and possibly dangerous, in agreement with the geophysical data recorded in the region. Systematic geochemical monitoring of the fumarolic gas discharges, coupled with a seismic and ground deformation network, is highly recommended in order to monitor a possible new eruptive phase.