THE INFLUENCE OF EPISODIC SHALLOW MAGMA DEGASSING ON HEAT AND CHEMICAL TRANSPORT IN VOLCANIC HYDROTHERMAL SYSTEMS

Springs at La Soufrière of Guadeloupe have been monitored for nearly four decades since the phreatic eruption and associated seismic activity in 1976. The breakthrough curves (BTCs) for chloride and temperature exhibit distinctive differences. For a proximal spring (~75 m from the volcanic dome), the BTC for chloride is very spiky, reflecting pulsatory magma degassing, whereas the BTC for temperature drops smoothly from ~70 °C to ~20 °C. The thermal anomaly is significantly damped and lagged in other La Soufrière springs. We conceptualize degassing vapor/gas mixtures as square-wave sources of chloride and heat, and we use this as input to a new semi-analytic solution to demonstrate that chloride and heat pulses with the same timing and duration result in good matches between measured and simulated spring temperatures and concentrations. While the concentration of chloride pulses is variable, the local boiling temperature of 96 °C was assigned to all thermal pulses. Because chloride is a conservative tracer, chloride breakthrough is only affected by one-dimensional advection and dispersion. The thermal tracer is damped and lagged relative to chloride due to conductive heat exchange with the overlying and underlying strata. Joint analysis of temperature and chloride allows estimation of the onset and duration of degassing pulses, refining the chronology of recent magmatic intrusion.