Degassing of SO2 and CO2 at Mount Etna (Sicily) as an indicator of pre-eruptive ascent and shallow emplacement of magma

We studied soil CO2 emissions together with crater SO2 fluxes from Mt Etna during the period July 1997 to March 1999. This period was characterized by high levels of volcanic activity, and ended with the onset of a 10 month-long sub-terminal eruption on February 4, 1999. Soil CO2 degassing was measured at two sites (P39 on the lower SW flank of Etna and P78 on the E flank) known for being connected to deep faults that allow the escape of magmatic gas. Site P39 is inferred to drain gas from a deep (>15 km) magma reservoir, whereas site P78 is connected to a shallower (5-10 km) reservoir. Based on the temporal variations of measured soil CO2 and crater SO2 data, five intervals of anomalous degassing were recognized. Each of them is interpreted as due to massive gas release from a magma batch that is moving toward the surface. The time lag between the occurrence of degassing anomalies of CO2 and those of SO2 agrees well with the different depth of exsolution of the two gases. Our data also indicate a step-like migration of magma to the surface, which bears on the existence of at least three different temporary storage levels within Etna's feeder system. Lastly, the observed anomalies preceded by months to weeks increases in the summit activity of Mt Etna, including the February 4, 1999 eruption. Comparison between the amounts of degassing magma during each interval of anomalous degassing, based on SO2 fluxes, and erupted volumes of lava during the same periods, seems to indicate that the 1999 eruption was largely fed by magma that started entering the upper feeder system of Etna about 1 year earlier. (C) 2001 Elsevier Science B.V. All rights reserved.