CO2 output and delta C-13(CO2) from Mount Etna as indicators of degassing of shallow asthenosphere

An estimated average CO2 output from Etna's summit craters in the range of 13 +/- 3 Mt/a has recently been determined from the measured SO2 output and measured CO2/SO2 molar ratios. To this amount the CO2 output emitted diffusely from the soil (approximate to 1 Mt/a) and the amount of CO2 dissolved in Etna's aquifers (approximate to 0.25 Mt/a) must be added. Data on the solubility of CO2 in Etnean magmas at high temperature and pressure allow the volume of magma involved in the release of such an amount of this gas to be estimated. This volume of magma (approximate to 0.7 km(3)/a) is approximately 20 times greater than the volume of magma erupted annually during the period 1971-1995. On the basis of C-isotopic data of CO2 collected in the Etna area and of new hypotheses on the source of Mediterranean magmas, significant contributions of CO2 from non-magmatic sources to the total output from Etna are unlikely. Such large outputs of CO2 and also of SO2 from Etna could be due to an anomalously shallow asthenosphere beneath the volcano that allows a continuous escape of gases toward the surface, even without migration of magma.