Mantle CO2 degassing at Mt. Vulture volcano (Italy): Relationship between CO2 outgassing of volcanoes and the time of their last eruption

Abstract

Mantle volatiles are mainly lost from the Earth to the atmosphere through subaerial and submarine volcanism. Recent studies have shown that degassing of mantle volatiles also occurs from inactive volcanic areas and in tectonically active areas. A new challenge in Earth science is to quantify the mantle-derived flux of volatiles (e.g., CO2) which is important for understanding such diverse issues as the evolution of the atmosphere, the relationships between magma degassing and volcanic activity, gas pressure and seismogenic processes, and the hazards posed by volcanic lakes. Here we present a detailed study of mantle-derived CO2 budget from Mt. Vulture volcano in the Apennines, Italy, whose latest eruption occurred 141 ± 11 kyr ago. The relationship between δ13CCO2 and total dissolved carbon at Mt. Vulture volcano indicates that the emitted CO2 is a mixture of a biogenic end-member with an average δ13CCO2 of about −17h and a mantle-derived CO2 end-member with δ13CCO2 values from −3h to +2h. These values of mantle- derived δ13CCO2 are in the range of those for gas emitted from active volcanoes in the Mediterranean. We calculated the contribution of individual components (CO2 in groundwater, in lakes and from main pools) to the total CO2 budget in the area. We used new measurements of water flow, combined with literature data, to calculate the CO2 flux associated with groundwater, and measured the gas flux from the main pools on the volcanic edifice. Finally, we calculated the CO2 flow in the lakes based on the gradient concentration and eddy diffusivity. The total mantle-derived CO2 budget in the area is 4.85×108 molyr−1, which is more than double previous estimates. This is higher than those observed in younger volcanic systems elsewhere, thereby supporting the existence of actively degassing mantle melts below Mt. Vulture volcano. A structural map highlights the tectonic control on CO2 flow across the Mt. Vulture volcanic edifice. Indeed, the tectonic discontinuities that controlled the magma upwelling during the most recent volcanic activity are still the main active degassing structures. The new estimate of CO2 budget in the Mt. Vulture area, together with literature data on CO2 budget from historically active and inactive Italian volcanoes, suggests a power-law functional relationship between the age of the most recent volcanic eruption and both total discharged CO2 (R2 = 0.73) and volcano size-normalized CO2 flux (R2 =0.66). This relation is also valid by using data from worldwide volcanoes highlighting that deep degassing can occur over very long time too. In turn, the highlighted relation provides also an important tool to better evaluate the state of activity of a volcano, whose last activity occurred far in time. Finally, our study highlights that in the southern Apennines, an active degassing of mantle-derived volatiles (i.e., He, CO2) occurs indiscriminately from west to east. This is in contrast to the central– northern Apennine, which is characterized by a crustal radiogenic volatile contribution, which increases eastward, coupled to a decrease in deep CO2 flux. This difference between the two regions is probably due to lithospheric tears which control the upwelling of mantle melts, their degassing and the transport of volatiles through the crust.