Accidental gas emission from shallow pressurized aquifers at Alban Hills volcano (Rome, Italy): Geochemical evidence of magmatic degassing?

摘要

Recent studies suggested that Alban Hills (Rome) is a quiescent and not an extinct volcano, as it produced Holocene eruptions and several lahars until Roman times by water overflow from the Albano crater lake. Alban Hills are presently characterized by high P_(CO2) in groundwaters and by several cold gas emissions usually in sites where excavations removed the superficial impervious cover. Gas consists mostly of CO_2 with minor H_2S and the diffuse CO_2 soil flux is locally very high. Accidental gas blowouts, occurred during shallow well drillings (tens to hundreds m depth) in zones with no surface gas manifestations, indicate the presence of gas pressurized aquifers confined underneath impermeable layers, within both the volcanic rock pile and the underlying Pleistocene loose sediments. Degassing mostly occurs in correspondence of bordering faults of buried horsts cut in the Mesozoic carbonate basement, hosting the main aquifer. Carbon isotopic composition (δ~(13)C_(CO2)) suggests that CO_2 is at least partly originated by thermal decarbonation of these limestones. ~3He/~4He isotopic ratio of the gas (up to 1.9 Ra) is the same or even slightly higher than that of olivine and clinopyroxene fluid inclusions of the Alban Hills volcanic products, indicating a possible magmatic source for the gas. Low R/Ra values, compared to MORB and island arc magmas, are characteristic of the potassic Roman Comagmatic Province and reflect a deep involvement of crastal material in the magma genesis. The lack of high temperature fumaroles can be explained by an efficient meteoric cold water penetration and circulation in the volcano permeable terrains.