Mantle-derived fluids discharged at the Bradanic foredeep/Apulian foreland boundary: the Maschito geothermal gas emissions (southern Italy)

Abstract

The mephitis of Maschito, known since historical times as Lago Fetente (Smelly Lake) -although the lake is now dry-, is located twenty kilometers from the Mt. Vulture volcanic edifice (Southern Italy). It is placed along the same regional tectonic discontinuity where some maars are located, close to the boundary between the foredeep and the Apulian foreland. About 300 m2 of surface is lacking in flora, while dead animals are frequently found all around it. The smelly exhalations are mainly composed of CO2 (∼ 98 %), and, in lesser amounts, of H2S, N2, CH4 and other hydrocarbons. He, Ne and Ar occur in trace amounts. The CO2 isotopic composition is in the range of that of the main active Italian volcanic gases. The helium isotopic ratio (4.7 Ra) fits with the values measured in Mt. Vulture volcano and particularly with the olivine and pyroxene fluid inclusions of mantle xenoliths ejected during its last volcanic activity (140,000 years). The 40Ar/36Ar isotopic ratio of ∼320 supports some minor non-atmospheric contributions. The C/3He ratio (2.9x109) is in the typical range of magma released fluids, while δ13C(CH4) and δD(CH4) values fall in the field of thermogenic methane. T The amount of CO2 released is about 3200 tons/year. The flux of mantle-derived helium (> 7 x1010 atoms m-2s-1) is at least three orders of magnitude higher than that of a stable continental crust. This study strongly supports the possibility that Maschito manifestations are fed by a geothermal system, which is powered by a degassing melt, bearing in mind that the Maschito gas emissions fall along the same fault system of the Monticchio maars, which formed during Mt. Vulture volcano’s last activity.