Helium and carbon isotopes in the dissolved gases of Friuli Region (NE Italy): Geochemical evidence of CO2 production and degassing over a seismically active area

The first geochemical data showing the existence of an active degassing activity over a large seismically active sector of the Southern Alps ( Friuli Region, NE Italy) are presented. The dissolved gases, helium and carbon isotopic systematics of 46 water samples taken from 13 sites running along E-W and NE-SW faults besides the natural degassing of a 5000 km(2) wide area are investigated. The chemical composition of the dissolved gases revealed that a CO2-rich gas phase feeds the local groundwaters. He-3/He-4 ratios ( R) normalized to the atmospheric He-3/He-4 ratio (Ra=1.39 x 10(-6)) and corrected for the atmospheric contamination (R/Rac), range from 0.29 to 1 as a result of a two component (radiogenic and atmospheric) mixing. The delta C-13 values of total dissolved inorganic carbon (TDIC) ranging from -15.28 to -0.75% vs. PDB, show the occurrence of multiple gas-water interactions. The mixing between the atmospheric air and a crustal source and the gas-water interactions occurring at various extents appears to be the main control on the observed He-C systematics. The natural CO2 degassing was evaluated by a soil gas survey carried out by a grid of about 100 measuring sites located over the area that generated destructive seismic sequences (e.g. the Gemona sequence of 1976; main shocks M6.4 of 6th May and M6.1 of 11th and 15th September). The results obtained show that a significant amount of crustal-originated gases are released over the continental area of Eastern Southern Alps. The evidence that carbon dioxide is associated with radiogenic-type helium denotes the lack of the mantle as primary energy and degassing source, highlighting the possibility that CO2 is produced by thermo-mechanical processes occurring at seismogenic depth. The information provided here may be used to start up a long-term geochemical monitoring of this seismically active area and could be able to detect the modi. cations occurring to the circulating fluids to gain a better insight on the relationships between the fluids' geochemistry and the activity of the local seismogenic faults. (C) 2009 Elsevier B.V. All rights reserved.