Noble Gases and Carbon Isotopic Composition of Fluid Inclusions from the Larderello Geothermal Field (Italy), Preliminary Data

Abstract

The intracontinental geothermal field of Larderello (Tuscany, Italy) is a vapour-dominated system, which has been exploited for electricity production throughout this Century. At Lardereil0, studies on hydrothermal mineral assemblages and related fluid inclusions evidenced the presence of several fluid types of different sources and a complex evolution of the geothermal system, from early stage circulation, related to granite emplacement, to present-day condition (Valori et al., 1992; Cathelineau et al., 1994). In this study we measured N2/Ar ratio and the isotopic compositions of noble gases and carbon of CO2 extracted by undervacuum crushing of fluid inclusions in order to: 1) define the origin of the palaeogeothermal fluids, 2) compare noble gas and CO2 isotopic compositions of the fluid trapped in the inclusions with present-day discharged fluid and evaluate if the source of these fluids has been changed with time. The stratigraphy of the deep geothermal wells (up to 4.5 km below the ground level, b.g.l.) schematically consists of (from top to bottom): 1) a cover of siliciclastic sediments, carbonatic rocks, evaporite and ophiolite rocks (Late Triassic to Pliocene), 2) a complex of tectonic slices which include the lower formation of the sedimentary rocks and the uppermost part of the metamorphic basement, 3) a metamorphic basement (Palaeozoic), consisting of phyllitic-quartzitic rocks, micaschists and gneisses. The deepest wells at Larderello have also encountered contact metamorphic rocks and granite dykes and intrusion with ages between 1.3 and 3.8 Ma. Geophysical data suggest that a large intrusion is present below the Larderello area; the geothermal activity can be related to the presence of this intrusion. The productive horizons were found in Triassic carbonate and anhydrite formations (shallow reservoir) of the upper units and in some permeable levels of the Palaeozoic metamorphic basement (deep reservoir).