Active contact metamorphism and CO2-CH4 fluid production in the Larderello geothermal field (Italy) at depths between 2.3 and 4 km.

Several zones can be distinguished in the Larderello geothermal field (southern Tuscany, Italy) based on the nature of the fluid, mineralogical assemblages and the fluid flow as a function of depth. The deepest levels of the field are characterized by mineralogical assemblages produced by currently active thermal metamorphism that is thought to have started a few million years ago, e.g., during or later than the main intrusions that are dated from 1.6 to 3.8 Ma. At depths between 2 and 4 km below ground level (b.g.l.), the rocks are characterized by the assemblage quartz-biotite-tourmaline. This zone is characterized by present-day productive layers of superheated steam. Fluid inclusions from these deep zones are dominated by C-H-O-N and occur as CO 2-CH 4-N 2-H 2O vapors, with CO 2 as the dominant gas. The CO 2-CH 4 ratio and the water content, which are not affected by immiscibility or mixing processes, are compatible with values expected from calculations of water-graphite equilibrium. A thermal metamorphic origin is suggested for the genesis of the aqueous-carbonic fluid, especially the volatiles (CO 2-CH 4). Such fluids may be considered as the dominant type, compared to other fluid types in the deeper levels, especially within the seismic high-amplitude reflector called K horizon. The water component of the aqueous-carbonic vapors has a rather low salinity (< 5 wt.% eq. NaCl) and may correspond either to water produced by dehydration reactions or to waters that have extensively equilibrated with the metamorphic host rocks. Three other fluid sources may also contribute to the variety of fluids found in the biotite zone: (i) deep Li-rich brines considered as having a magmatic origin, (ii) brines resulting from water-halite interactions within the evaporites, (iii) meteoric derived fluids, which penetrated downwards. The carbonic vapors were generated and trapped under lithostatic conditions of 100 to 120 MPa and high temperatures between 600 and 420 °C, depending on the depth and the distance to the granitic intrusives. These fluids may be considered as representative of the dominant fluid at depth within the pressurized zones that have been inferred by geophysical investigations (K horizon). After cooling, and in many cases decompression, aqueous fluid inclusions were trapped at pressures fluctuating between lithostatic and hydrostatic. The estimated pressure corresponds to depths between 2 and 4 km, which is in agreement with the transition from lithostatic to hydrostatic conditions. The end of the proposed fluid path corresponds to an evolution towards the present-day temperature and hydrostatic pressure conditions.