Temperature and Compositional Changes in the Hydrothermal Fluids in the Mt. Amiata Geothermal Area: Evidence from Fluid Inclusion Data

Abstract

Two geothermal fields, Bagnore and Piancastagnaio, are in the southern sector of the Quaternary Amiata volcano (southern Tuscany, Italy). Fluid inclusions trapped in quartz, adularia and calcite found in core-sample in deep drilled wells (between 1690 and 3220 m below the ground level) of the geothermal fields provided information on the physical-chemical characteristics of circulating fluids in past. Three types of fluid inclusions related to the geothermal activity were recognized: type I (liquid-rich) inclusions trapped an aqueous fluid with CO2 concentration (1.27-2.68 mol/kg) that is significantly higher than that of present-day fluids (0.11-0.05 mol/kg). Type II (liquid-rich) inclusions formed after type I, and trapped a fluid with lesser CO2 (<0.85 mol/kg). Type III (vapor-rich) inclusions record an early fluid circulation under boiling conditions and in some samples coexist with type I inclusions. The decrease of CO2 concentration from type I inclusions to type II inclusions, and then to present-day concentrations can be related to boiling with gas loss and/or mixing. In general type I and II inclusions show variable salinities (0.11-3.90 mol/kg of Na+ and Cl-) and Th (150-335?C), which can also be associated with boiling and mixing processes. In particular, the data suggest that mixing between a hot low-salinity fluid and a cooler saline fluid may has occurred in samples where present-day temperatures are in the range 195-280?C. On the other hand, the present-day hottest areas (295-320?C) appear to be nearly in thermal stability from the time of fluid inclusion trapping and mixing processes are not evident.