Fluid inclusion evidence on the direct exsolution of magmatic brines from a granite intrusion beneath the eastern sector of Larderello geothermal field (Italy)

Abstract

This work is based on a fluid inclusion investigation on two core samples (sampled at 3017m and 4150m of depth) of a granite intrusion drilled by the Travale 1 sud geothermal well, in the eastern sector of Larderello geothermal field. Fluid inclusion data demonstrate that the earliest fluids circulating in the intrusion were high-temperature brines exsolved directly from the crystallizing magma during its ascent in the shallow crust. The progressive release of magmatic fluids from the crystallizing granitic magma probably occurred under confining lithostatic pressure of about 200 MPa. Two phase fluid inclusions represent the hydrothermal meteoric water-dominated fluids that penetrated in the granite after opening of the system, induced by cooling and consequent transition from ductile behaviour to brittle fracturing. Type 1b two phase fluid inclusions, having temperature of homogenization that agree well with down-hole measured temperatures, may be considered to approximate the fluid that is actually circulating in the granite in recent times.