Thermobarometric and Microthermometric insights into the Monte Amiata magmatic reservoir

Monte Amiata is a Pleistocene hybrid volcano with lava flows and domes outpoured between 300 and 232 ka [Laurenzi et al., 2015] by a SSW-NNE fissure direction [Ferrari et al., 1996; Brogi, 2008; Marroni et al., 2015]. Volcanic activity took place in two-short lived eruptive episodes [Ferrari et al., 1996; Conticelli et al., 2015; Marroni et al., 2015], apparently separated by a level characterised by a strong weathering alteration [e.g., Certini et al., 2006; Wilson et al., 2008; Principe & Vezzoli, 2021]. The Basal Trachydacitic Complex (BTC) represent the early period and is distributed in the lowermost portion of the volcano and they are characterised by lava flows of trachydacitic composition, with abundant angular crustal xenoliths ranging from phyllites to micashist and hornfels [van Bergen, 1984; Ferrari et al., 1996; Orlando et al., 2010] and rare and small ellipsoidal microgranular igneous enclaves. The Dome and massive Lava flow Complex (DLC) represent the late period of activity concentrated, all but one (Trauzzolo dome), along the feeding apical fracture. They are characterised by lavas and minor ejecta of trachydacitic to trachytic and latitic compositions, with rare crustal xenoliths but abundant microgranular ellipsoidal to rounded microgranular igneous enclaves (decimetric to metric in size) with trachytic to potassic trachybasaltic composition [Ferrari et al., 1996; Conticelli et al., 2015; Marroni et al., 2015; La Felice et al., 2017]. Two small olivine-latitic lava flow (OLF) close the eruptive event [Ferrari et al., 1996; Conticelli et al., 2015; Marroni et al., 2015]. The region around the Amiata and Radicofani volcanoes was characterised by an uplift of about 2,000 m, possibly caused by a magmatic intrusion of uncertain age [Acocella & Mulugeta, 2001]. The igneous intrusion that triggered the doming was suggested to be either a crustal-derived anatectic magma [Gianelli et al., 1988], on one side, or mantle-derived magma [Conticelli et al., 2015], on the other one. These two hypotheses implicate different thermodynamic and thermochemical conditions related with the history of the magmatic reservoir from the initial intrusion to the eruption event. The heat source of Monte Amiata volcanic field feed, with its thermal energy, two water-dominated high-enthalpy geothermal reservoirs. A shallow one represented by a carbonate-evaporite formation of the Tuscan Nappe with temperature ranging between 150 to 230 °C, and a deeper one hosted in a highly-fractured layer of the metamorphic basement with temperature > 300 °C [Sbrana et al., 2021]. This study investigates the P–T conditions of equilibrium of Monte Amiata volcanic rocks that might help to shed some lights on the heat source of the related geothermal field. To achieve this goal we used a multimethod approach, using traditional experimental derived algorithms, a machine-learning method, and eventually the melt inclusion investigation. The aim is to reconstruct the temperature condition and the storage level of the magmatic reservoir during the eruption. For Monte Amiata lavas, classical geothermobarometer and modern machine learning tools on minerals and glass revealed a temperature in the range between 950 and 1170 °C, with a largely variable storage pressure between 0.1 and 0.8 GPa. Microthermometry experiments on sanidine melt inclusions revealed temperatures in the range between 990 and 1075 °C. Sanidine crystals ideally represents the lower temperature limits of the magmatic reservoir. The P–T condition suggest a volcanic nature referred to the magmatic reservoir, supporting the idea of a mantle-derived magmas and a storage depth from 7 to 25 Km.