Thermal fluid discharges from submarine springs at the Formiche di Grosseto islets (Tyrrhenian sea, Tuscany, Italy) and their relation with regional anti-apennine tectonic lineaments

The Formiche di Grosseto (southern Tuscany, Italy) islets are a reef system of about 1,500 m2, 9 nautical miles offshore the outfall of the Ombrone river and are constituted by Liassic carbonate rocks of the Tuscan Series (Calcare Massiccio) and are part of the Giannutri-Formiche di Grosseto (GFR) ridge separating Neogene basins of the Tuscan Shelf. In 2005, during a monitoring survey several thermal springs discharging from the sea bottom close to Formiche islets were noticed. Thermal fluids discharge from several points mainly located along the N-NE scarp bordering the main reef at depths varying from 6 to 32 m. and an outlet temperature of about 41 °C was measured. Accordingly, continental Tuscany is characterized by a large amount of Ca-SO4 thermal water discharges, such as Bagni Osa and Roselle located inland in the proximity of the Formiche di Grosseto area, whose chemistry is to be related to mixing of fluids from different sources, i.e. hydrothermal and seawater (Bagni Osa) and/or hydrothermal and meteoric (Roselle). For these considerations, the occurrence of CO2-rich submarine thermal discharges at Formiche di Grosseto may conveniently be related to the regional anti- Apennine tectonic lineaments.