Geochemical features of the gas phase extracted from sea-water and rocks of the Marsili seamount (Tyrrhenian sea, Italy): implications for geothermal exploration projects

Abstract

A new target for worldwide geothermal exploration and exploitation is represented by the submarine hydrothermal activity. A geologically young (Upper Pliocene – Pleistocene) area in the Tyrrhenian sea, on the west shore of Italy, provides important heat sources from some seamounts. The heat flow data are characterized by positive anomalies comparable to those of onshore geothermal fields. A geothermal exploration program of the Marsili seamount, the largest volcanic edifice (~30 km in length) in Europe, suggests the presence of a geothermal activity testified by oxy- and hydroxide-deposits predominantly made up of Fe- and Mn-rich sediments, crusts and nodules. Several rock samples of Marsili volcanic activity were collected during dredging and coring projects. These rocks are lavas, pillows and dikes with a calc–alkaline affinity and ranging in composition from basalt to andesite to trachy-andesite. Some tephra layers were also sampled at the top of the volcano showing shoshonitic to latitic bulk compositions and variable amounts of crystals. Importantly, isotopic ratios of noble gases extracted from selected solid samples have provided novel constrains on the Marsili magmatism. In November 2007 and July 2011, water-column studies were carried out aboard of the R/V Urania and Astrea. Isotope analyses of the gases dissolved in water samples collected at the top the Marsili have evidenced that the He/4He isotope ratio, i.e. a clear indicator of hydrothermal input, is in excess with respect to the background and it is also associated with the anomalous behaviour of hydrothermal-derived gases (CO2, CO, CH4). Although some hydrothermal emissions are known to occur offshore the Aeolian subaerial volcanoes, results from our isotope analyses are the first to confirm the hydrothermal activity of Marsili. The highest 3He values were measured over the shallowest part of the seamount, where hydroxide deposits were found. The chemical composition of the dissolved gases clearly shows the presence of CO2 and CH4 over a wide water column depth range. The same anomalies were found in 2007 and 2011, depicting the presence of a persistent plume related to the deep hydrothermal activity of Marsili. A high heat flow (250 mW/m2) is measured at the uppermost portion of the volcano, reaching the maximum value (500 mW/m2) in the central parts. It coincides with gravity and magnetic anomalies, suggesting the presence of magmatic bodies intruding within shallow and thinned crustal levels. Although direct thermal measurements on the Marsili seamount are still lacking, our results give important constraints on the submarine volcanic and hydrothermal activity. Moreover, they support an ongoing project (carried out by Eurobuilding SpA company on the Italian Minister of Economic Development permission) aimed to drill the first offshore geothermal well at the top of the seamount that may potentially represent one of the most abundant energy resources worldwide; this mainly in response to the large amount and virtually infinite recharge of the circulating fluids. To conclude, it is worth noting that besides the scientific information concerning the geothermal activity, the technologies available at the present time will permit on-site geothermal exploitation and 68 production of electric energy (~1 GW) from a seamount located at ~100 km far from the Italian coasts.