Shallow-water geothermal activity offshore Panarea, Aeolian island arc, Italy

Abstract

Panarea island represents a partially emerged arc volcano forming part of the Aeolian volcanic chain in the southern Tyrrhenian Sea. The island rises from the western sector of a submarine platform representing the eroded summit of a large submarine stratovolcano. Seafloor observations on the eastern portion of the submarine platform have been conducted using a remotely operated vehicle. The seafloor survey resulted in the discovery of a new field of hydrothermal venting between the Secca dei Panarelli and Basiluzzo islet that is typified by quiescent gas venting and the discharge of thermal waters at water depths ranging from 60 to 100 m. The geothermal activity is associated with a wide range of seafloor morphological features, many of which are transient in nature and will not likely be preserved in the geological record. This includes widespread anhydrite-gypsum deposits that have been drilling to a maximum penetration of 5 m below seafloor using a lander-type drilling device. The massive to brecciated hydrothermal deposits contain > 80 % anhydrite and gypsum in addition to alteration minerals formed during subseafloor replacement and infiltration of the volcaniclastic host. Advanced-argillic alteration of the volcaniclastic material under acidic conditions resulted in the formation of kaolinite, with variable amounts of alunite, boehmite, and barite being present, whereas argillic alteration at near-neutral conditions caused smectite and rhodochrosite formation. Minor amounts of sulfide minerals, including pyrite, marcasite, sphalerite, and galena. The deposits show an enrichment of Ag, As, Cd, Hg, Pb, and Zn, attesting to the metal-carrying capacity of the thermal liquids. The isotopic composition of the sulfate minerals suggests that the anhydrite-gypsum deposits were formed by a mixture between thermal waters and seawater at temperatures of 132‒170⁰C. A model for fluid evolution of the Panarea hydrothermal field suggests mixing of an upwelling thermal liquid with shallow circulating seawater. Heating of seawater during mixing triggered the precipitation of anhydrite, which subsequently partly converted to gypsum. The thermal liquids likely contained volatiles derived from a magmatic source. The formation of massive sulfate deposits in shallow water on a partially emergent arc volcano make Panarea a distinct new type of seafloor hydrothermal system.