Exceptional discovery at Panarea island. The unique recent iron ooids deposit known in the world: multidisciplinary study to constrain their hydrothermal/volcanic origin.

Exploration strategy aimed at discovering clues of submarine hydrothermal systems is strongly relied on the occurrence of surface evidences of subsurface geothermal energy. In the subduction-related volcanic arc setting of the Aeolian Islands, a primary role is played by the submarine hydrothermal activity that generated in different steps iron oxydroxydes and sulfur deposits over the whole archipelago. In the last years, the Fe-rich depositions representa research topic of interest in providing genetic implications on similar terrestrial as well as Martian deposits. In this scenario, during a cruise carried out by ISPRA and INGV on board of the research vessel Astrea, a sand deposit was sampled off the Panarea Island. The sample turned out to be an exceptional discovery, because it represents the unique, recently formed, not diagenetic and not reworked iron ooids deposit, known to date in the world. Iron ooids are an important component of the sedimentary record since Precambrian times; their formation, however, is not yet fully understood. Results of the preliminary studies of the Panarea ooids - able to constrain their hydrothermal origin and to provide a first genetic model- are here reported. The deposit occurs over an area marked by diffuse submarine hydrothermal discharge consisting of diffuse venting of thermal waters andgas bubbles streams. An integrated textural, mineralogical and geochemical characterization of iron ooids was performed. The results showa good matching with those from ancient sedimentary counterparts. Panarea ooids are made by concentric goethite laminae deposited around nuclei of particles present on the seafloor. The process was abiotic, rapid and ruled by CO2-rich hydrothermal fluids. Compared with the other components of the hydrothermal system, the iron coating shows some HREE enrichment. In this regard, a genetic model in which CO2-rich hydrothermal fluids interact with the hosting rocks in a rhyolitic reservoir and mobilizing incompatible trace elements, including REE and others, is here proposed. As a matter of fact, the hydrothermal fluids become carriers of iron, silica, REE and LILE, as well as of low amount of Nb and Zr. As soon they rise up through the sea-floor sediments and interact with the colder sea water, in an oxidizing sediment/seawater interface, the oxyhydroxides flocculate. The overall results constrain the hydrothermal/volcanic origin of the coated grains, define them as the precursor grains of older iron oolite sand may also contribute to interpret analogue particles recently discovered on Mars. This contribution combines monitoring data and a multidisciplinary approach for understanding environmental events that frequently occurred during the Earth evolution.