HYDROTHERMAL MANGANESE CRUSTS FROM ENARETE AND PALINURO SEAMOUNTS IN THE TYRRHENIAN SEA

Manganese crusts were recovered from Enarete and Palinuro Seamounts in the Tyrrhenian Sea. The crusts consist of porous, black, layered Mn o xides up to 45 mm thick. lit some cases, the surface has a black metal lic sheen similar to that observed in crusts from the Tonga-Kermadec A re. The crusts overlie substrates such as calcareous sediment, siltsto ne, and oyster shells. They consist dominantly of IO-A manganate and 7 -Angstrom manganate, with minor quartz, calcite, aragonite, illite, mo ntmorillonite, plagioclase, and goethite. The sample having the highes t Mn content contained 52.8% Mn, 0.15% Fe, 46 ppm Ni, 84 ppm Cu, 26 pp m Zn, 6 ppm Pb, and 2,130 ppm Ba, with a Mn/Fe ratio of 347. It also h ad a low rare-earth element (REE) abundance (La 2.7 ppm) and a negativ e Ce anomaly (Ce/La ratio 0.5). A TV profile on Palinuro showed the oc currence of yellow (Fe oxyhydroxide or nontronite), black (Mn oxide), and white (sulfate) halos in the sediment and opaqueness in the overly ing water. Nontronite and massive sulfides were also recovered at some locations on Palinuro. The morphology, mineralogy, and composition of these crusts as well as ancillary geological evidence confirm that th e crusts are of hydrothermal origin. Similar crusts occur in a number of island-arc environments, particularly in the Pacific, and are known to be formed by low-temperature hydrothermal venting. The origin of t he Tyrrhenian Sea is complex but it is believed to be a back-arc basin formed above a NW-dipping subduction zone. The Aeolian islands and se amounts consist of 16 volcanic edifices which form an anticlockwise ar e around the Marsili abyssal plain. Palinuro is the youngest of these. Our data support the idea that the Mn deposits formed as a result of submarine hydrothermal activity associated with subduction-related pro cesses.