Hydrothermal manganese crusts from Enarete and Palinuro seamounts in the Tyrrhenian Sea

Abstract

Manganese crusts were recovered from Enarete and Palinuro Seamounts in the Tyrrhenian Sea. The crusts consist of porous, black, layered Mn oxides up to 45 mm thick. In some cases, the surface has a black metallic sheen similar to that observed in crusts from the Tonga‐Kermadec Arc. The crusts overlie substrates such as calcareous sediment, siltstone, and oyster shells. They consist dominantly of 10‐Å manganate and 7‐Å manganate, with minor quartz, calcite, aragonite, illite, mont‐morillonite, plagioclase, and goethite. The sample having the highest Mn content contained 52.8% Mn, 0.15% Fe, 46 ppm Ni, 84 ppm Cu, 26 ppm Zn, 6 ppm Pb, and 2,130 ppm Ba, with a Mn/Fe ratio of 347. It also had a low rare‐earth element (REE) abundance (La 2.7 ppm) and a negative Ce anomaly (Ce/La ratio 0.5). A TV profile on Palinuro showed the occurrence of yellow (Fe oxyhydroxide or nontronite), black (Mn oxide), and white (sulfate) halos in the sediment and opaqueness in the overlying 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 the 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 the 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 seamounts consist of 16 volcanic edifices which form an anticlockwise arc 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 processes.

Key words:

• manganese crusts
• submarine hydrothermal activity
• Tyrrhenian Sea