Geochemistry and U-series dating of Holocene and fossil marine hydrothermal manganese deposits from the Izu-Ogasawara arc

Abstract

This study investigated Holocene and fossil hydrothermal manganese deposits in the Izu-Ogasawara arc. Mineralogically, these deposits comprise 10Å and 7Å manganate minerals, and the fossil samples showed higher 10Å stabilities. Chemical compositions of the Holocene samples are typical of other hydrothermal manganese deposits, including low Fe/Mn ratios, low trace metals, and low rare earth elements. Although the fossil samples generally have similar chemical characteristics, they exhibit significant enrichment in Ni, Cu, Zn, Cd, Ba, REE, Tl, and Pb contents. Furthermore, the chondrite-normalized REE patterns showed more light REE enrichment trends. These chemical characteristics suggest post-depositional uptake of these metals from seawater. U-Th dating of a Holocene hydrothermal manganese deposit from the Kaikata Seamount indicated 8.8±0.94ka for the uppermost layer and downward growth beneath the seafloor with a growth rate of ca. 2mm/kyr. This is approximately three orders of magnitude faster than that of hydrogenetic ferromanganese crusts. U-Pb age of a fossil hydrothermal manganese deposit from the Nishi-Jokyo Seamount showed 4.4±1.6Ma, which was contemporary with basaltic volcanism (5.8±0.3Ma). Hydrothermal manganese deposits contain high concentrations of high value Mn, but only small amounts of valuable minor metals; their ages constrain the periods of past hydrothermal activity and provide a vector to explore for polymetallic sulfide deposits.