During RN Moana Wave and RIV Hakurei-maru cruises since 1982, iron-manganese deposits have been described for more than 150 dredge hauls taken within an area of 200 x 200 km in the back-arc region of the northwestern Pacific. Hydrogenetic crusts with variable thicknesses occur on inactive seamounts up to 200 km away from the active volcano chain. The thickness of crusts generally increases with increasing distance from the active volcano chain and with increasing age of substrate volcanic rocks, assuming a constant and continuous slow growth. The large variation of thickness in a single site is explained by small-scale collapse of the volcanic edifice as the result of mass wasting, which commonly results in unexpectedly thin development of crusts. Element concentrations of hydrogenetic crusts vary within small ranges with geographic location in the area and through crusts with depth from surface. Hydrothermal deposits are clearly different from those crusts in chemistry and mineralogy.
A wide variety of iron and manganese oxide deposits have been described from the Izu-Bonin-Mariana (IBM) Arc, a submarine island-arc system in the northwest Pacific (Usui et al., 1994; Usui et al., 1997; Usui and Someya, 1997). This paper briefly describes the mode of occurrence of iron-manganese deposits on inactive seamounts in the back-arc region of the Izu-Bonin volcanic chain, and discusses controls on the abundance and composition of the deposits. During Moana Wave Cruise MW9507 in the back-arc region in 1995, a number of iron-manganese crusts and nodules were collected from 74 dredge hauls out of 116 in an area of about 200x200km covering four seamount chains. Other sample descriptions were made during previous R/V Hakurei-maru cruises since 1982 by the Geological Survey of Japan (GSJ) and other organizations. On-site description by submersible observations and sampling with Shinkai 2000 are also included.