L.I. Anikeeva1, V.E. Kazakova1, G.M. Gavrilenko2, V.A. Rashidov2

1VNII Oceangeologia, 190121 St. Petersburg, Russia
2Institute of Volcanology and Seismology, Far East Division. Russian Academy of Sciences, Petropavlovsk- Kamchatskii, 683006, Russia

Chemical composition of hydrothermal hydrogenous ferromanganese crusts occurring in different sections of the West Pacific transition zone is variable because hydrothermal solutions contribute differently to crust formation. The hydrothermal origin of the Fe-Mn crusts from the back-arc basins of the WPTZ is evidenced by the contrasting variations of Mn and Fe content, low concentrations of non-ferrous and earth elements (domination of the middle/heavy rare earths elements, positive Eu anomaly, increased Sm/Nd rate), high content of Li and Ba. The formation of aureoles of oxide Fe-Mn crusts is associated with modern hydrothermal activity accompanying sulfide ore outcrops at the sea bottom. All geochemical varieties of the crusts of the ocean’s rift systems have been discovered: essentially Mn- and Fe-Mn crusts and ferriferous and nontronite ones containing silicon. As for the hydrothermal crusts, their distance from the hydrotherm discharge areas is controlled by Mn/Fe. High contents of Fe are indicative for high-temperature hydrothermal process and associated shows of deep-sea polymetallic sulfides. The studied Fe-Mn crusts of unstable chemical and mineral composition are of hydrothermal-sedimentary nature at different degrees of involvement of the hydrogenous factor, which is most vividly demonstrated by the samples from the Kuril branch. These formations are the product of hydrothermal activity at the sea bottom and may be regarded as indicators for under bottom sulfide formation in the Kuril branch, similar to the major ore shows in the southern segments of WPTZ, i.e., the Sunrise (the Izu-Bonin arc) and the Jade (the Okinawa trough).