Chemical composition of ferromanganese manifestations from the Sea of Okhotsk

New data on microstructures and mineral and chemical compositions of ferromanganese crusts sampled from the western slope of the Kuril Island Arc in the Sea of Okhotsk during cruises of R/V Vulkanolog are discussed. The study of the crusts using analytical electron microscopy methods revealed that their manganese phase is represented by vernadite, Fe-vernadite, todorokite, asbolane, and asbolane-buserite, while iron phase consists of hematite, hydrohematite, ferroxyhite, and magnetite. Lithic mineral assemblage includes apatite, quartz, epidote, and montmorillonite. According to chemical analysis most of the crusts contain significant part of volcanogenic and hydrothermal material. It is evident from elevated values of Mn/Fe and (Mn+Fe)/Ti ratios, low concentrations of some trace elements, and positive Eu anomaly.

Rocks of the Western Kamchatka-Koryak continental margin volcanogenic belt: age, chemical and mineral compositions

An isotope-geochemical study of Eocene-Oligocene magmatic rocks from the Western Kamchatka-Koryak volcanogenic belt revealed lateral heterogeneity of mantle magma sources in its segments: Western Kamchatka, Central Koryak, and Northern Koryak ones. In the Western Kamchatka segment magmatic melts were generated from isotopically heterogeneous (depleted and/or insignificantly enriched) mantle sources significantly contaminated by quartz-feldspathic sialic sediments; higher 87Sr/86Sr (0.70429-0.70564) and lower 143Nd/144Nd [eNd(T) = 0.06-2.9] ratios in volcanic rocks from the Central Koryak segment presumably reflect contribution of an enriched mantle source; high positive eNd(T) and low 87Sr/86Sr ratios in magmatic rocks from the Northern Koryak segment area indicate their derivation from an isotopically depleted mantle source without significant contamination by sialic or mantle material enriched in radiogenic Sr and Nd. Significantly different contamination histories of Eocene-Oligocene mantle magmas in Kamchatka and Koryakia are related to their different thermal regimes: higher heat flow beneath Kamchatka led to crustal melting and contamination of mantle suprasubduction magmas by crustal melts. Cessation of suprasubduction volcanism in the Western Kamchatka segment of the continental margin belt was possibly related to accretion of the Achaivayam-Valagin terrane 40 Ma ago, whereas suprasubduction activity in the Koryak segment stopped due to closure of the Ukelayat basin in Oligocene.