Geochemistry of Ferromanganese Crusts of the Bering Sea

Abstract

Ferromanganese crusts found in the Bering Sea on the Volcanology Massif, Alpha Fault Zone, and Shirshov Submarine Ridge that cover the surface of rocky volcanic structures are most likely the product of postvolcanic activity. The results indicate that the studied ferromanganese formations developed under the influence of two factors: slow precipitation of metals from ordinary seawater and metal-enriched hydrothermal solutions. In microstructural and mineralogical terms, the composition of Fe–Mn crusts of the Bering Sea proved rather monotonous. The ore part is represented mainly by ferruginous vernadite and, rarely, hematite in combination with amorphous silica, and to a lesser extent, montmorillonite, calcite, and aragonite. The manganese mineral todorokite, considered a reliable sign of hydrothermal origin of ore crusts, was not detected in our samples. In the studied samples, the reduced cerium anomaly (0.87) was established only in one sample, and in other samples, its value varies within 1.08–1.89, which is typical of the upper horizons in the ocean water column. At the same time, the europium anomaly is close to neutral, so in seven samples, its value is 0.96–1.03 (average 1.0) and only in three samples is it slightly elevated (1.05–1.07), which can be considered a very weak sign of hydrothermal activity. In addition, the presence of gold microinclusions in the ferromanganese phase can indirectly indicate the possible influence of hydrothermal factor on the crust composition.