Platinum Group Elements in Ferromanganese Crusts of the Northwest Pacific Ocean

A geochemical study of seven ferromanganese (Fe–Mn) crusts dredged from the surface of guyots and near-fault zones in Kamchatka water area of the Pacific Ocean was carried out: on slopes of seamounts in the Stalemate fault zone, to the west of it, and near the Kuril–Kamchatka Trench, as well as on guyots of the Emperor Seamount chain. Data were obtained on the content of platinum group elements (PGE) in crusts. In the ratio of trace elements (Co + Ni + Cu) and Fe–Mn matrix, as well as the composition of rare earth elements, the studied crusts have characteristics typical of hydrogenic formations. All samples showed a positive cerium anomaly, and a clear correlation of Y, La, and Nd with P2O5 was revealed. The content of platinum group metals in the studied samples is lower than in Fe–Mn crusts of the Pacific Ocean according to published data. The studied samples exhibited the highest enrichments in platinum (60–279 ppb), with its content correlating with the Mn/Fe ratio and Ni content. Sources of platinum group metals in Fe–Mn crusts can be ocean water, cosmic dust, and underlying basalts. No cosmic spherules and edaphogenic particles of noble metals were found in the studied crusts. Environmental factors influencing the composition of Fe–Mn crusts were considered. An explanation is proposed for the low content of platinum group metals and cobalt in the studied Fe–Mn crusts of the NW Pacific Ocean compared to the Pacific Prime Crust Zone. The obtained data on the platinum content in ferromanganese crusts of the near-Kamchatka segment of the Pacific Plate are important for assessing the influence of composition of sediments in the plate subducting under Kamchatka on the composition of present-day island-arc volcanic rocks.