Change of the Composition of Solution During Its Migration Through the Sedimentary Cover in the Modern Hydrothermal System Center (Juan de Fuca Ridge, Pacific Ocean, ODP Hole 858B)

— The article presents the results of studying alteration of the hydrothermal solution composition during its percolation in the Pleistocene sediments drilled by ODP Hole 858B (38.6 m deep) in the northern segment of Middle Valley, Juan de Fuca Ridge (Dead Dog hydrothermal field), 20 m away from the hydrothermal source with a temperature 276°C. Influence of sediments in Units I, IIB, and IID from this hole on the solution composition variation during the solution–sediment interaction has been established. The greatest influence on the solution composition occurred during its interaction with sediments of Units IIB and IID under high-temperature conditions (200–350°C) expressed in enrichment of the solution with a large amount of chemical elements. A possible alteration in the solution composition is shown in the process of solution–sediment interaction during its percolation in the sedimentary cover 250–300 m thick.