Mercury in the Sedimentary Cover and Basalts of the Basement in the Recent Hydrothermal Activity Area, Middle Valley, Juan de Fuca Ridge

The content and distribution of mercury in Holocene–Upper Pleistocene turbidites, hemipelagic sediments intercalating therein, as well as basement basalts are studied. Samples of sediments were taken from the core of Holes 858A, 858B, 858C, 858D, and 858F. Basalt samples were taken from Holes 858F and 858G drilled during Leg 139 ODP in the Middle Valley (Juan de Fuca Ridge) in the Dead Dog hydrothermal field with a high heat flow (4–20 W/m2) and numerous vents with temperature ranging from 234 to 276°C. Samples of sediments and basalts with the background Hg content were taken from the core of Holes 855A, 855C, and 855D are located beyond the hydrothermal system in the base of the fault scarp on the eastern Middle Valley. In rocks, the content of Hg and its occurrence form were determined by the atomic absorption spectrometry with thermal atomization method; the chemical composition, by the XFA and ICP-MS methods. Sections of the sedimentary cover and basalt basement are marked by an alternation of “layer cake” type units with low and high contents of Hg. Mercury occurs in rocks in the physically adsorbed and mineral forms. The Hg concentration in some parts of the sedimentary section is anomalously high: up to 9696 ppb in Hole 858B and 7260 ppb in Hole 858C. In metalliferous sediments, the Hg content is 3130 ppb. Its maximum content (up to 23200 ppb) is recorded in basalts.