Chemical Composition of Sediments and Its Changes at the Modern Hydrothermal System Center in the Middle Valley, Juan de Fuca Ridge, Hole 858B ODP

The article presents the results of studying changes in the chemical composition of Pleistocene sediments from Hole 858B (depth 38.6 m, temperature gradient 10‒11°C/m) drilled in the Middle Valley, Juan de Fuca mid-ocean ridge (Northeast Pacific) in the Dead Dog hydrothermal field 20 m away from a “black smoker” (temperature 276°C). The content of macroelements in these sediments was studied by the XRD method. For the first time, data on a large set of trace elements were obtained from this object using the ICP-MS method. The chemical composition of sediments changed in the process of solution–sediment interaction during a rapid downsection increase of temperature. In the upper part of Sequence I (1.97‒10.41 m), the sediments are commonly altered slightly at a temperature of about 17°C. Changes in the content of macro- and trace elements are prominent in the lower part of Sequence I (12.70‒25.31 m, average temperature 112‒197°C). The chemical composition of sediments is most altered in Sequences IIB and IID (25.31‒38.6 m, temperature from 112‒197 to 320‒330°С). Changes in the content of the majority of macro- and trace elements in the sediments of Sequences IIB and IID are similar, except for a stronger decrease of chemical elements in the sediments of Sequence IID, which is marked by a significantly lower content of Cu, Zn, Ga, Rb, Sr, Ag, Cd, Sb, Cs, Ba, Tl, and Bi (i.e., the majority of detected elements except U) than in background sediments. Sediments in Sequence IID, as in Sequence IIB, contain less Ca, Na, K, and P, but more Mg. A lower content of the above-listed elements in these sediments can be attributed to their evacuation during the solution–sediment interaction and concentration in the solution, whereas a higher content of these elements is likely related to their input into sediments from the solution and, accordingly, their depletion in the solution. The results of studying the chemical composition of metalliferous sediments (Sequence III, 0‒1.97 m) and sulfide layer IV (10.41‒12.70 m), as well as the chemical composition of unaltered background sediments from Holes 855A, 855C, and 855D are presented.