Peculiarities of clay minerals formation in the Pleistocene sediments under specific tectonomagmatic and hydrothermal conditions of the Central Hill (Escanaba Trough, Gorda Ridge, Pacific Ocean). Communication 1. Hole ODP 1038B

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Using a complex of analytical methods, clay minerals were studied in Pleistocene sediments from Hole ODP 1038B, 120.50 m deep, drilled on the northwestern edge of the Central Hill, located in the Escanaba Trough (Gorda Ridge) near a hydrothermal source with a temperature of 108°C, as well as in Pleistocene background terrigenous sediments from reference Hole ODP 1037B, drilled in the Escanaba Trough, 5 km south of Central Hill. The association of terrigenous clay minerals in sediments from Hole 1037B consists of mixed-layer smectite-illites, smectite, chlorite, illite, and kaolinite. In sediments from Hole 1038B in the interval from the bottom surface to a depth of 5–7 m, clay minerals are terrigenous. In the rest of the sedimentary section, clay minerals are represented by newly formed biotite, chlorite, and dioctahedral smectite. Their formation occurred under the conditions that arose during the intrusion of basaltic melt into the Escanaba trough with the formation of a laccolith and the subsequent rapid cooling of its flank; the intrusion was accompanied by the ascent of high-temperature hydrothermal fluid in the central discharge channel, interacting with the adjacent sediments. As a result, at the high-temperature stage of this interaction, finely dispersed biotite was formed in the sediments due to the original terrigenous clay minerals, K-feldspar and amphiboles. Then, at the rapid cooling of the hydrothermal fluid to a temperature presumably 270–330°C, partial replacement of biotite by chlorite. With further rapid cooling of the hydrothermal fluid to a temperature of 200°C and below and its mixing with sea water seeping into the sediments of the Central Hill, smectite was formed.