The results of the study of clay mineral alterations in Upper Pleistocene sediments of the southern trough in the Guaymas Basin (Gulf of California) due to the influence of hydrothermal solutions and heat produced by sill intrusions are discussed. Core samples from DSDP Holes 477 and 477A were taken for the analysis of clay minerals. Application of the method of modeling X-ray diffraction patterns of oriented specimens of the finely dispersed particles made it possible to establish the phase composition of clay minerals, determine their structural parameters, and obtain reliable quantitative estimates of their contents in natural mixtures. The modeling data allowed us to characterize reliably the transformation of clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin. In Upper Pleistocene sandy–clayey sediments of the southern trough, changes in the composition of clay minerals occurred under the influence of a long-living hydrothermal system. Its lower part (interval 170.0–257.5 m) with maximum temperatures (~300°C) was marked by the formation of chlorite. Terrigenous clay minerals are not preserved here. Saponite appears at a depth of 248 m in the chlorite formation zone. Higher in the sedimentary section, the interval 146–170 m is also barren of terrigenous clay minerals. Sediments of this interval yielded two newly formed clay minerals (chlorite and illite), which were formed at lower temperatures (above 180°C and below 300°C, approximately up to ~250°C), while the relatively low-temperature upper part (110–146 m) of the hydrothermal system (from ~140°C to ~180°C) includes the mixture of terrigenous and newly formed clay minerals. Terrigenous illite is preserved here. Illitization of the mixed-layer illite–smectite was subjected to illitization. The terrigenous montmorillonite disappeared, and chlorite–smectite with 5–10% of smectite layers were formed. In the upper interval (down to approximately 110 mbsf), the composition of terrigenous clay minerals remains unchanged. They are composed of the predominant mixed-layer illite–smectite and montmorillonite, the subordinate illite, mixed-layer chlorite–smectite with 5% of smectite layers, mixed-layer kaolinite–smectite with 30% of smectite layers, and kaolinite. This composition of clay minerals changed under the influence of sill intrusions into the sedimentary cover at 58–105 m in the section of Hole 477. The most significant changes are noted in the 8-m-thick member above the sill at 50–58 m. The upper part of this interval is barren of the terrigenous mixed-layer illite–smectite, which is replaced by the newly formed trioctahedral smectite (saponite). At the same time, the terrigenous dioctahedral smectite (montmorillonite) is preserved. The composition of terrigenous clay minerals remains unchanged at the top of the unit underlying the sill base.

The paper presents the results of study of the chemical composition of hydrothermally altered and primary (unaltered) sediments from DSDP holes 477, 477A, 478, 481, and 481A drilled in the Guaymas Basin (Gulf of California). There is a newly obtained data for this region on a great number of trace (including rare earth) elements. To determine the compositional transformation of the ascending hot solutions as they pass through the sedimentary cover, we carried out a combined analysis of transformation of the chemical composition of hydrothermally altered sediments and solutions discharging into the sea. In the course of solution–sediment interaction, various elements behave differently during the compositional transformation of hot solutions percolating through the sedimentary cover in the southern trough of the Guaymas Basin. Consequently, concentrations of Cu, Zn, Cd, K, Li, As, Rb, and Ba are subjected to highest changes—some of them are intensely concentrated in the solutions, while others are depleted. The latter group also includes elements, such as Sb, Cs, and Tl. The impact of sedimentary cover is moderate (depletion effect) on the concentration in solutions of Ag, Ca, Fe 2+ , Mo, Bi, and Br and is almost absent on the concentration of Mn, Co, Si, Be, Sr, Na, Mg, S, Cl, F, Ni, Al, Pb, Hf, Ta, W, and REE. During the interaction of sediments with high-temperature hydrothermal solutions, Ti, P, Sc, V, Cr, Ga, Y, Zr, Nb, Th, and U remain therein as immobile elements and do not pass into solutions migrating across the sedimentary cover and discharging into the southern trough of the Guaymas Basin. The paper also discusses influence of the sedimentary cover on the compositional transformation of solutions percolating in sediments overlying the hydrothermally active spreading zone in the Gulf of California (southern trough of the Guaymas Basin).