Mg-rich clay mineral formation associated with marine shallow-water hydrothermal activity in an arc volcanic caldera setting

Abstract Shallow-water hydrothermal activity, represented by venting of hydrothermal fluid around 200 °C, occurs in the Wakamiko submarine crater at 200 m water depth in Kagoshima Bay, southwest Japan. The crater is the center of large eruptions that formed a volcanic caldera, which is semi-submerged at present. The crater is covered with thick volcanic sediments of felsic composition. We studied the distribution and chemical composition of hydrothermal clay minerals that are abundant in the sediment collected by piston coring. We also conducted chemical analysis of pore fluids squeezed from the sediment to understand hydrothermal interactions that resulted in formation of these clay minerals. The PC-2 core (340 cm in length) collected in the vicinity of a high-temperature fluid venting site was characterized by abundant Mg-saponite that is limited to a layer between 270 and 300 centimeters below the seafloor (cmbsf) and montmorillonite throughout the core below 55 cmbsf. Vertical profiles of pore fluid chemistry suggest that saponite formation is related to the interface between the seawater and the hydrothermal component in the sediment layer. Formation temperatures of the montmorillonite were estimated to be 118–163 °C, based on oxygen isotope thermometry. Formation of the montmorillonite is attributed to hydrothermal interaction between seawater-dominant pore fluid and volcanic glass. The formation temperature of the saponite was estimated to be ~ 164 °C, based on oxygen isotope thermometry. Formation of the saponite is attributed to hydrothermal interaction between seawater-dominant pore fluid and the montmorillonite, which had been formed at a prior stage. The PC-1 core (240 cm in length) collected from a relatively low-temperature fluid shimmering site was characterized by the occurrence of kerolite in the lower section (210–240 cmbsf). Vertical profiles of pore fluid chemistry suggest that the kerolite formation occurred at the interface between seawater and the hydrothermal component of the sediment layer. Formation temperature of the kerolite was estimated to be about ~ 211 °C, based on oxygen isotope thermometry. Formation of the kerolite is attributed to precipitation from a fluid that was a mixture of a hydrothermal component and seawater. This study revealed the occurrence of Mg-rich clay minerals, saponite and kerolite, beneath a submarine hydrothermal field that developed within sediment of felsic composition. During hydrothermal interactions that formed these clay minerals, seawater penetrated into the sediment and was an important Mg source. Formation of Mg-rich clay minerals, saponite and kerolite, are controlled by pore fluid chemistry, which varies from a seawater-dominant to hydrothermal-dominant component. Exclusive formation of Mg-rich clay minerals at different sites could be explained by different water–rock ratios of the hydrothermal interaction — saponite formation at low water-rock ratio and kerolite precipitation at high water–rock ratio. Occurrence of Mg-rich clay minerals provides clues to the hydrological structure in sediment-covered hydrothermal systems in an arc volcanic caldera setting.