Chemical composition of sediments from marine shallow-water hydrothermal mounds in Wakamiko submarine crater revealed by multiple prompt gamma-ray analysis

Multiple prompt gamma-ray analysis (MPGA), a new radionuclide quantification method with high energy resolution, was applied to sediments from a marine shallow-water hydrothermal mound. Surface sediments around mounds were collected from the Wakamiko submarine crater located in Kagoshima Bay, southwest Japan, where the hydrothermal system was different from those at other marine shallow-water hydrothermal mounds. All samples were desalted and MPGA measurements were performed for 4,500 s (real time). Event data were obtained using eight Canberra-Eurisys CLOVER Ge detectors. The size of the list data that included all multiple events was from 1.19 to 19.2 GB. We could obtain concentrations of 32 S, 55 Mn, 75 As, 157 Gd, 39 K, 149 Sm, 56 Fe, and 85 Rb with high sensitivity by MPGA. Furthermore, 32 S, 85 Rb, and 55 Mn have characteristic concentrations only at a mound, not at control sampling sites, suggesting that sediments were sulfurated by magmatic fluids at this mound.