Geochemistry of Sulfide Chimneys and Basement Pillow Lavas at the Southern Mariana Trough (12.55°N–12.58°N)

Three submarine hydrothermal sites (Snail, Y and Pika sites) in the southern Mariana area were investigated to clarify the geochemical difference between off-axis and on-axis submarine hydrothermal activities and volcanic rocks. The Snail and Y sites are located on the axis of the spreading ridge, and the Pika site is located on the off-axis region. Low-temperature venting of submarine hydrothermal fluids was observed at the Snail and Y sites. High-temperature black smoker activities were found at the Pika site. For further geochemical study, sulfide chimneys and mounds were collected from each site by manned submersible Shinkai 6500 . Basement short drill cores of volcanic rocks and sulfides were also obtained from the Snail and Pika sites using a multi-coring system. All drilled pillow lavas were tholeiitic andesite with a narrow range of chemical variation. Significant enrichments of Rb, Ba and Th were absent in both on-axis (Y and Snail sites) and off-axis (Pika site) samples, suggesting the least incorporation of subducting sediments to the magmatic system. Concentrations of Au and Ag in the sulfide chimney were within the range of massive sulfides at the mid-ocean ridge rather than typical arc-type massive sulfides. It is found that sulfur isotope compositions of sulfides were different between the on-axis and off-axis samples: on-axis samples had heavy δ³⁴S (+2.9–+8.7‰) and off-axis samples (–0.3 to +3.8‰) were similar to the local magmatic value. Such a regional difference probably results from changes of deep hydrothermal processes during the water–rock interaction rather than differences in tectonic settings.