FLUCTUATION OF CHEMICAL-COMPOSITIONS OF THE PHASE-SEPARATED HYDROTHERMAL FLUID FROM THE NORTH FIJI BASIN RIDGE

On the central ridge of the North Fiji Basin (southwest Pacific), hydr othermal activities were located in 1989. It was the second discovery of the hydrothermal fluid undergo phase separation. During a dive miss ion of Shinkai 6500 in 1991, we revisited the hydrothermal area and co llected fluid samples froin the same anhydrite chimney, Fluid composit ion of the end member shows no significant changes during this two yea rs. Other fluid ventings 150 m apart show similar chemistry, suggestin g that the phase separation and segregation processes occur throughout the hydrothermal area. Small fluctuation in chemical composition of t he fluids from two sites is attributed to subseafloor fluid-rock inter action after the phase segregation.