Age of the Beppu Hydrothermal System, Japan

Abstract

Thermal and chemical data, obtained from a 300 m well drilled in the southern part of the Beppu hydrothermal field, have revealed that a sodium-chloride type water of 150 ?C flows laterally through the layer of volcanic breccia at around 250 m depth, and that potassium is highly concentrated in breccia below 250 m deep as K-feldsper replacing the primary plagioclase phenocrysts. These suggest that the potassium metasomatism of the breccia has occurred by a long period of water-rock interaction in the flow path. A potassium budget in the flow system indicates the duration time (age) of the flow system to be 50,000 years, based on the quantities; potassium concentrations of thermal waters flowing in and out the system, residence time of water in the system, initial and present potassium concentrations of rock, and porosity of rock.