Isotope geochemical study of the origin and formation mechanism of carbonate minerals in geothermal wells and surrounding hot spring waters in the western Unzen area

Abstract

In areas of volcanic and geothermal activity, caprock and other impermeable layers regulate the flow direction of geothermal fluids. However, it is unclear how this impermeable layer was formed or at what depth. This impermeable layer is partly formed by carbonate minerals. In the Unzen area, the temperature logging results of geothermal wells showed that high temperature fluids move as lateral flow. In this study, we focused on the carbonate minerals in the area and determined the origin of the fluids from their isotope compositions. This study explores the formation mechanisms of carbonate layers through reactions between CO₂-rich water and rocks within a volcanic geothermal system. Samples were collected from two geothermal wells and adjacent hot spring water in the western Unzen area for chemical and isotope analysis. The UZ-7 well, located near Mt. Unzen, features carbonate mineral layers at depths of approximately −300 m and −600 m. The isotope compositions of carbon and oxygen within these layers indicate a magmatic origin for the carbon, with the fluid mixture consisting of meteoric water, magmatic fluid, and fossil seawater. Deep fluids in the western Unzen region likely undergo vapor–brine separation at about −600 m elevation, where vapor escapes as fumarole gas and hot spring water in the Unzen hot spring area. The brine, approximately 240 °C, then flows westward, emerging at Obama hot spring. This lateral movement and heating of surrounding strata to about 150 °C at depths of −300 m and −600 m facilitate extensive calcite precipitation (up to 100 m thick) through rock interactions nearing chemical equilibrium. This process also results in significant kaolinite and chlorite deposition, potentially forming two impermeable caprock layers. The UZ-4 well, situated 2 km west of UZ-7, also exhibits carbonate layers, albeit on a reduced scale, likely due to insufficient rock reactions from progressively cooler brine temperatures.