Origin of chlorine in saline groundwater in the Northeast Japan Arc inferred from halogen compositions: implications for the role of magmatic fluids

Understanding the characteristics and origin of groundwater is important for evaluating groundwater, subsurface resources and their utilization, and assessing their relationships with volcanic activity and earthquakes. Northeast Japan is located in a typical subduction zone of the Pacific Plate and hosts numerous volcanoes from the volcanic front to back-arc region. In the Tohoku region, saline groundwater is widely distributed from coastal to inland areas, and 281 samples of this were collected for this study. Hydrogen and oxygen isotopic data indicate that the main possible sources of the chlorine are seawater, magmatic fluids, and marine pore water, including water associated with oil and gas fields. The spatial distribution of Br/Cl and I/Cl ratios shows characteristic variations across the fore-arc to back-arc regions. In particular, from 10 km east to 25 km west of the volcanic front, the saline groundwater commonly has low Br/Cl ratios (< 1 × 10⁻ 3 , i.e., lower than seawater) and relatively high I/Cl ratios ((30–700) × 10⁻⁶; i.e., higher than seawater), suggesting that fluid affected by volcanic activity may contribute to the observed halogen signature. A three-component mixing analysis based on halogen ratios for seawater, magmatic fluid, and marine pore water (including water associated with oil and gas fields) was conducted using hypothetical endmembers to estimate the origins of the Cl. These results show that relatively high magmatic Cl contributions generally characterize samples collected within 25 km west of the volcanic front and within 20 km of the nearest Quaternary volcano. However, even within 20 km of the nearest Quaternary volcano, the contributions of magmatic Cl vary widely, suggesting that magmatic fluids are supplied locally through fractures and faults. The depth above which 90% of earthquakes occur (D90) tends to be shallower where saline groundwater samples have a high contribution of magmatic Cl, indicating a spatial association between shallow seismicity and magmatic fluid input.