Origin of high pH hot spring water in the Tanzawa area, Japan

Abstract

The serpentinite seamounts discovered in the Mariana Trench in 1981 are one of the largest discoveries in the history of deep-sea floor research. One of the major characteristics of the serpentinite seamounts in the Mariana forearc region is the high pore water with a pH=12.6. In the Tanzawa area located in the northwestern part of Kanagawa Prefecture and its vicinity, groundwater with a water temperature higher than the average temperature and a pH=11.3 has been used as a hot spring for a long time. The high pH of the hot spring water can be explained by the reaction with serpentinite, but since serpentinite is not exposed in the Tanzawa area and its vicinity, the cause of the high pH of the hot spring water gushing out in this area has not been investigated. However, geophysical surveys such as seismic tomography have indicated the possibility of the existence of serpentinite in the Kanto area (Kamiya 2012). In this study, we attempted to elucidate the origin of high pH hot spring waters in the Tanzawa and surrounding areas by using elemental indicators of lithium, which behaves characteristically when reacting with serpentinite.