Relationship between helium isotopes and heat flux from hot springs in a non-volcanic region, Kii Peninsula, Southwest Japan

Umeda, Koji; Sakagawa, Yukihiro*; Ninomiya, Atsushi; Asamori, Koichi

In order to examine the potential correlation between heat and helium isotope signatures in a non-volcanic region, we estimated the local heat flux using Peclet number analysis of thermal profile from deep boreholes in the Kii Peninsula, Southwest Japan. There is a distinct correlation between the common logarithms of He/He ratio and heat flux. Here heat flux values higher than 90 mW/m are estimated from the deep boreholes and the upward flow of deep fluids predominates over the groundwater recharge of the local hydrology. These results support the recently proposed source model that shows that the high temperature hot springs with significantly higher He/He ratio in the Kii Peninsula originate due to the effective transfer of mantle helium and heat from the subcrustal lithosphere by aqueous fluids generated by dehydration of the subducting Philippine Sea slab.