Characteristics and origins of non-meteoric-water components contained in hot springs in the central Japan

Abstract

While many of hot springs are originated from meteoric water, it is known that some springs contain much non-meteoric water. However, characteristics and origins of non-meteoric water components contained in those springs have not been well understood. To clarify them, we collected source spring water samples from 62 hot springs in the central Japan. Among them, 29 springs were inferred to contain much non-meteoric-water on the basis of d-excess, total dissolved solid, and δ values. Then, using precipitation isoscape model, we estimated isotope compositions of meteoric water component at recharge zones for every springs. All the mixing lines, which connect each plot of sampled spring waters and plot of their corresponding meteoric end-members on δ-diagram, suggest that non-meteoric components have intermediate isotopic signatures between seawater and andesitic magma water. In the present study, we proposed to call such non-meteoric-origin fluids as seawaterrock equilibrium waters. Focusing on their isotopic compositions and water quality especially, Cl, B, and Li , we could classify them into three groups. The first group i.e., weak equilibrium waters was characterized by relatively low δ 18 O, high δ 2 H, and high Cl concentration and was found in the plain areas near the sea. The second i.e., medium equilibrium waters was characterized by medium δ 18 O and δ 2 H, low Cl concentration and high B/Cl ratio and was found at the mountain-plain transitional areas. The third i.e., strong equilibrium waters was characterized by high δ 18 O, low δ 2 H, high Cl concentration, and high Li/Cl ratio and was found in inland mountainous areas. It s shown that the strong equilibrium water is closely associated with subduction of oceanic plate.