2005 Volume 111 Issue 12 Pages 751-764
A large-scale travertine deposit is developed at Shionoha hot spring (Kawakami Village, Nara Prefecture). The water from the vent is relatively low in temperature (38~39ºC), high in CO2 and Ca2+, and DO-free. Chemical and stable isotopic compositions of the vent water indicate that water of a meteoric origin is circulated in deep, and that deep-originated CO2 is reacted with rock. The vent water, during flowing a 70 m-long valley, first deposits Fe-rich deposits and then calcitic travertines. Changes of dissolved Fe2+ and Ca2+ in the water along the valley are consistent with distributions of the Fe-rich deposits and the calcitic travertine.
Near the vent, Fe-rich deposit is formed partly due to quick uptake of oxygen from the atmosphere. However, its texture showing upward-branching filaments of 10-20 μm thick indicates that metabolism of iron-oxidizing bacteria probably controls formation of the iron precipitate. Results of XRD, EPMA and TEM-EDS indicate that the Fe-mineral is amorphous ferrihydrite. Calcite precipitation becomes dominant in the lower valley, and forms travertine domes. The travertines exhibit various textures, including of submilimeter-milimeter order lamination, which consists of bundles of needle-shape calcite crystals intercalating with fine-grained layers. Observation result indicates that about 40 bands were formed during a period of 41 days. Therefore, the banded texture was defined as daily rings. Depositional rates were evaluated about 5 mm/year for the Fe-rich deposit, and 20 cm/year for a laminated travertine.
