ABSTRACT

Oxygen isotopic ratios were measured for the silica which was precipitated from the solution in process of evaporative concentration of Otake geothermal water at 76 and 88°C. The oxygen isotopic ratios of the silica precipitates were extremely out of isotopic equilibrium with the solution in which the colloidal silica was observable. The fractionation factors for the “silica-solution in evaporative concentration system” were compared with those obtained for “the silica-flowing geothermal water system” where the silica is in oxygen isotopic equilibrium with the water. The difference is explained by isotopic zoning of the growing amorphous silica particles. The rearrangement and interlinkage of adsorbed silica on the surface of the amorphous silica contributes to attaining oxygen isotopic equilibrium between the silica precipitate and water. Amorphous silica phase once formed does not re-equilibrate with the solution at later stages. Hence the apparent oxygen isotopic fractionation between the bulk silica precipitate and the solution at the final stage seems to be out of equilibrium.