Effect of Silica Phase Transformations on Hydrogen and Oxygen Isotope Ratios of Coexisting Water

Hydrothermal treatment of silica gel was carried out to study the effect of silica phase transformations on the isotopic property of coexisting water. Phase transformations from amorphous silica to silica-X and quartz were confirmed in 200˚C and 5 MPa treatments. Silica-X was observed at 1d, 2d, 3d, 5d, and quartz appeared at 5d and was dominant in the 10d sample. The hydrogen and oxygen isotope ratios of the treated silica and coexisting aqueous solution approached each other. Major isotopic exchange between the silica and the solution was completed soon after the beginning of the treatment (< 3 h), and was followed by a continuous and gradual shift of silica δ18O. The rapid and gradual shifts were interpreted as isotope exchange of easily exchangeable sites, such as Si-OH, and resistant sites in the SiO2 structure, respectively. Oxygen isotope fractionation of the silica and solution treated for 10d was similar to the calculated fractionation of water and amorphous silica or quartz at 200 ˚C.