ABSTRACT

Recent studies have reported slight but definite differences in Δ′¹⁷O between the lithosphere and hydrosphere. In the present study, we precisely and accurately quantify the Δ′¹⁷O values of geothermal H₂O and mid-ocean ridge basalt (MORB) with normalization on the VSMOW-SLAP scale to further substantiate these differences and to discuss the isotopic evolution of the hydrosphere throughout the geologic time scale. With a Δ′¹⁷O value of -60 ± 13 × 10^-6, the Δ′¹⁷O value of MORB is comparable with that in other silicates reported in previous studies. However, the Δ′¹⁷O value of geothermal H₂O tended to decrease from +31 × 10^-6 to -51 × 10^-6, which are the usual Δ′¹⁷O values in meteoric water and silicates, respectively, in accordance with the ¹⁸O-enrichment. These results imply an active oxygen isotope exchange between silicates and geothermal H₂O under high-temperature conditions at depth. This is supported by previous studies which report the ¹⁷O-enrichment of silicate altered by hydrothermal H₂O. Considering this direct evidence for depletion of ¹⁷O, we conclude that the ¹⁷O-depleted H₂O has been supplied continuously to the hydrosphere. Additionally, low-temperature interaction between the silicates and H₂O besides high-temperature hydrothermal interaction must be assumed to explain the observed Δ′¹⁷O of the terrestrial hydrosphere. We conclude that the Δ′¹⁷O of the terrestrial hydrosphere should have been variable throughout the geologic time scale owing to the various oxygen exchange interaction between the lithosphere and hydrosphere.