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 H
2O 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 H
2O 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 H
2O under high-temperature conditions at depth. This is supported by previous studies which report the
17O-enrichment of silicate altered by hydrothermal H
2O. Considering this direct evidence for depletion of
17O, we conclude that the
17O-depleted H
2O has been supplied continuously to the hydrosphere. Additionally, low-temperature interaction between the silicates and H
2O 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.
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