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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.
Express Letter

High-precision Δ′17O measurements of geothermal H2O and MORB on the VSMOW-SLAP scale: evidence for active oxygen exchange between the lithosphere and hydrosphere

Takashi Sambuichi, Urumu Tsunogai, Kazushige Kura, Fumiko Nakagawa, Takeshi Ohba
Geochemical Journal, Vol. 55, No. , P. e25-e33, 2021

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.

KEYWORDS

triple oxygen isotopes, geothermal H2O, MORB, VSMOW-SLAP scale, rock-water interaction

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