First direct evidence of the O–H isotopic composition of slab-derived water in the subduction zone, revealed by fluid inclusions in high-pressure metamorphic rocks

Abstract The O and H isotopic compositions of water provide clues to their origin and the water cycle in the deep Earth, and those of waters from hot springs can indicate their deep, non-meteoric origin. Quartz veins in high-pressure type metamorphic rocks can provide crucial information on paleo-subduction zone fluids. We investigated the O and H isotopic compositions of fluid inclusions in quartz veins collected from the Sanbagawa metamorphic belt, SW Japan, that are thought to preserve a trace element signature from slab-derived water. A total of 300–1000 mg of purified quartz grains were crushed, and the O and H isotopic compositions of the extracted water were analyzed using cavity ring-down spectroscopy (CRDS). The O isotopic compositions of the residual host quartz were determined by continuous-flow isotope ratio mass spectrometry (CF-IRMS). The fluid inclusions in schistosity-parallel quartz veins, which yield the trace element signatures of slab-derived water, have O and H isotopic compositions that do not plot along the meteoric water line but are consistent with previous numerical models of slab-derived water. Furthermore, the depths of the slab-derived water predicted by the numerical models are consistent with the peak pressures of the host metamorphic rocks. In contrast, fluid inclusions in quartz veins that cut across the main schistosity in the host rocks, which likely formed during the exhumation of the rock, have an isotopic composition intermediate between those of meteoric and slab-derived waters. The estimated temperature of equilibrium between the water and quartz O isotopic compositions is not consistent with the peak metamorphic temperature of the host rock, suggesting disequilibrium between the bulk quartz vein and trapped fluids. These results are the first direct evidence of the stable isotopic composition of slab-derived water, and thereby highlight the importance of direct determination of paleo-fluid composition using fluid inclusions.