5:15 PM - 7:15 PM
[SGC37-P06] First direct constraints of the stable isotope composition of the slab-derived fluids: evidence from the fluid inclusions in high-P type metamorphic rocks.
Keywords:geofluid, fluid inclusion, stable isotope, high pressure metamorphic rock, Arima-type hydrothermal fluid
Oxygen and hydrogen isotope compositions of aqueous fluids provide clues about their origin and water cycle from the deep Earth. Arima-type hydrothermal fluids, typically observed in the Arima-Takarazuka area in SW Japan are characterized by high salinity, high δ18O, high 3He/4He ratio, and low δD even despite their occurrence in the non-volcanic fore-arc region (e.g., Matsubaya et al., 1973). Measurements of spa waters and model calculations suggest their origin to be dehydration of subducting slab (Kusuda et al., 2014; Adachi & Yamanaka, 2024).
Quartz veins in high-pressure type metamorphic rocks can provide crucial information about the paleo subduction zone fluid activities. Yoshida et al. (2015) performed crush-leach analysis of the fluid inclusions in quartz veins collected from the Sanbagawa metamorphic belt, showing their chemical composition are consistent with the surface-observed “slab-derived fluids” with respect to some fluid-compatible elements (Li/Cl and B/Cl) that are thought to be indicators of slab-origin (Ohsawa et al., 2010; Kazahaya et al., 2014). As such, fluid inclusions in metamorphic rocks are good candidate for investigating deep fluids in the subduction zone. In this study, we have investigated oxygen and hydrogen isotope compositions of fluid inclusions in the same quartz veins investigated by Yoshida et al. (2015). 300-1000 mg of the purified quartz grains were crushed and the extracted water were analyzed for oxygen and hydrogen isotopes by cavity ring-down spectroscopy (CRDS) which was developed to apply speleothems carbonate (Uemura et al., 2016). The host quartz oxygen isotopes were also determined by continuous-flow isotope ratio mass spectrometry (CF-IRMS) (Ijiri et al., 2014). Fluid inclusions in the quartz veins showed distinct isotopic compositions from the meteoric water line, but in harmony with the numerical calculations from the previous studies. Furthermore, the depths of slab-derived fluids expected from the numerical calculation were also consistent with the peak pressure of the host metamorphic rocks. Additionally-analyzed quartz vein formed during the exhumation of the rock showed an intermediate isotopic composition between meteoric water and slab-derived fluids. Equilibrium temperatures calculated for the fluid and quartz oxygen isotopic composition were not consistent with the peak metamorphic temperature of the host rock, indicating that the non-equilibrium nature of the bulk quartz vein and trapped fluids. These results become the first direct evidence of the stable isotope composition of the slab-derived fluids, and also suggest the importance of direct determination of paleo fluid composition using fluid inclusions.
Quartz veins in high-pressure type metamorphic rocks can provide crucial information about the paleo subduction zone fluid activities. Yoshida et al. (2015) performed crush-leach analysis of the fluid inclusions in quartz veins collected from the Sanbagawa metamorphic belt, showing their chemical composition are consistent with the surface-observed “slab-derived fluids” with respect to some fluid-compatible elements (Li/Cl and B/Cl) that are thought to be indicators of slab-origin (Ohsawa et al., 2010; Kazahaya et al., 2014). As such, fluid inclusions in metamorphic rocks are good candidate for investigating deep fluids in the subduction zone. In this study, we have investigated oxygen and hydrogen isotope compositions of fluid inclusions in the same quartz veins investigated by Yoshida et al. (2015). 300-1000 mg of the purified quartz grains were crushed and the extracted water were analyzed for oxygen and hydrogen isotopes by cavity ring-down spectroscopy (CRDS) which was developed to apply speleothems carbonate (Uemura et al., 2016). The host quartz oxygen isotopes were also determined by continuous-flow isotope ratio mass spectrometry (CF-IRMS) (Ijiri et al., 2014). Fluid inclusions in the quartz veins showed distinct isotopic compositions from the meteoric water line, but in harmony with the numerical calculations from the previous studies. Furthermore, the depths of slab-derived fluids expected from the numerical calculation were also consistent with the peak pressure of the host metamorphic rocks. Additionally-analyzed quartz vein formed during the exhumation of the rock showed an intermediate isotopic composition between meteoric water and slab-derived fluids. Equilibrium temperatures calculated for the fluid and quartz oxygen isotopic composition were not consistent with the peak metamorphic temperature of the host rock, indicating that the non-equilibrium nature of the bulk quartz vein and trapped fluids. These results become the first direct evidence of the stable isotope composition of the slab-derived fluids, and also suggest the importance of direct determination of paleo fluid composition using fluid inclusions.