[A-HW19] Hydrology & Water Environment

[AHW19-01] Modeling of water-rock interactions in water isotopic ratio formation and the deep water cycle

★Invited Papers

*Ikuya Adachi¹, Tsutomu Yamanaka² (1.Graduate School of Life and Environmental Sciences, University of Tsukuba, 2.Faculty of Life and Environmental Sciences, University of Tsukuba)

Keywords:water isotope, water-rock interaction, deep water cycle, lithospheric water, spa water, slab-derived water

Hydrogen and oxygen stable isotopes of water are useful tools to trace hydrological cycle near the Earth's surface. Isotopic signatures of natural spa waters sourced from hot and/or mineral-rich springs (including flowing/pumping wells) potentially provide important clues about hidden water cycle extending deep into the Earth's interior, because some of them are deep-seated and non-meteoric. However, their original isotopic-signatures are usually renewed by contamination with meteoric water and thus poorly known. Here we show isotopic evolutionary tracks of the lithosphere-enclosed waters by a simple but versatile model, and then reconstruct the original (i.e., before mixing with meteoric water) signature of the spa waters in a great lift region of central Japan, where two oceanic plates doubly subduct. The model describes the exchange of hydrogen/oxygen isotopes among the following four pools: pore water, interlayer water in clay particle, constitution water (OH group) of hydrous mineral, and O atom constituting mineral crystal. Using a set of likely conditions, the model computes temporal evolution of isotopic compositions in the four pools during progressive discharge with elevated temperature; degree of isotopic equilibrium among the pools is regulated by temperature-dependent rate constant. To consider uncertainties in the conditions, a Monte Carlo simulation was carried out with randomly selected values assuming possible range or probability density function. Although in this study the model is applied for cases of subducting slab that contains ocean-origin water, it is possible to apply for non-subduction and meteoric-origin cases. The predicted evolutionary tracks have some variety but can be approximated by a single curve with an uncertainty band, herein called the ‘ocean-originated lithospheric water curve’ (OLWC), as expressed by δ²H = 60 / (δ¹⁸O − δ¹⁸O_fin) + δ²H_off , where δ¹⁸O_fin is the δ¹⁸O at final evolutionary stage, which mainly depends on rock δ¹⁸O and equilibrium fractionation factors, and δ²H_off is the deuterium offset, which primarily reflects variability of mineral composition and terrigenous clay fraction as well as dissociation/formation of gas hydrate. Assuming δ¹⁸O_fin=11±1 and δ²H_off=0±10, 94% (n=217) of subseafloor pore-water data can be represented by OLWC. Although the percentage for submarine-mud-volcano pore-water reduces to 82% (n=204), the outliers are strongly affected by mixing with modern Mediterranean seawater or formation of gas hydrate. Giving an isotopic signature of meteoric end-member (δ_MW) that corresponds to individual spa on the basis of a meteoric groundwater isoscape, the signature of lithospheric end-member (δ_LW) is reconstructed with a mixing line. The mixing ratios (or relative contributions) of meteoric and lithospheric (i.e., non-meteoric) end-members can be estimated using the signature of spa water (δ_spa), δ_MW and δ_LW. Mixing analysis indicates that the lithospheric water accounts for 19-95% of each spa water we investigated. Nonlinear relationships of reconstructed δ¹⁸O with Cl and Li concentrations justify our hypothesis that the lithospheric waters are on the trail of evolution from seawater to magmatic water, rather than their mixtures. Comparison of reconstructed results with model predictions under plate subduction settings suggest that many of progressively evolving lithospheric waters are released from subducting Philippine Sea (PHS) slab and a few from Pacific (PAC), while waters at early evolutionary stage found in two specific areas (i.e., Niigata and southwestern Gunma) come from shallower marine sediments moderately heated by tertiary volcanic rocks. We anticipate long-term monitoring of isotopic signature of spa water to be a help to detect changes in Earth's interior as well as to properly manage unique water resources.