[S-IT18] GEOPHYSICAL PROPERTIES AND TRANSPORT PROCESSES IN THE DEEP CRUST AND MANTLE

[SIT18-11] Effect of sodium salt on rutile solubility in aqueous fluids: Insights into titanium transport in subduction zones

*Naoko Takahashi^1,2, Tatsuki Tsujimori^1,3, Seiji Kamada^1,4, Michihiko Nakamura¹ (1.Graduate School of Science, Tohoku University, 2.Graduate School of Science, The University of Tokyo, 3.Center of Northeast Asian Studies, Tohoku University, 4.Frontier Research Institute for Interdisciplinary Sciences, Tohoku University)

Keywords:Hydrothermal diamond anvil cell, Rutile, Solubility, Aqueous fluid, Sodium salt

Most arc tholeiitic and calc-alkaline basalts display TiO₂ and Zr concentrations exceeding those that could be derived from partial melting of depleted mantle [1]. This peculiar geochemical signature indicates that small amounts of Ti and Zr are transported from a slab and added to the source mantle by aqueous fluids, hydrous melts, or supercritical fluids [1]. The titanium transport from the slab to the mantle wedge could be controlled by the solubility of TiO₂ in fluids. However, our knowledge of TiO₂ solubility and speciation in aqueous fluids with complicated compositions remains insufficient. We investigated the solubility of rutile (TiO₂) in Na₂CO₃–H₂O, NaHCO₃–H₂O, Na₂SO₄–H₂O, and NaF–H₂O fluids at high temperatures up to 1012 °C and approximate pressures up to 1.7 GPa by in-situ observations of the complete dissolution of rutile crystals in a hydrothermal diamond anvil cell (HDAC). We compared the rutile solubility in NaF solutions with previous data and additionally examined the enhancing effect of ligands such as OH^-, CO₃^2-, HCO₃^-, and SO₄^2- on the solubility.

In-situ observations of the dissolving rutile grain at a constant temperature of 800 °C for 13 minutes showed that the equilibration was achieved in a few minutes after the target temperature was attained at a heating rate of ~30 °C min^–1. To avoid the kinetic delay of dissolution, we reduced the heating rate to ~3 °C min^–1 when the heating temperature approached approximately 50 °C within the complete dissolution temperature. The addition of the sodium salts (Na₂CO₃, NaHCO₃, Na₂SO₄, and NaF) to H₂O resulted in a remarkable increase in the solubility of rutile as compared to those in pure H₂O previously determined with the piston-cylinder method. The solubility of Ti in 1.0 m Na₂CO₃ solution increased by a factor of ∼40, while the NaF solutions showed a less enhancing effect than previously expected. Because the addition of those sodium salts to fluids is assumed to increase Na⁺ ions and the anionic species and change pH in the fluids, the enhanced rutile solubility can be explained by the formation of Ti aqua hydroxo-complexes or Ti complexes with sodium or the anionic ligands. This dissolution mechanism contributes to interpreting the extraordinary high Ti mobility in alkaline carbonic fluids within exhumed high- to ultrahigh-pressure terranes in orogenic belts. In addition, the speciation of Ti might control whether the dissolved Ti is trapped by precipitation at the slab–mantle interfaces or transported by metasomatic fluids into the mantle wedge.

Reference: [1] Schmidt and Jagoutz (2017) Geochem. Geophys. Geosyst., 18, 2817–2854.