[S-IT15] Mass and energy transport properties and processes in the crust and the mantle

[SIT15-P02] The effect of NaCl and CsCl on silica solubility in aqueous fluids at high pressures and high temperatures

*Naoko Takahashi^1,2, Bjorn Mysen² (1.Graduate School of Science, The University of Tokyo, 2.Earth and Planets Laboratory, Carnegie Institution for Science)

Keywords:silica, salt aqueous solution, diamond anvil cell, Raman spectroscopy

Knowledge of the dissolution behavior of silica in aqueous fluids is fundamental to understanding the transport and redistribution of silica in the Earth’s crust and upper mantle. Despite numerous studies on the solubility of silicate minerals in fluids at elevated temperatures and pressures, a full understanding of their solubility in complex mixtures containing volatile species and other dissolved solute components, especially salts, is still limited. For instance, Shmulovich et al.[1] showed that CsCl in fluids produces the effect of enhancing silica solubility (“salting-in”) up to an experimental maximum pressure of 0.9 GPa. This is distinct from the salting-in effect of NaCl up to 0.5 GPa[1,2] and no salting-in effect of CaCl₂ at 0.5–1.4 GPa[1,3], suggesting potential interactions between alkali ions and silica species[3,4] or alterations of solvent properties of water by salts. However, the effects of salts on silica speciation and hydrogen-bonded structures of aqueous CsCl solution have not been investigated by spectroscopic techniques at high pressures and high temperatures. To address this issue, we conducted a Raman spectroscopic study on silica speciation in pure H₂O,NaCl solutions, and CsCl solutions in equilibrium with quartz up to 800 °C and 1.6 GPa using an externally-heated hydrothermal diamond anvil cell. We observed that the intensities of the Q⁰ species (Qⁿ terminology represents the tetrahedral silicate center connected to n bridging oxygen atoms) band decreased with increasing NaCl concentrations in fluids (X_NaCl=0.02–0.20), while it remained largely unchanged at X_CsCl=0.15 compared to H₂O. These results align with the previous findings on the distinct effects of the different salts on silica solubility. In addition, no obvious changes were detected in the symmetric S–O stretching vibrations of Q⁰ species and Si–O–Si vibrations of silica oligomers between the investigated different solutions, leaving the effect of Na⁺/Cs⁺ or NaCl/CsCl on silica complexation and polymerization at high pressures and high temperatures inconclusive. On the other hand, the Raman OH stretching bands showed a more pronounced low-frequency contribution with increasing NaCl concentrations, and for CsCl solutions than for NaCl solutions. These observations do not hold the simplistic view that the dominant presence of the low-frequency components in the Raman OH stretching bands indicates low concentrations of isolated water dipoles, which would reduce dielectric constant of solutions and thus lower neutral aqueous silica solubility. Our results underscore the complex role of different types of cations in subduction-zone fluids buffered by various lithologies on silica solubility and transport.

References: [1] Shmulovich et al. (2006) Geofluids, 6, 154–167, [2] Newton and Manning (2000) Geochim. Cosmochim. Acta, 550, 119699, [3] Makhluf et al. (2023) Am. Mineral., 108, 1852-1861. [4] Newton and Manning (2016) Geofulids, 16, 342–348.