4:00 PM - 4:15 PM
[SCG46-08] Effect of dissolved silica on the electrical conductivity of H2O-NaCl fluid in the lower crust
★Invited Papers
Keywords:Electrical conductivity, NaCl aqueous fluid, Dissolved silica, Molecular dynamics simulation
Fluids have a significant impact on the properties and dynamics of the crust; therefore, understanding the forms of fluids present in the crust is essential. Generally, the electrical conductivity of fluids is higher than that of most rocks, which is critical for estimating the volume fraction, distribution, and the properties of fluids. To identify the fluids responsible for the observed high electrical conductivity, it is necessary to understand their electrical conductivity of fluids at elevated temperatures and pressures in the crust.
The electrical conductivity of H2O-NaCl fluids at elevated temperatures and pressures, in the lower crust, has been determined from both molecular dynamics simulations and experiments (Sakuma and Ichiki, 2016; Sinmyo and Keppler, 2017). However, the chemical composition of fluids can be altered by the dissolution of rocks in the lower crust, and the conductivity of pure H2O-NaCl fluids alone cannot account for the observed behavior. For example, the conductivity of H2O-NaCl fluids in quartzite (Shimojuku et al., 2014) was significantly lower than that of pure H2O-NaCl fluids. This difference was attributed to the presence of SiO2-NaCl complexes in the fluids (Newton and Manning, 2016); however, it remains unclear whether the electrically neutral dissolved silica can influence the mobility of Na and Cl ions.
In this study, our objective is to investigate the influence of dissolved silica on the electrical conductivity of H2O-NaCl fluids. The conductivity was calculated using molecular dynamics simulations under conditions representative of the lower crust: 800 K and 1 GPa. Dissolved silica species and concentrations were determined based on the model proposed by Manning (2018). The potential parameters of SiO2 monomer and dimer of our previous model (Yokoyama and Sakuma, 2018) were employed and slightly modified to inhibit species dissociation. The electrical conductivity was calculated using the self-autocorrelation function of the electric current arising from the mobility of Na and Cl ions.
As a result, the conductivity slightly decreased in the presence of Si(OH)4 monomer, but this decrease was insufficient to explain the observed reduction in conductivity in fluid-bearing quartzite experiments. This may be attributed to the weak electrostatic interactions between electrically neutral Si(OH)4 monomers and charged Na and Cl ions. In the presentation, we will further explore the effect of Si2O(OH)6 dimers and their implications for understanding conductivity variation in fluid-bearing quartzite.
References: Manning, C.E. (2018) Annu. Rev. Earth Planet. Sci., 46, 67-97.; Newton, R.C. and C.E. Manning (2016) Geofluids, 16, 342-348.; Sakuma, H. and M. Ichiki (2016) J. Geophys. Res. Solid Earth, 121, 577-594.; Shimojuku, A., T. Yoshino, D. Yamazaki (2014) Phys. Earth Planet. Int., 198, 1-8.; Sinmyo, R. and H. Keppler (2017) Contrib. Mineral. Petrol., 172, 4; Yokoyama, T. and H. Sakuma (2018) Geochim. Cosmochim. Acta, 224, 301-312.
The electrical conductivity of H2O-NaCl fluids at elevated temperatures and pressures, in the lower crust, has been determined from both molecular dynamics simulations and experiments (Sakuma and Ichiki, 2016; Sinmyo and Keppler, 2017). However, the chemical composition of fluids can be altered by the dissolution of rocks in the lower crust, and the conductivity of pure H2O-NaCl fluids alone cannot account for the observed behavior. For example, the conductivity of H2O-NaCl fluids in quartzite (Shimojuku et al., 2014) was significantly lower than that of pure H2O-NaCl fluids. This difference was attributed to the presence of SiO2-NaCl complexes in the fluids (Newton and Manning, 2016); however, it remains unclear whether the electrically neutral dissolved silica can influence the mobility of Na and Cl ions.
In this study, our objective is to investigate the influence of dissolved silica on the electrical conductivity of H2O-NaCl fluids. The conductivity was calculated using molecular dynamics simulations under conditions representative of the lower crust: 800 K and 1 GPa. Dissolved silica species and concentrations were determined based on the model proposed by Manning (2018). The potential parameters of SiO2 monomer and dimer of our previous model (Yokoyama and Sakuma, 2018) were employed and slightly modified to inhibit species dissociation. The electrical conductivity was calculated using the self-autocorrelation function of the electric current arising from the mobility of Na and Cl ions.
As a result, the conductivity slightly decreased in the presence of Si(OH)4 monomer, but this decrease was insufficient to explain the observed reduction in conductivity in fluid-bearing quartzite experiments. This may be attributed to the weak electrostatic interactions between electrically neutral Si(OH)4 monomers and charged Na and Cl ions. In the presentation, we will further explore the effect of Si2O(OH)6 dimers and their implications for understanding conductivity variation in fluid-bearing quartzite.
References: Manning, C.E. (2018) Annu. Rev. Earth Planet. Sci., 46, 67-97.; Newton, R.C. and C.E. Manning (2016) Geofluids, 16, 342-348.; Sakuma, H. and M. Ichiki (2016) J. Geophys. Res. Solid Earth, 121, 577-594.; Shimojuku, A., T. Yoshino, D. Yamazaki (2014) Phys. Earth Planet. Int., 198, 1-8.; Sinmyo, R. and H. Keppler (2017) Contrib. Mineral. Petrol., 172, 4; Yokoyama, T. and H. Sakuma (2018) Geochim. Cosmochim. Acta, 224, 301-312.