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[SIT20-07] HDAC experiments on silica solubility and speciation in Na2CO3–H2O fluids at high pressure and temperature
Keywords:hydrothermal diamond anvil cell, Raman spectroscopy, silica species, solubility
Silica is the most dominant solute in subduction-zone fluids. Recent thermodynamic models predict the presence of deprotonated monomer, SiO(OH)3-(aq), the monomers, Si(OH)4(aq), and the dimers, Si2O(OH)6(aq), as primary silica species in the alkaline fluids equilibrated with subducted silicate rocks [1]. However, the silica speciation in alkaline fluids at high P–T conditions remains unclear due to the complexities of equilibrium constants among silica species. Aranovich et al. [2] reported the higher silica solubility in Na2CO3 solutions at 500–700°C and 0.4–0.5 GPa than that expected solely from the known species in the extended Deep Earth Water (DEW) model [3], suggesting the possible presence of deprotonated dimer, Si2O(OH)6-(aq). To confirm the speciation of silica in Na2CO3 solutions at high P–T conditions, we conducted a Raman spectroscopic study on silica speciation in pure H2O and Na2CO3 solutions (0.5 and 1.0 m [mol/kg H2O]) up to 750°C and 2 GPa using a hydrothermal diamond anvil cell. Moreover, we measured the solubility of quartz by direct observation of dissolving quartz grains.
The measured Raman spectra showed an asymmetric and broad band at ~770 cm-1, which can be assigned to symmetric Si–OH stretching of the monomeric species SiO(OH)3-(aq) and Si(OH)4(aq). Besides, we newly detected the intense broad bands at ~600 cm-1 consisting of at least three components, which represent the bending vibrations of bridging oxygen Si–O–Si of several oligomers. The lowest frequency component at ~530 cm-1 was interpreted as the asymmetric breathing mode of ring trimer. While the uncertainty remains in assigning the broad bands at ~1080 cm-1 to a single species, we presume the main contributions from deprotonated or oligomeric species. The ratio of the oligomer band area at ~600 cm-1 to the monomer band area at ~770 cm-1 of each experimental spectrum increases according to the increase pressure and temperature at quartz-saturated conditions.
The determined quartz solubility in pure H2O at 718°C and 1.0 GPa fell in the range of 0.62 to 0.82 m, which is almost consistent with the value of 0.72 m calculated by the DEW model. On the other hand, the silica solubility in the Na2CO3 solutions at experimental conditions increased to the range of 1.5 to 3.0 m with increasing Na2CO3concentration and temperature. A preliminary comparison among our experimental results and thermodynamic models has led the following conclusions. (1) the silica solubility in the Na2CO3 solutions of the present experiments was several times higher than the value calculated based on the thermodynamic properties of aqueous silica species in the DEW model. (2) Although the addition of thermodynamic parameters of deprotonated dimer Si2O(OH)6-(aq) can readily explain the high solubility of silica in the Na2CO3 solutions, our Raman spectroscopic data suggest the presence of several other oligomeric species that contributed to the high silica solubility. Therefore, additional oligomeric silica species should be considered in the estimation of silica solubility in alkaline fluids at deep-crust and upper mantle conditions.
References: [1] Connolly and Galvez (2018) Earth Planet. Sci. Lett., 501, 90–102, [2] Aranovich et al. (2020) Chem. Geol., 550, 119699, [3] Huang and Sverjensky (2019) Geochim. Cosmochim. Acta, 254, 192–230.
The measured Raman spectra showed an asymmetric and broad band at ~770 cm-1, which can be assigned to symmetric Si–OH stretching of the monomeric species SiO(OH)3-(aq) and Si(OH)4(aq). Besides, we newly detected the intense broad bands at ~600 cm-1 consisting of at least three components, which represent the bending vibrations of bridging oxygen Si–O–Si of several oligomers. The lowest frequency component at ~530 cm-1 was interpreted as the asymmetric breathing mode of ring trimer. While the uncertainty remains in assigning the broad bands at ~1080 cm-1 to a single species, we presume the main contributions from deprotonated or oligomeric species. The ratio of the oligomer band area at ~600 cm-1 to the monomer band area at ~770 cm-1 of each experimental spectrum increases according to the increase pressure and temperature at quartz-saturated conditions.
The determined quartz solubility in pure H2O at 718°C and 1.0 GPa fell in the range of 0.62 to 0.82 m, which is almost consistent with the value of 0.72 m calculated by the DEW model. On the other hand, the silica solubility in the Na2CO3 solutions at experimental conditions increased to the range of 1.5 to 3.0 m with increasing Na2CO3concentration and temperature. A preliminary comparison among our experimental results and thermodynamic models has led the following conclusions. (1) the silica solubility in the Na2CO3 solutions of the present experiments was several times higher than the value calculated based on the thermodynamic properties of aqueous silica species in the DEW model. (2) Although the addition of thermodynamic parameters of deprotonated dimer Si2O(OH)6-(aq) can readily explain the high solubility of silica in the Na2CO3 solutions, our Raman spectroscopic data suggest the presence of several other oligomeric species that contributed to the high silica solubility. Therefore, additional oligomeric silica species should be considered in the estimation of silica solubility in alkaline fluids at deep-crust and upper mantle conditions.
References: [1] Connolly and Galvez (2018) Earth Planet. Sci. Lett., 501, 90–102, [2] Aranovich et al. (2020) Chem. Geol., 550, 119699, [3] Huang and Sverjensky (2019) Geochim. Cosmochim. Acta, 254, 192–230.