Abstract
The dissolved silica structures in quartz-saturated 0.50 and 1.50 m [mol kg H2O–1] Na2CO3 and 0.47 m NaOH solutions at up to 750 °C and 1.5 GPa were investigated by in-situ Raman spectroscopy using a Bassett-type hydrothermal diamond anvil cell. The solubility of quartz in the solutions was determined by in-situ observations of the complete dissolution of the grain. The Raman spectra of the quartz-saturated Na2CO3 and NaOH solutions at high pressures and temperatures exhibited the tetrahedral symmetric stretching band of silica monomers. The lower frequency and broader width of the band than those in pure H2O indicated the presence of both neutral and deprotonated monomers. In addition, we newly confirmed the intense bridging oxygen band and the tetrahedral symmetric stretching band of Q1 (silicate center having a single bridging oxygen atom) in the spectra of the Na2CO3 solutions. The integrated intensity ratios of the bridging oxygen band to the monomer band increased with the addition of Na2CO3 and NaOH to fluids, corresponding to an elevation of the measured quartz solubilities. These observations indicate that the formation of silica oligomers in addition to neutral and deprotonated monomers explains the high dissolved silica concentrations in the solutions. The presence of deprotonated monomers under the experimental conditions suggests that deprotonated oligomers exist in the solutions, because the production of the latter more significantly reduces the Gibbs free energy. The anionic silica species and oligomers formed in alkaline silicate fluids may act as effective ligands for certain metal ions or complexes in deep subduction zones.
Highlights
Aqueous fluids facilitate the mass transport of elements for the deep hydrothermal processes involved in metamorphism, metasomatism, and ore formation in the crust and upper mantle of the Earth
There was considerable uncertainty in the estimated quartz volume, the measured solubilities agreed, within the errors, with the values calculated based on the Deep Earth Water (DEW) model (0.72 and 0.77 m, respectively; Sverjensky et al 2014; Huang and Sverjensky 2019) that used the solubility and speciation data for aqueous silica from high pressure experiments for characterizing the HKF coefficients
The present study suggests that alkaline fluids can dissolve a significant amount of silica compared to pure H2O at elevated P–T conditions, which is explained by the formation of neutral and deprotonated monomers and the subsequent formation of oligomers
Summary
Aqueous fluids facilitate the mass transport of elements for the deep hydrothermal processes involved in metamorphism, metasomatism, and ore formation in the crust and upper mantle of the Earth. Contributions to Mineralogy and Petrology (2022) 177:36 and Burnham 1965; Manning 1994), and this increase in the solubility is associated with the polymerization of aqueous silica monomers to form dimers or more polymerized species (e.g., Newton and Manning 2002, 2008; Zotov and Keppler 2002; Mysen 2010; Mysen et al 2013). These experiments were conducted at near-neutral pH conditions, whereas the solubility behavior of quartz in high pH fluids could provide some insights into the nature of alkaline fluids in subducting lithologies such as crustal pelite (Galvez et al 2015, 2016), crustal basalt (Galvez et al 2016), and sediments (Connolly and Galvez 2018). The information available on the Raman observations is insufficient to understand silica solubility and speciation in N a2CO3 and NaOH solutions under high P–T conditions
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