The effect of NaCl on aqueous silica in mantle fluids: constraints from quartz solubility experiments in NaCl-H2O at 1000 °C, 1.5 GPa

Silica is among the most abundant solutes in mantle fluids, and NaCl is the dominant dissolved salt; however, the effect of NaCl on silica solubility and complexing is poorly known. We investigated the solubility of quartz in NaCl-H2O at 1000 °C and 1.5 GPa using hydrothermal piston-cylinder methods. Solubilities were measured by weight loss of ground and polished natural single quartz crystals of high purity. Quartz crystals, H2O, and NaCl were sealed in Pt capsules by arc-welding. Typical run times were 1 to 2 hr. Quartz solubility declines with added NaCl according to the exponential y=0.124e-5.65x, from XSiO2 = 0.192 ± 0.023 in pure H2O, to 4.69×10-3 at XNaCl = 0.596. To facilitate comparison to other quartz-solubility studies, concentrations were converted to activities using models for mixing of silica monomers and dimers (NM02, Newton & Manning, 2002, GCA, 66, 4165), and for H2O and NaCl (Aranovich and Newton, 1999, CMP, 125, 200). In the NM02 model, aSiO2 corresponds the mole fraction of the monomer. Results show that log aSiO2 decreases nonlinearly with log aH2O until SiO2 concentrations decline to XNaCl=0.1, consistent with the presence of additional silica species in solution. However, at lower concentrations the decrease is linear. This slope corresponds to the monomer hydration state, n, which can be seen from the equilibrium SiO2(quartz)+nH2O=Si(OH)4●(n-2)H2O (monomer). A value of n=4.0 (including 4OH- and 2 solvating H2O) obtains for monomers in CO2-H2O; however, at 800 °C, 1 GPa, n=1.8 in NaCl-H2O. Here, we observe n=1.7 at 1000 °C, 1.5 GPa. The similar n implies that NaCl interaction with aqueous silica is nearly identical at these higher P and T. The higher solubility of quartz in NaCl-H2O vs. CO2-H2O at the same H2O activities suggests that the NaCl-SiO2 complexing also persists to these high P-T conditions. These results suggest that both CO2 and NaCl will shift the critical end point in the system SiO2-H2O to higher P-T, but that NaCl will do so to a smaller extent than CO2.