Figure 2. Experimental and calculation results of dissolved silica concentrations.

Results of ΣSiO₂ (=SiO_2(aq)+HSiO₃⁻+NaSiO_3(aq)) are shown as a function of temperature at variable pH. Black and grey circles are the measured ΣSiO₂ in fluid samples of the opx and olivine experiments, respectively. The results of total silica concentrations in the opx experiments are also given in ref. 5. The numbers annotated to the experimental data are the calculated in situ pH values (see Methods). Solid lines are ΣSiO₂ values in chemical equilibrium at 400 bars according to the following reaction between serpentine and saponite/talc (serpentine–talc buffer): serpentine+2SiO_2(aq)↔talc (saponite)+H₂O. Dotted lines are ΣSiO₂ values in chemical equilibrium at 400 bars according to the following reaction between serpentine and brucite (serpentine–brucite buffer): serpentine+H₂O↔3brucite+2SiO_2(aq). The concentrations of HSiO₃⁻ and NaHSiO_3(aq) were calculated for different pH values and at a constant Na⁺ concentration (100 mmol kg⁻¹) using the equilibrium constants of the following reactions: SiO_2(aq)+H₂O↔HSiO₃⁻+H⁺ and HSiO₃⁻+Na⁺↔NaHSiO_3(aq). Horizontal broken lines show the solubility of amorphous silica at 0 °C and 100 bars for each pH value.