The aim of this section is to predict the system evolution under equilibrium conditions, and then use these predictions to identify potential kinetic effects that may explain the observed sequential sodic and potassic alterations. Thermodynamic calculations are challenging since our experiments involve reaction between a complex electrolyte solution with evolving composition and mineral solid-solutions with a miscibility gap (Fig. 3A). We used GEM-Selektor (details in the Supplementary Data), since the suitability of this package to effectively model this complex thermodynamic situation has been demonstrated23,24. At 600 ˚C, the sanidine composition used in our experiments is within the miscibility gap between K-feldspar and albite, though it is close to the single-phase boundary (Fig. 3A). The Lippmann diagram in Fig. 3B depicts the aqueous ion concentrations (X(Na+,aq); green solutes line) in equilibrium with a given solid solution composition25 (X(Ab); red solidus line). In the presence of a Na+-rich fluid (X(Na+,aq) > 0.85), the equilibrated solid is an albite-rich feldspar (Fig. 3B). At a X(Na+,aq) of 0.85, a peritectic point is reached where the aqueous phase co-exists with two solid solutions: albite with X(Ab) = 0.88, and K-feldspar with X(Ab) = 0.35 (Fig. 3C). When X(Na+,aq) decreases slightly below 0.85, the Na content of the equilibrated solid solution decreases dramatically, forming a K-feldspar with X(Ab) ≤ 0.35 (Fig. 3B). Hence, these calculations show that K-rich (X(Ab) of 0.13 to 0.22) feldspars can coexist with Na-rich aqueous solutions ((X(Na+,aq) of 0.72 to 0.83; Fig. 3B): K-rich fluids are not required to drive potassic alteration.
Progressive replacement of sanidine has been modelled by aliquot titration of sanidine in a fixed amount of solution at experimental conditions (2 kbar, 600 °C). Three different equilibrium scenarios were considered in terms of solution used, i.e., pure H2O, and NaCl and NaF solutions (Figs. 3D-F). In pure H2O (Fig. 3D), dissolution of sanidine quickly results in equilibration of the solution with a feldspar of composition close to that of the titrated sanidine. In the NaCl solution (Fig. 3E), albite is predicted to form first, resulting in decreased Na+ and increased K+ in solution. When the sanidine/water molar ratio is around 0.25, two different products (Na-poor K-feldspar and a Na-rich albite) formed at the peritectic point. The NaF solution showed a similar evolution, but the equilibrium peritectic point was reached at lower sanidine/water ratio (Fig. 3F).
Hence, the equilibrium simulations show that unless sanidine dissolves congruently and buffers fluid composition (Fig. 3D), sanidine should be replaced by two feldspars at the peritectic point (Figs. 3E-F), with a final fluid composition with X(Na+,aq) = 0.85. These predictions tally with previous experiments18, whereby an Ab60Or40 feldspar was replaced by coarse-grained, coexisting albite and K-feldspar upon reaction with H2O/HCl solutions (Fig. 3G).
However, our experiments display different textures and final product compositions than those predicted by equilibrium thermodynamic modelling (Figs. 2, 3H). The most significant difference is that there is no evidence for co-precipitation of two feldspars in our experiments: albite first replaces sanidine, and then this newly formed albite –and some sanidine– are replaced by K-rich feldspar along a separate, independent reaction front (Fig. 3H). This indicates that mineral formation in this system is governed by interfacial fluids at the reaction fronts with compositions different from the bulk solution.