Alkali ion exchange between vapor and feldspar phases

Abstract

The high alkali chloride contents of fluid inclusions and most connate waters suggest that metamorphic rocks and many igneous rocks have crystallized in the presence of rather concentrated solutions of Na and K salts. Equilibria involving alkali ions in solution in a fluid phase and alkali silicates should therefore be of interest to petrologists. The system KAlSi ₃ O ₈ -NaAlSi ₃ O ₈ -NaCl-KCl-H ₂ O has been investigated at temperatures of 350 degrees to 700 degrees C. at a pressure of 2000 atmospheres with the fluid phase 2M in total alkali chloride. Alkali feldspars were the only crystalline phases encountered in these experiments. Phase assemblages observed were: 1) a univariant 2-phase assemblage consisting of a single alkali feldspar and an alkali chloride solution, 2) an invariant 3-phase assemblage consisting of 2 alkali feldspars and an alkali chloride solution. The proportion of K to (K+Na) in the vapor phase that coexists with 2 alkali feldspars in the invariant phase assemblage decreases with falling temperature from 0.260 at 670 degrees C. to less than 0.16 at 400 degrees C. This is consistent with, and provides an explanation for, the small amount of K relative to Na found in the waters of hot springs. The crest of the alkali feldspar solvus is near 680 degrees C. at a feldspar composition of Ab (sub 7O+ or -10) Or (sub 30+ or -10) . A comparison of melt-crystal equilibria and vapor-crystal equilibria suggests that fractionation of alkalies between a fluid phase and crystalline feldspars is strongly dependent upon temperature but only slightly dependent upon the physical condition of the fluid. Melt-crystal and vapor-crystal equilibria appear to be nearly the same for the alkali feldspars at a given temperature. Because the alkali ratio in a vapor phase that coexists with 2 feldspar phases varies with temperature, the presence of a thermal gradient in a 2-feldspar rock requires the presence of compositional gradients with respect to the alkalies in the vapor phase. Alkali ions will tend to diffuse through the vapor in response to these gradients, and alkali metasomatism within the rock mass will take place. In general, the cooler rock will be enriched and the warmer rock depleted in K-feldspar. This mechanism of alkali transport has been demonstrated on a small scale in the laboratory. The presence of Ca-feldspar as a constituent in coexisting feldspar phases has a significant effect on the alkali ratio in a coexisting vapor phase. At constant temperature, an increase in the Ca-content of the feldspar phases will result in a higher K/Na ratio in the vapor phase. Original inhomogeneities in Ca-content within a mass of sedimentary rocks will result in compositional gradients with respect to alkalies in the vapor phase that will favor alkali metasomatism during metamorphism. Rocks originally rich in Ca will tend to be depleted in K-feldspar, and rocks originally poor in Ca will tend to be enriched in K-feldspar. It seems likely that alkali metasomatism will take place in the presence of an alkali-bearing vapor phase as a natural consequence of temperature, pressure, and compositional gradients in the earth9s crust.