A THERMODYNAMIC STUDY OF HIGH TEMPERATURE METASOMATISM IN THE K-FELDSPAR-ANDALUSITE SYSTEM

During the ascent of magma to shallow portions of the crust, decompression of the melt or the crystallization of anhydrous phases may cause one or more magmatic volatile phases (MVP) to be exsolved. The MVPs are most commonly chlorine-rich waters that are hypothesized to be responsible for some areas of hydrothermal alteration and the transfer of metals to sites of ore deposition. A dearth of data exists on the properties of the high-salinity liquid phase (brine) at elevated temperatures and pressures, thus, this study is intended to fill in the gaps in the experimental database. The phase stability and equilibrium fluid compositions in the K-feldspar-muscovite-andalusite-quartz system were ascertained from 550ºC to 750ºC and 50 to 80 MPa. . The stable mineral phase was determined by surface morphology and by the K/Al and Al/Si ratios as determined by using a JEOL JSM-5610LV Scanning Electron Microscope with Energy Dispersive Spectrometry (SEM-EDS). The phase stability fields were plotted as a function of temperature and log₁₀ (KCl/HCl) ratios of the fluids. The results of this study show that the K-feldspar/andalusite equilibrium boundary at 600ºC and 50 MPa is log₁₀ KCl/HCl 0.50±0.10. At 80 MPa and temperatures of 600ºC, 650ºC, and 700ºC the equilibrium boundary is at log₁₀ KCl/HCl values of 0.818±0.25, 0.818±0.25, and 0.318±0.24, respectively. The equilibrium KCl/HCl values of the brine are much lower than in a brine-undersaturated vapor supercritical system. It appears that as salinity increases, the equilibrium a_KCl/a_HCl values of the fluid decrease due to variations of the KCl and HCl activity coefficients. Our data can be used to update thermodynamic mineral datasets to improve models of high-temperature mineral alterations in the presence of high-salinity fluids. Additional experiments will be conducted to further narrow down the phase boundaries, followed by thermodynamic calculations to determine the KCl/HCl activity coefficient ratios..