K-feldspar-muscovite-andalusite-quartz-brine phase equilibria: an experimental study at 25 to 60 MPa and 400 to 550°C

Abstract

Felsic magmas may evolve one or more water or chlorine-rich fluid phases which can transport heat and solutes into associated hydrothermal systems and can contribute to alteration and ore deposition. To understand the role of a high-salinity aqueous phase in the magmatic hydrothermal environment, the composition of a subcritical, vapor-undersaturated high-salinity liquid phase (brine) in equilibrium with K-feldspar-muscovite-quartz and muscovite-andalusite-quartz was determined for pressures and temperatures ranging from 25 MPa and 400°C to 60 MPa and 550°C, with total Cl (NaCl + KCl + HCl) concentrations ranging from 3.42 to 8.56 (moles of solute/kg solution). Values of log 10 (KCl/HCl) have been obtained for the equilibria: 1.5 K-feldspar + HCl = 0.5 muscovite + 3 quartz + KCland muscovite + HCl = 1.5 andalusite + 1.5 quartz + 1.5 H 2O + KCl. For the K-feldspar-muscovite-quartz-brine equilibrium, log 10 (KCl/HCl) = 1.6 ± 0.1, 0.81 ± 0.06, 0.54 ± 0.04 and 0.42 ± 0.08 at 25 MPa and 400°C, 40 MPa and 450°C, 50 MPa and 500°C, and 60 MPa and 550°C (pressures and temperatures of the experiments), respectively. For the muscovite-andalusite-quartz-brine equilibrium, log 10 (KCl/HCl) = 0.63 ± 0.1, −0.063 ± 0.06, 0.17 ± 0.05, and 0.25 ± 0.08 at the pressures and temperatures of the experiments, respectively. Comparison of our results with previous studies conducted at higher pressures and with lower-salinity aqueous phases show that the mineral stability fields in the K-feldspar-muscovite-andalusite-quartz system shift to lower KCl/HCl values with increasing salinity and decreasing pressure.