FELDSPAR-BRINE ALTERATION AT HIGH-TEMPERATURES

A magmatic volatile phase (MVP) may be exsolved from a silicate melt through decompression during ascent and/or the crystallization of anhydrous mineral phases. MVPs are commonly chloride-rich fluids and are hypothesized to induce hydrothermal alteration in and around igneous intrusions. The feldspars are common alteration products in these systems with K-feldspar (potassic alteration zone) often found in areas directly above cupolas of the igneous intrusions. Albite-bearing alteration assemblages are found on the flanks of the intrusions, below the cupolas, or more distally. The alterations of minerals near the intrusion are likely to be influenced wholly or partially by a subcritical, vapor undersaturated high-salinity liquid phase (brine). Whereas the K-feldspar-MVP system is well studied, there is a dearth of data on albite-brine systems at elevated temperatures and pressures. This study addressed mineral phase stability and equilibrium brine compositions in the albite-andalusite-quartz-brine system (65 wt. % NaCl equivalent; NaCl-HCl-H₂O) at 600, 650, and 700 °C and 80 MPa. The stable mineral phase was ascertained through an examination of crystal morphology, optical microscopy, and by the compositions of the run products. The albite-andalusite (+ quartz) phase boundary was found to be at NaCl/HCl atomic values in the brine of 87.5±12.5, 67.5±7.5, and 40±10 at 600, 650, and 700 °C, respectively. Apparent equilibrium constants, not accounting for activity coefficients, for the reaction: albite + HCl = 0.5 andalusite + NaCl + 2.5 quartz + 0.5 H₂O were calculated as 49±7, 38±4, and 23±6 at 600, 650, and 700 °C, respectively. The alteration of albite to andalusite occurs at much lower HCl concentrations than is required to alter K-feldspar to andalusite for equivalent molar concentrations of chloride salts. Thus, HCl concentrations of brines within the potassic or sericitic alteration zones can be much higher than in albite alteration zones. We hypothesize that albite-rich mineral zones on the flanks of igneous intrusions may be the result of regional flow of low acidity meteoric fluid or of a descending magmatic brine with an HCl content already neutralized by mineral alteration reactions near the cupola.