Fluid-melt interactions involving Cl-rich granites: Experimental study from 2 to 8 kbar

Abstract

Experiments have been conducted in the system Na 2O-K 2O-Al 2O 3-SiO 2-H 2O-Cl 2O −1at 2, 4 kbar (800 and 1000°C) and 6, 8 kbar (800°C) to determine thermodynamic constraints on fluid-melt interaction and to determine the effects of pressure ( P) and temperature ( T) on Cl partitioning between aqueous fluid and subaluminous and peralkaline haplogranite melts. The Cl concentration of the run product glasses was determined by electron microprobe, and the H 2O concentration of some glasses was determined by ion microprobe. The Cl concentration of the aqueous fluid was computed by mass balance and was also analyzed by chloridometer for several runs; agreement between the two methods is within ±9 relative %. D Cl (wt% Cl in aqueous fluid/wt% Cl in granite melt) increases strongly with increasing P for P < 6 kbar , increasing concentrations of Cl in the system, and decreasing T. Previous work, however, indicates the effect of T on D Cl in F-bearing granites within this P–T range is opposite to that observed here. The experimental results indicate that the concentration of Cl in granitic melts may reach a maximum limiting value as Cl-rich magmas crystallize. Granitic systems exhibiting a maximum concentration of Cl in the melt must either contain crystalline alkali chlorides, molten alkali chlorides, or coexisting liquid plus vapor or the melt must contain the solubility limit for Cl. The experimental data suggest that at 2 kbar and 800°C a single, supercritical fluid is stable, if the fluid contains ≤67 wt% NaCl and KCl. Conversely, at 2 kbar and 1000°C two immiscible phases (i.e., saline liquid and relatively alkali chloride-poor vapor) coexist with haplogranite melt if the combined liquid and vapor contain from approximately 10 to 55 wt% NaCl and KCl. Computed activities of H 2O in the experimental melts ( Burnham, 1981; Nekvasil, 1986) constrain the activity of H 2O in the associated saline fluids at magmatic P and T. At 2 kbar and 800°C, H 2O behaves ideally with up to 10 mol% NaCl and KCl in the aqueous fluid. As the NaCl and KCl concentration of the aqueous fluid increases from 10 to 40 mol%, the activity of H 2O exhibits small but increasingly positive deviations from ideality. Application of experimental data to Cl-rich and F-poor, mineralizing granitic systems suggests that Cl will be most strongly enriched in ore fluids at relatively low temperature (between 800 and 1000°C) and relatively high pressure (for P < 6 kbar ). The data also suggest that inasmuch as granitic systems contain a single aqueous fluid at T ≤ 800°C, 2 kbar, and with ≤67 wt% NaCl and KCl in fluid, the deposition of ore minerals from such fluids cannot be a direct result of boiling.