EXSOLUTION OF IMMISCIBLE VAPOR AND LIQUID-PHASES FROM A CRYSTALLIZING SILICATE MELT - IMPLICATIONS FOR CHLORINE AND METAL TRANSPORT

The compositional variation of a Cl-bearing aqueous fluid exsolving from a crystallizing silicate melt is evaluated for the system haplogranitic melt-(Na, K)Cl-H2O, with attention to the effect of vapor-liquid immiscibility on the aqueous fluid. Above critical pressures for the aqueous fluid (P > 1.5 kb at 800-degrees-C), where the fluid is single phase, the Cl concentration in the aqueous fluid (C(cl)aq) gradually decreases with crystallization of a silicate melt under Rayleigh fractionation, as demonstrated by previous studies. However, the variation of C(Cl)aq is quite different below critical pressures, where vapor-liquid immiscibility occurs, in particular at low pressures (P < 1.3 kb at 800-degrees-C). Under such conditions, a vapor phase will exsolve when the initial Cl/H2O ratio in a melt is small and C(Cl)aq will increase with crystallization. A high Cl/H2O ratio at low pressure results in liquid exsolution and decreasing C(Cl)aq with crystallization. Eventually both vapor and liquid phases will exsolve simultaneously regardless of the initial exsolving phase. During this simultaneous exsolution, the composition of the exsolving vapor and liquid phases, the mass ratio of these phases, and the Cl concentration of the silicate melt will all remain constant. Comparison of the model and published experimental data indicates that the simultaneous exsolution of vapor and liquid may occur from a normal rhyolite crystallized at pressures below the critical pressure for the aqueous fluid. Therefore, simultaneous exsolution of the immiscible aqueous phases is likely to be a common phenomenon in magmatic hydrothermal systems at conditions for the vapor-liquid immiscibility. Model calculations at 0.6 and 1.2 kb indicate that major amounts of Cl in the system will be distributed to the liquid phase particularly at the lower pressure. This suggests that the majority of chloride-complexed metals exsolving from a magma may be transported in the liquid phase under shallow crustal conditions.