Estimation of aqueous HCl and Cl concentrations in felsic systems

Two models are presented which can be used to make order of magnitude estim ates of the concentration of HCl and Cl in the magmatic volatile phase (MVP ). One model is based primarily upon aplite chemistry, and the other is bas ed primarily on apatite chemistry; the models are not entirely independent, and additional information can be gained by using the models in conjunctio n with one another. Combining both methods allows for the calculation of a model bulk Cl concentration of magmatic vapor. Both models are applied to a natural system: the Billy Lake granite and Rush Creek quartz-monzodiorite in the Sierra Nevada Batholith of eastern California. The bulk composition of aplite and other granitic rocks of(vapor-saturated) near-minimum melt co mpositions can be used to estimate maximum HCl/total Cl for the magmatic va por phase. The composition of the aplites in the Billy Lake-Rush Creek (BLR C) system has been used to determine model HCl/KCl acid HCl/NaCl for the va por in equilibrium with the aplite melts; both ratios are on the order of u nity (1.1). These values suggest that the maximum HCl/Cl similar to 1/3, co rresponding to a concentration of HCl in the magmatic vapor of similar to 0 .02 molal if the magmatic system was saturated with a brine phase. Apatite compositions can be used to estimate the HCl concentration in the magmatic vapor as a function of temperature. Based on the P2O5 and SiO2 concentratio ns in the aplites, and the solubility of apatite in silicate melts, apatite began to crystallize at 820 degrees C in the aplites, which also yields an estimate of 0.02 molal for the concentration of HCl for the BLRC system by the apatite method. Within error, both models produce the some result. (C) 1999 Elsevier Science B.V. All rights reserved.