Water solubility and chlorine partitioning in Cl-rich granitic systems: Effects of melt composition at 2 kbar and 800°C

Abstract

Experiments have been conducted in the system Na 2O-K 2O-Al 2O 3-SiO 2-H 2O-Cl 2O −1 at 2 kbar and 800°C to determine the effects of melt composition on the solubility of water in haplogranite melt and on the distribution of Cl between aqueous fluid and haplogranite melt. The melts studied include peraluminous, subaluminous and peralkaline haplogranites and quartz-albite and quartz-orthoclase compositions. The solubility of water at 2 kbar and 800°C in water-saturated haplogranite melts varies strongly as a function of melt composition. Water solubility in melt decreases in the order strongly peralkaline haplogranite &> quartz-albite composition ≈ moderately peralkaline haplogranite ≈ peraluminous haplogranite ≈ subaluminous haplogranite &> quartz-orthoclase composition. D Cl (wt% Cl in fluid/wt% Cl in melt) is a strong function of system composition. D Cl increases as the concentration of Cl in the system increases. A comparison of the distribution of Cl between haplogranite melts of subaluminous, peraluminous, and peralkaline compositions and aqueous fluids shows that with similar concentrations of Cl in the system; D Cl is largest for subaluminous melts ( A/NK ≈ 1) having relatively high K 2O concentrations. As the A/NK of melt increases or decreases from 1 or as the N/NK of the melt increases, both D Cl and the concentration of Cl in the fluid decrease. The experimental data imply that complexes involving Cl and Na in hydrous, subaluminous granite melts are dominant. Furthermore, the data imply that in hydrous, peraluminous granite melts, Cl complexes with network modifying A1 3+ and in hydrous, peralkaline granite melts Cl complexes with network modifying Na + ± K + . On a molar basis, Cl apparently complexes with network modifying A1 3+ in peraluminous melts as strongly as it does with Na + and K + in peralkaline melts. Experimental studies of Cl partitioning in granitic systems suggest that the strongest enrichment of magmatic-hydrothermal fluids in Cl and ore metals complexed with Cl will most likely occur in fluids exsolved from subaluminous granite magmas that are relatively enriched in K 2O, SiO 2, and Cl, and relatively depleted in F, CO 2, and Na 2O.