Experimental constraints on the partitioning of Cl between topaz rhyolite melt and H2O and H2O + CO2 fluids: New implications for granitic differentiation and ore deposition

Abstract

The partitioning of Cl between superliquidus melts of topaz rhyolite and fluids consisting of H 2 O and H 2 O + CO 2 has been determined experimentally at pressures of 0.5 to 5 kbar and temperatures of 725° to 950°C with f O 2 controlled near NNO. The major element and Cl contents of a starting material glass and the run product glasses were determined by electron microprobe. The concentration of Cl and the major elements in the fluid are calculated by mass balance. Cl partitions in favor of the fluid for all experimental run conditions. For natural abundances of Cl in a magmatic system, ( $ ̌ −2000 ppm Cl in melt and $ ̌ −40 g Cl/kg fluid), D Cl ranges from 2 to 20. These values are much lower than those previously reported in the literature. D Cl increases as the Cl content of the melt and fluid increases, as the F content of the system decreases, as T and P increase, as the melt becomes more peraluminous, and as ( H 2 O H 2 O + CO 2 ) of the fluid increases. Peraluminous granitic melts that are in equilibrium with aqueous fluids at 800°C and 2 kbar achieve a maximum concentration of Cl in the melt at 2500 ± 100 ppm Cl (independent of the F content). In experiments with Cl ranging from 40 to 300 g Cl/kg fluid (2000 to 2500 ppm Cl in melt) the D Cl ranges from 20 to 130 and is consistent with literature values. The experimental data also provide strong constraints on the speciation of Cl in granitic melts and coexisting fluids. As a topaz rhyolite melt that contains < −2500 ppm Cl equilibrates with HCl-enriched solutions, Cl is dissolved into the melt and H + exchanges with the cations Na, K and Fe, which leave the melt as chloride complexes. During the equilibration of HCl-enriched fluids with melts that contain > −2500 ppm Cl, no additional Cl dissolves into the melt and no additional Na, K or Fe are removed from the melt. HCl does not become a dominant fluid species until the melt achieves the solubility maximum for Cl. For fluid-saturated, silicic melts of granite composition with H 2 O activities near 1, the strong partitioning of Cl into the fluid also causes partitioning of some ore metals, trace elements and the major elements Na, K and Fe (± Ca) into the fluid: This is maximized for granitic melts that are strongly peraluminous, low in F and highly enriched in Cl; partitioning of Cl into aqueous fluids is maximized at high P and T . If the H 2 O-rich fluid is diluted with CO 2 ( x h 2 O fl > − 0.45) the concentration of Cl in the fluid decreases strongly.