Partitioning of F and Cl between magmatic hydrothermal fluids and highly evolved granitic magmas

The distribution of F and Cl between aqueous fluids and melts of haplogranite and topaz rhyolite composition was determined experimentally at 0.5 to 5 kbar and 775° to 1,000°C. The distribution coefficients, Di, for F and Cl were calculated as the parts per million by weight (ppmw) of iin the fluid/parts per million by weight (ppmw) of iin the melt. DClranges from 0.8 to 85; however, under typical geologic conditions, Cl partitions more strongly into an aqueous fluid relative to F-bearing granitic melts. Cl partitions increasingly in favor of the fluid as F in the fluid and magma decrease and as the XH 2Ofl(molar H 2O/H 2O + CO 2in fluid), temperature, pressure, and Cl in the fluid and melt increase. Cl partitions in favor of haplogranite meltsthat contain >7 wt % F and ⩽1,200 ppm Cl at 1,000°C and 2 kbar. DFranges from 0.2 to >1.0; however, F typically is concentrated in granitic melts relative to aqueous fluids. F concentrates more strongly into topaz rhyolite melt as pressure, F in the fluid, and melt, temperature, and the XH 2Ofldecrease. However, F partitions in favor of aqueous fluidsrelative to topaz rhyolite melts at 800°C and 2 kbar, if the melt contains ⩾7 wt % F. Computations indicate that extreme enrichments in F (>4 wt %) and Cl (>5,000 ppm) may occur in magmas and in associated magmatic hydrothermal fluids during the end stages of crystallization of topaz rhyolite magmas and magmas associated with Climax-type molybdenum deposits if the initial H 2O, CaO, and ferromagnesian contents in the magma are low, and if the pressure at which water saturation occurs is high.