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, D _i , for F and Cl were calculated as the parts per million by weight (ppmw) of i in the fluid/parts per million by weight (ppmw) of i in the melt.

D _Cl ranges 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 X _H2O ^fl (molar H₂O/H₂O + CO₂ in fluid), temperature, pressure, and Cl in the fluid and melt increase. Cl partitions in favor of haplogranite melts that contain >7 wt % F and ⩽1,200 ppm Cl at 1,000°C and 2 kbar.

D _F ranges 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 X _H2O ^fl decrease. However, F partitions in favor of aqueous fluids relative 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 H₂O, CaO, and ferromagnesian contents in the magma are low, and if the pressure at which water saturation occurs is high.