Hydroxyl, Cl, and F partitioning between high-silica rhyolitic melts-apatite-fluid(s) at 50–200 MPa and 700–1000 °C

Hydrothermal experiments were conducted with fluid- and apatite-saturated, high-silica rhyolitic melts at ca. 700–1000 °C and 50–200 MPa to determine the distribution of H2O/OH, Cl, and F between melt, apatite, aqueous vapor, brine, or vapor plus brine. Seed grains of fluorapatite (1–3 µm diameter) were added to starting charges to serve as apatite nucleation sites. CaHPO4 and Ca(OH)2 were used to stimulate apatite crystallization, and temperature was cycled daily, ±10 to ±15 °C, to promote growth of relatively equant apatite crystals large enough for electron probe microanalysis (EPMA). The experiments were conducted with gold capsules and run in cold-seal pressure vessels on a hydrothermal line and an internally heated gas pressure vessel for durations of 165 to 1149 h.