Partitioning behavior of chlorine and fluorine in felsic melt-fluid(s)-apatite systems at 50 MPa and 850-950 °C

- Chlorine partitions in favor of fluids > apatite > rhyodacitic melt at 50 MPa and ca. 900 °C.
- Fluorine partitions in favor of apatite > rhyodacitic melt at 50 MPa and ca. 900 °C.
- Chlorine partitioning between apatite-melt-fluids varies with the Cl content of the system.
- Chlorine partitioning between apatite-melt-fluids varies with the molar (Al2O3/Na2O + K2O + CaO) and (Na2O/Na2O + K2O) of rhyodacitic melt.
- Chlorine partitions more strongly in favor of apatite relative to rhyodacitic melt or fluids at 50 versus 200 MPa.

The partitioning of Cl between apatite, felsic melts, and aqueous fluids (vapor ± brine), and the partitioning of F between apatite and felsic melt were investigated experimentally at 50-58 MPa and 850-950 °C. Chlorine partitioning between apatite, melt, and fluid(s) is a function of the Cl content of the system, melt composition, Cl solubility in the melt, and pressure. For most of the 23 equilibrium experiments, the Cl content of apatite increases as a simple and linear function of the Cl concentration of the melt. The partitioning behavior of Cl between apatite and melt (DClapat/mt by wt. fractions of [Cl in apatite/Cl in melt]) ranges from 3 to 32. Furthermore, weight fractions of DClapat/fluid(s) range from 0.07 to 1.3 and most are less than unity. Both DClapat/mt and DClapat/fluid(s) vary with the molar (Al2O3/(Na2O + K2O + CaO)) ratio of the melt at 50 MPa. Chlorine partitions more strongly in favor of melt as the molar (Al2O3/(Na2O + K2O + CaO)) ratio decreases below 0.9, and Cl distribution between apatite and fluid(s) varies non-linearly with the molar (Na2O/(Na2O + K2O)) ratio of the coexisting silicate melts. Weight-based values of DClfluid(s)/mt range from 3 to 31. Chlorine partitioning exhibits non-Nernstian behavior; the distribution of Cl between fluid(s) and melt at 50 MPa varies with the Cl concentration of the system and the Cl solubility in the silicate melt. Similar behavior was observed previously with this rhyodacitic melt composition at 200 MPa (Webster et al., 2009b). Comparison with prior research at 200 MPa shows that Cl is distributed increasingly in favor of apatite, relative to felsic melt or vapor ± saline liquid, as pressure decreases from 200 to 50 MPa.The partitioning of F between apatite and silicate melt is masked by analytical imprecision. No distinct relationships with system composition or pressure are apparent for molar DXFapat/mt ranging from 13 to 219; F partitions in favor of apatite relative to melt at all conditions.We estimate pressures of apatite crystallization and Cl contents of melts and fluid(s) coexisting with apatite by comparing these 50-MPa results with published data involving apatite, rhyodacitic melt, and fluid(s) at 200 MPa. These experimental constraints are applied to prehistoric and recent Cl-enriched apatites and silicate melt inclusions of magmas of Augustine volcano to investigate pre-eruptive Cl concentrations of magma and pressures of apatite crystallization.