Partitioning behavior of chlorine and fluorine in the system apatite-silicate melt-fluid

The partitioning behavior of Cl among apatite, mafic silicate melt, and aqueous fluid and of F between apatite and melt have been determined in experiments conducted at 1066 to 1150 °C and 199-205 MPa. The value of D_Cl^apatite/melt (wt. fraction of Cl in apatite/Cl in melt) ≈0.8 for silicate melt containing less than ∼3.8 wt.% Cl. At higher melt Cl contents, small increases in melt Cl concentration are accompanied by large increases in apatite Cl concentration, forcing D_Cl^apatite/melt to increase as well. Melt containing less than 3.8% Cl coexists with water-rich vapor; that containing more Cl coexists with saline fluid, the salinity of which increases rapidly with small increases in melt Cl content, analogous to the dependency of apatite composition on melt Cl content. This behavior is due to the fact that the solubility of Cl in silicate melt depends strongly on the composition of the melt, particularly its Mg, Ca, Fe, and Si contents. Once the melt becomes “saturated” in Cl, additional Cl must be accommodated by coexisting fluid, apatite, or other phases rather than the melt itself. Because Cl solubility depends on composition, the Cl concentration at which D_Cl^apatite/melt and D_Cl^fluid/melt begin to increase also depends on composition. The experiments reveal that D_F^apatite/melt ≈3.4. In contrast to Cl, the concentration of F in silicate melt is only weakly dependent on composition (mainly on melt Ca contents), so D_F^apatite/melt is constant for a wide range of composition.The experimental data demonstrate that the fluids present in the waning stages of the solidification of the Stillwater and Bushveld complexes were highly saline. The Cl-rich apatite in these bodies crystallized from interstitial melt with high Cl/(F + OH) ratio. The latter was generated by the combined processes of fractional crystallization and dehydration by its reaction with the relatively large mass of initially anhydrous pyroxene through which it percolated.