Chlorine and fluorine partitioning between apatite and sediment melt at 2.5 GPa, 800 °C: A new experimentally derived thermodynamic model

The partitioning behavior of Cl and F between apatite and sediment melt has been investigated by performing piston-cylinder experiments at 2.5 GPa, 800 degrees C using a hydrous experimental pelite staring material (EPSM) with similar to 7 wt% H2O and variable Cl (similar to 0, 500, 1000, 2000, or 3000 ppm) and F (similar to 0, 700, or 1500 ppm) contents, relevant for subduction zone conditions. Cl and F partitioning between apatite and melt is non-Nernstian, with DClAp-melt varying from 1.9-10.6 and DFAp-melt varying from 16-72. In contrast, Cl and F partition coefficients between phengite/biotite and melt (DClPhen-melt, DClBi-melt, DFPhen-melt, and DFBi-melt) were determined to be 0.24 +/- 0.01, 0.86 +/- 0.05, 1.4 +/- 0.1, and 3.7 +/- 0.4, respectively. The Nernstian partitioning of Cl and F between phengite/biotite and melt suggests ideal mixing of F, Cl, and OH in phengite, biotite, and melt. Exchange coefficients for F, Cl, and OH partitioning between apatite and melt were determined, with K-d(ClOHK)Ap-meltd = 19-49, K-dFOHK(Ap-melt) = 164-512, and Kd(F-Cl)(Ap-meltd) = 7-21. The evident variation of K-d values was attributed to non-ideal mixing of F, Cl, and OH in apatite. A regular ternary solution model for apatite was developed by modeling the variation of K-d values for experiments from this study and those from Webster et al. (2009) and Doherty et al. (2014). Positive values (similar to 15 to similar to 15 kJ/mol) obtained for Margules parameters W-Cl-OH(Ap), and W-F-OH(Ap) at low-pressure conditions (0.2 GPa, 0.05 GPa, and 900 degrees C) are in contrast to zero or negative values at 2.5 GPa, 800 degrees C. Based on a thermodynamic framework for F, Cl, and OH exchange between apatite and melt, using values for -Delta(r)G(Cl-OH)(o) (P,T), -Delta(r)G(F-OH)(o)(P, T), -Delta(r)G(F-Cl)(o)(P, T), W-Cl-OH(Ap), W-F-Cl(Ap), and W-F-OH(Ap) obtained through regression, F and Cl contents in melt can be derived from apatite compositions.