PARTITIONING OF FLUORINE AND CHLORINE BETWEEN APATITE AND AQUEOUS FLUIDS AT HIGH-PRESSURE AND TEMPERATURE - IMPLICATIONS FOR THE F AND CL CONTENT OF HIGH P-T FLUIDS

Experiments in a piston-cylinder apparatus equilibrated natural fluorapatite with aqueous HCl, NaCl, NaOH, Na2CO3 and CO2-H2O mixtures at 1-2.0 GPa and 950-1050-degrees-C. Post-experiment apatite compositions were determined by electron microprobe and fluid compositions were calculated from mass-balance.This enabled calibration of the fluorine and chlorine contents of apatite coexisting with a variety of fluids at high P and T, and provides better constraints on the halogen composition of aqueous fluids which may exist in the deep crust and upper mantle. Results obtained at 2.0 GPa and 1050-degrees-C indicate that the ratio X(ClAp)/X(OHAp) in apatite (X(FAp), X(ClAp) and X(OHAp) are the mole fractions of fluor-, chlor- and hydroxyapatite) is proportional to the chlorine content of the fluid up to approximately 1 (H2O-CO2), approximately 5 (H2O-HCl) and approximately 15 (H2O-NaCl) wt% Cl. In contrast, the ratio X(FAp)/X(OHAp) was found to vary in proportion to the fluorine content of the fluid only at concentrations of less-than-or-equal-to 0.15 (H2O-HCl) and less-than-or-equal-to 0.4 (H2O-NaCl) wt% F. Arising from changes in the activities of aqueous HCl and HF, the results of this study demonstrate that fluid composition, in addition to the absolute abundances of F and Cl in the fluid, will determine the amounts of these elements incorporated into coexisting apatite. Specifically, relatively low Cl and F concentrations in acidic (H2O-HCl) fluids are required to obtain high ratios of X(ClAp)/X(OHAp) and X(FAp)/X(OHAp) in apatite, whereas much higher abundances of Cl and F in basic fluids (i.e., Na-bearing) are required to achieve the same result. In addition, resulting from dilution of the aqueous phase with CO2 and the subsequent reduction in the activity of H2O, apatites with high ratios of X(ClAp)/X(OHAp) and X(FAp)/X(OHAp) may be produced by CO2-H2O fluids with relatively low levels of Cl or F. Application of these results to apatites from a suite of ultramafic xenoliths from southeastern Australia indicates chlorine abundances of 500-3500 ppm in the fluid phase and points to chlorine as a potentially important component of some mantle fluids.