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 fluora patite with aqueous HCl, NaCl, NaOH, Na2CO3 and CO2-H2O mixtures at 1- 2.0 GPa and 950-1050-degrees-C. Post-experiment apatite compositions w ere determined by electron microprobe and fluid compositions were calc ulated from mass-balance.This enabled calibration of the fluorine and chlorine contents of apatite coexisting with a variety of fluids at hi gh 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 rati o 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 th e chlorine content of the fluid up to approximately 1 (H2O-CO2), appro ximately 5 (H2O-HCl) and approximately 15 (H2O-NaCl) wt% Cl. In contra st, the ratio X(FAp)/X(OHAp) was found to vary in proportion to the fl uorine content of the fluid only at concentrations of less-than-or-equ al-to 0.15 (H2O-HCl) and less-than-or-equal-to 0.4 (H2O-NaCl) wt% F. A rising from changes in the activities of aqueous HCl and HF, the resul ts of this study demonstrate that fluid composition, in addition to th e absolute abundances of F and Cl in the fluid, will determine the amo unts of these elements incorporated into coexisting apatite. Specifica lly, 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(OH Ap) in apatite, whereas much higher abundances of Cl and F in basic fl uids (i.e., Na-bearing) are required to achieve the same result. In ad dition, 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 flui ds with relatively low levels of Cl or F. Application of these results to apatites from a suite of ultramafic xenoliths from southeastern Au stralia indicates chlorine abundances of 500-3500 ppm in the fluid pha se and points to chlorine as a potentially important component of some mantle fluids.