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

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Experiments in a piston-cylinder apparatus equilibrated natural fluorapatite with aqueous HCl, NaCl, NaOH, Na 2CO 3 and CO 2 H 2O mixtures at 1–2.0 GPa and 950–1050°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°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 ∼ 1 (H 2O CO 2), ∼ 5 (H 2O HCl) and ∼ 15 (H 2O 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 ≤ 0.15 (H 2O HCl) and ≤ 0.4 (H 2O 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 (H 2O 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 CO 2 and the subsequent reduction in the activity of H 2O, apatites with high ratios of X ClAp/X OHAp and X FAp/X OHAp may be produced by CO 2 H 2O 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.