Oxygen isotope fractionations involving apatites: Application to paleotemperature determination

The increment method is adapted to calculate oxygen isotope fractionations in phosphate minerals by taking calcite as a reference. The results show a significant difference in oxygen isotope partitioning between OH-apatite, Cl-apatite and F-apatite. The substitution of halogen for hydroxyl in apatites can thus have a considerable influence on oxygen isotope properties. The obtained order of 18O enrichment is: Cl-apatite < F-apatite < OH-apatite < monazite < xenotime. At 25°C, the oxygen isotope fractionation between OH-apatite and F-apatite is 0.7‰, and that between OH-apatite and Cl-apatite is 1.8‰. Three sets of internally consistent fractionation factors for the systems phosphate-water, quartz-phosphate and calcite-phosphate are acquired, respectively, for the temperature range of 0° to 1200°C. The inspection of existing isotope data demonstrates that the theoretical calibrations involving apatites are applicable to the paleotemperature determination of ancient oceans.