Oxygen isotope fractionation in phosphates: the role of dissolved complex anions in isotope exchange^†

Abstract

Oxygen isotope fractionation factors for phosphates were calculated by means of the increment method. The results suggest that Ag₃PO₄ and BiPO₄ are enriched in ¹⁸O relative to AgPO₄, and the three phosphates are consistently depleted in ¹⁸O relative to Ba₃[PO₄]₂; fluorapatite and chlorapatite exhibit a similar behaviour of oxygen isotope fractionation with consistent enrichment of ¹⁸O relative to hydroxyapatite. The valence, radii and coordination of metal cations play a quantitative role in dictating the ¹⁸O/¹⁶O partitioning in these phosphates of different compositions. The calculated fractionation factors for the Ag₃PO₄–H₂O system are in agreement with experimental determinations derived from enzyme-catalysed isotope exchange between dissolved inorganic phosphate and water at the longest reaction durations at low temperatures. This demonstrates that the precipitated Ag₃PO₄ has completely captured the oxygen isotope fractionation in the dissolved inorganic phosphate. The calculated fractionation factors for the F/Cl-apatite–water systems are in agreement with the enzyme-catalysed experimental fractionations for the dissolved phosphate–water system at the longest reaction durations but larger than fractionations derived from bacteria-facilitated exchange and inorganic precipitation experiments as well as natural observations. For the experimental calibrations of oxygen isotope fractionation involving the precipitation of dissolved phosphate species from aqueous solutions, the fractionation between precipitate and water is primarily dictated by the isotope equilibration between the dissolved complex anions and water prior to the precipitation. Therefore, the present results provide a quantitative means to interpret the temperature dependence of oxygen isotope fractionation in inorganic and biogenic phosphates.

Keywords:

• biogenic apatite
• dissolved phosphate
• exchange kinetics
• fractionation factor
• isotope fractionation
• oxygen-18
• precipitated phosphate
• structural carbonate

Acknowledgements

This paper is dedicated to Prof. J. Hoefs for his lifelong contributions to stable isotope geochemistry. I thank him for hosting me to study at the University of Göttingen in Germany. Thanks are due to Ruth Blake and Zhengrong Wang for their comments that greatly helped to improve this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

This study was supported by the Natural Science Foundation of China [grant number 41621022].