Oxygen Isotope Fractionations Between Dissolved Phosphate and Apatite at 22-45 oC : Effects of P:Ca Ratio and Recrystallization

Continued expansion of the range of applications of phosphate O isotope ratios, for example, to studies of biogeochemical cycling of P and microbial P metabolism, will require full elucidation of the O isotope systematics and reaction pathways of the PO_4(aq)-organophosphate-apatite-water system within living organisms as well as in abiotic environments. Although previous studies provide many empirical determinations of the overall fractionation between biogenic phosphates (e.g., teeth, shells) and body fluids, the oxygen isotope effects of important intermediate steps, and O isotope exchange reactions involving PO_4(aq) and water, and fractionations between PO_4(aq) and apatite, have received little or no study. Oxygen isotope fractionations between PO_4(aq) and apatite have been determined in controlled experiments at 22-45 ^oC. PO_4(aq) - apatite fractionations from long-term (4 yrs ) room temperature experiments were ~1 permil. Over shorter experimental run periods (1 week to 4 months), fractionations between PO_4(aq) and apatite were initially more complex and evolved over time with progressive recrystallization of the apatite. PO_4(aq) - apatite fractionations were observed to be influenced by the initial P:Ca ratio as well as initial precipitation rates. The implications of these experimental results for application of phosphate O isotope ratios to studies of P diagenesis, authigenic apatite precipitation, and biomineralization will be discussed.