Oxygen isotope fractionation between apatite and aqueous-phase phosphate: 20–45 °C

Replacement of biogenic CaCO3 by apatite is an important mode of authigenic apatite formation in marine sediments, active along the Peru Margin today. Synthesis experiments mimicking this process were undertaken in abiotic inorganic systems in order to determine the oxygen (O) isotope fractionation between apatite and aqueous-phase inorganic phosphate [(PO4)aq]. CaCO3 solids were added to PO43− solutions made with 18O-labeled waters and O isotope fractionations between solid- and aqueous-phase PO43− were determined in a series of experiments at 20–45 °C. Results of δ18O analyses of apatite synthesis systems indicate that O isotope fractionation between solid- and aqueous-phase PO43− is: (1) not affected by variations in water δ18O values; (2) initially non-equilibrium; and (3) relatively small (∼ + 1‰). Fractionations determined from long-term (up to 6 years) room-temperature experiments were ∼ + 1‰. Due to initial rapid precipitation, short experimental run periods (1 week to 4 months), resulted in fractionations between apatite and (PO4)aq that were initially reversed from those of long-term experiments. Over time and with progressive recrystallization, these fractionations evolved to reach values consistent with long-term experiments.