Oxygen isotope exchange between dissolved phosphate and water at temperatures <= 135 degrees C: Inorganic versus biological fractionations

Oxygen isotope fractionations have been measured experimentally between dis solved orthophosphate ions H2PO4- and water from 50 degrees C to 135 degree s C. The derived equilibrium fractionation equation reveals that biogenic a patites are systematically O-18-depleted (8 parts per thousand at 20 degree s C) compared with inorganic dissolved phosphate. The activation energy is relatively high at 134 kJmol(-1) and the kinetics of the homogeneous inorga nic exchange reaction are very slow (k = 10(-13) to 10(-11) s(-1)) when ext rapolated to surface temperatures. In seawater, the oxygen isotope composit ion of dissolved phosphate is not in equilibrium with water; its compositio n should directly reflect source contributions or biological recycling. Any deviation from the composition of the inorganic sources may be attributed to biological recycling of this nutrient. The original isotopic composition s of fossils may be modified in less than 10(5) years at a temperature of 2 0 degrees C through mechanisms of dissolution and precipitation in the pres ence of crustal aqueous fluids. The differences in oxygen isotope fractiona tions between biogenic or inorganic phosphate and water are applied to re-e valuate the significance of delta(18)O values of Phanerozoic and Precambria n phosphate deposits. Copyright (C) 1999 Elsevier Science Ltd.