The effects of phosphate speciation on both rates of isotopic exchange
and oxygen isotope equilibrium fractionation factors between aqueous
phosphate and water were examined over the temperature range 70 to
180degreesC. Exchange between phosphate and water is much faster at low
pH than at high pH, an observation that is similar to what has been
observed in the analogous sulfate-water system. Oxygen isotope
fractionations between protonated species like H3PO4 and H2PO4- that are
dominant at relatively low pH and species like PO43- and ion pairs like
KHPO4- that are dominant at relatively high pH, range between 5 and 8%
at the temperatures of the experiments. In aqueous phosphate systems at
equilibrium, O-18/O-16 ratios increase with increasing degree of
protonation of phosphate. This effect can be explained in part by the
relative magnitudes of the dissociation constants of the protonated
species. Under equilibrium conditions, carbonate in solution or in solid
phases concentrates O-18 relative to orthophosphate in solution or in
solid phases at all temperatures, supporting the traditional view that
biogenic phosphate is precipitated in near oxygen isotope equilibrium
with body/ambient aqueous fluids with no attendant vital effects.
Copyright (C) 2003 Elsevier Ltd.