VARIATIONS OF THE OXYGEN-ISOTOPE FRACTIONATION BETWEEN NACO(3-) AND WATER DUE TO THE PRESENCE OF NACL AT 100-300-DEGREES-C

The oxygen isotope fractionation between NaCO3- and water in the prese nce of NaCl concentrations up to 4 M at temperatures between 100-degre es and 300-degrees-C has been investigated using a new, high-temperatu re titration technique. Dissolved carbonate is equilibrated at tempera ture with the experimental solution, and then the carbonate is quantit atively precipitated as BaCO3 by the injection of BaCl2 solution at te mperature. The isotopic composition of the BaCO3 reflects the composit ion of the free water, and hence reflects changes in the oxygen isotop e composition of the free water due to the presence of NaCl. Under the experimental conditions used in this study, NaCO3- is the dominant di ssolved carbonate species, and the oxygen isotope fractionation betwee n NaCO3- and water between 100-degrees and 300-degrees-C is given by: 1000 In alpha = 2.7 (10(6)T-2)-5.7 where T is temperature in K. Values of DELTAO-18(NaCl(solution-water)) are generally small and negative ( between - 1 and 0 parts per thousand). The small values of DELTAO-18(N aCl(solution-water)) obtained in this study are consistent with data f rom the studies of Kendall et al. (1983), Zhang et al. (1989), and Hor ita et al. (1993a), rather than the studies of Truesdell (1974) and Ka zahaya (1986), which indicate values of DELTAO-18(NaCl(solution-water) ) up to + 3 parts per thousand for 4 M NaCl solutions at temperatures as high as 250-degrees-C. The results have implications for the use of oxygen isotope geothermometers in the presence of brines, and hence t o the quantification of temperatures and water compositions in a varie ty of research areas, such as studies of ore deposits and sedimentary diagnesis, and to the quantification of water/rock ratios in geotherma l and metamorphic fluid flow systems.