Oxygen isotope salt effect at high pressure and high temperature

The isotope salt effects of NaCl, CaCl2 and dissolved minerals at high pressure and high temperature were experimentally studied. The influences of NaCl and CaCl2 on the oxygen isotopic fractionation between quartz and water and/or between calcite and water were measured at 5 Kbar and 15 Kbar with temperature from 300°C to 750°C. CaCl2 has larger isotope salt effect than NaCl. While the presence of NaCl systematically changes the isotopic fractionation between quartz and water, NaCl shows no influence on the isotopic fractionation between calcite and water.;The oxygen isotope salt effects of dissolved quartz, calcite and phlogopite at 15 Kbar and 750°C were measured in the systems quartz-calcite-water and phlogopite-calcite-water. Under this condition, the oxygen isotope salt effects of the three dissolved minerals change from about 0.7‰ to 2.1‰. In both three-phase hydrothermal systems, the equilibrium fractionation factors between the pairs of minerals are the same as those obtained by anhydrous direct exchange between each pair of minerals, proving the use of carbonate as exchange medium also provide correct isotopic fractionation for a mineral pair.;When the oxygen isotope salt effects of two minerals are the same and can cancel one another, the use of water as indirect exchange medium provide correct fractionation for the two minerals. However, when the oxygen isotope salt effects of two minerals are different, the use of water as indirect exchange medium will give erroneous fractionation between the two minerals. The isotope salt effect of dissolved mineral is also the main reason for the previous observation that the experimentally calibrated oxygen isotopic fractionations between a mineral and water are systematically 1.5 to 2‰ more positive than the results of theoretical calculation. Dissolved minerals greatly affect the isotopic fractionation in mineral-water system. If the presence of a solute changes the solubility of a mineral, the real oxygen isotope salt effect of the solute at high pressure and high temperature cannot be correctly derived by using the mineral as reference phase.