EXPERIMENTAL-DETERMINATION OF THE STABILITY-CONSTANTS OF NASO4- AND NAB(OH)(4)(0) IN HYDROTHERMAL SOLUTIONS USING A NEW HIGH-TEMPERATURE SODIUM-SELECTIVE GLASS-ELECTRODE - IMPLICATIONS FOR BORON ISOTOPIC FRACTIONATION

A new sodium-selective glass electrode was used to measure the association constants of the sodium-sulfate (NaSO4-) and sodium-borate (NaB(OH)(4)(0)) ion pairs at temperatures from 50 degrees to 2000 degrees C. These data were used to generate, within the framework of the revised HKF model, NaSO4- and NaB(OH)(4)(0) equation of state parameters. Calculations of NaB(OH)(4)(0) association constants at temperatures up to 500 degrees C and pressures up to 5 kbar demonstrates that this ion pair dominates boron speciation in sodium-rich hydrothermal fluids at pH greater than or equal to 8. Because it has been shown that boron isotopic fractionation in natural processes is strongly affected by its aqueous speciation, the formation of NaB(OH)(4)(0) complexes cannot be neglected when using boron isotopes to estimate water-rock interactions occurring in the continental and oceanic crust at moderate and high temperatures.