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 associa tion constants of the sodium-sulfate (NaSO4-) and sodium-borate (NaB(O H)(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. Cal culations of NaB(OH)(4)(0) association constants at temperatures up to 500 degrees C and pressures up to 5 kbar demonstrates that this ion p air dominates boron speciation in sodium-rich hydrothermal fluids at p H greater than or equal to 8. Because it has been shown that boron iso topic fractionation in natural processes is strongly affected by its a queous speciation, the formation of NaB(OH)(4)(0) complexes cannot be neglected when using boron isotopes to estimate water-rock interaction s occurring in the continental and oceanic crust at moderate and high temperatures.