THE BORON ISOTOPE GEOCHEMISTRY OF THE KIRKA BORATE DEPOSIT, WESTERN TURKEY

We have measured the boron isotope composition of seventeen samples of borate minerals (colemanite, ulexite, and borax) and the Sr-87/Sr-86 ratio in thirteen borate samples from the Kirka borate deposit in western Anatolia, Turkey. These Neogene deposits were formed by evaporation of playa lakes fed by geothermal springs. The delta(11)B values range from - 14.9 parts per thousand in colemanite to -1.6 parts per thousand in borax. To a first approximation the relative differences in the delta(11)B values of the borate minerals are consistent With their basic boron atomic configuration, but the magnitude of the boron isotope fractionation between the three minerals precludes their simultaneous precipitation from a brine with the same boron isotope composition and pH. Rather the data are consistent with precipitation of colemanite from a brine with lower pH than that required for ulexite precipitation, which in turn requires a lower pH than is needed for borax precipitation. The boron isotope data also suggest that the borate minerals did not maintain boron isotopic equilibrium with the brine after they precipitated. Rayleigh fractionation models indicate that during borax precipitation the delta(11)B value of the brine was slightly heavier than during precipitation of ulexite and colemanite.