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

We have measured the boron isotope composition of seventeen samples of berate minerals (colemanite, ulexite, and borax) and the Sr-87/Sr-86 ratio in thirteen berate samples from the Kirka berate deposit in west ern Anatolia, Turkey. These Neogene deposits were formed by evaporatio n of playa lakes fed by geothermal springs. The delta(11)B values rang e from - 14.9 parts per thousand in colemanite to -1.6 parts per thous and in borax. To a first approximation the relative differences in the delta(11)B values of the berate minerals are consistent With their ba sic 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 an d pH. Rather the data are consistent with precipitation of colemanite from a brine with lower pH than that required for ulexite precipitatio n, which in turn requires a lower pH than is needed for borax precipit ation. The boron isotope data also suggest that the berate minerals di d not maintain boron isotopic equilibrium with the brine after they pr ecipitated. Rayleigh fractionation models indicate that during borax p recipitation the delta(11)B value of the brine was slightly heavier th an during precipitation of ulexite and colemanite.