BORON ISOTOPE GEOCHEMISTRY OF THERMAL SPRINGS FROM THE NORTHERN RIFT-VALLEY, ISRAEL

A boron isotope study was carried out on groundwaters from the norther n Rift Valley, Israel. The relatively high salinity (18 000 mgCl l-1), boron content (3.9 mg l-1), and deltaB-11 value (44parts per thousand ) and the low B/Cl ratios of the saline Tiberias Hot Spring suggest di lution of deep-seated trapped CaCl2 brines with a deltaB-11 almost-equ al-to 44 parts per thousand, which is consistent with previous interpr etations based on Na/Cl (0.58) and deltaO-18 (-3parts per thousand) va lues. The deltaB-11 values of the springs in Hammat Gader decrease gra dually from deltaB-11 of 41.9parts per thousand to 20.9parts per thous and with increasing salinity (70 to 497 mgCl l-1), temperature (25-deg rees-50-degrees-C), and boron content (0.1-0.3 mg l-1). The deltaB-11 values suggest that boron is derived from leaching of calcium carbonat e or desorption of clay minerals of a deltaB-11 = 14parts per thousand and mixing with meteoric water of a high deltaB-11 value. The relativ ely low deltaB-11 values of two low salinity springs (1240 and 2700 mg Cl l-1) along the Sea of Galilee (deltaB-11 = 31.6parts per thousand a nd 31.8parts per thousand) and the lake itself (24parts per thousand) suggest that a significant fraction of boron in these waters is also d erived from rock leaching. The influence of boron that is leached from the rocks is limited and can be recognized mainly in boron-depleted g roundwater. The variations of deltaB-11, Na/Cl, and deltaO-18 paramete rs in the Hammat Gader springs demonstrate the sensitivity of each ele ment to different sources. Whereas oxygen-isotope compositions reflect the meteoric source, the major ions (e.g. Na/Cl and Br/Cl ratios) rec ord the influence of the deep-seated brines, and boron isotopes in dil uted groundwater are sensitive to water-rock interaction.