Helium isotopes in Dead Sea Transform waters

Abstract

The concentrations and isotopic compositions of helium, and relative abundances of CO2, were studied in brines and freshwater springs, groundwater wells, and gas bubbles, in and around the Dead Sea Transform (DST) fault system. The latter forms part of the plate boundary between the African and Arabian plates along which intensive tectonic and volcanic activity occurred and where a range of freshwater to hypersaline water bodies percolate.The atmosphere-normalized 3He/4He ratios (R/Ra) show a distinct geographic gradient, whereby fluids in the north part of the DST, near Lake Kinneret, are characterized by R/Ra≈2. These values decrease gradually southward until the Arava Valley, where they are approximately 0.15, reflecting a corresponding decrease in the mantle-derived He fraction, from about ~30% in the north, to a few percent in the south. Freshwater springs outside the DST, in the Galilee and Judea Mountains, are characterized by R/Ra ratios close to unity, reflecting the dominance of atmosphere-derived helium, although they still contain a few percent of a mantle component.The spatial pattern of change in R/Ra ratios is independent of regional tectonic activity (rifting), which is similar around the Lake Kinneret basin and the Dead Sea, and a correlation does not exist between such activity and He concentrations and CO2/3He ratios. In contrast, helium concentrations, and to a lesser extent CO2/He ratios, correlate with temperatures of the sampled fluids, reflecting a dilution process of a deep, hot mantle component as it percolates upwards toward the surface. The preferential injection of a mantle He toward the surface in the north DST is facilitated by deep faults and a shallower Moho boundary, which are also associated with enhanced volcanism in the north DST, and is superimposed over a regionally scaled mantle background value. Regionally, the observations reflect a transition from a domination of a dilute mantle end member in the north, which itself is the product of a mixture between mantle and crustal helium, to a dominant crustal (radiogenic) end member in the south.The R/Ra ratio in basaltic olivines and diopside from the Golan is on average 6.6±0.7, lower than typical mantle values (~8) but corresponding to comparable sub-continental mantle compositions.