HYDROGEN AND OXYGEN STABLE ISOTOPE ANALYSES OF FLUID INCLUSIONS IN HALITE IN CHARHAN SALT LAKE WITH GEOCHEMICAL IMPLICATIONS

The stalble isotope composition of primary fluid inclusions in a crystal represents the isotopic composition of the liquid in which that crystal formed. By studying the isotope composition of the primary fluid inclusions in evaporite minerals from ancient lake deposits, we can explore the history of ancient salt lakes. An analysis of the hydrogen and oxygen isotopic ratios of primary halite fluid inclusions was conducted in a long core, (about 47m) on the north shore of Dabusun Lake, Charhan saline pan, Qaidem basin, China.Brine samples were also collected in the vicinity of Dabusun lake. An analytical method suitable for micro-water samples, including fluid inclusions (typically less than 8 mg) has been established.The method involves reaction of water with zinc to produce H2and water with guanidinc hydrochloride to produce CO₂. The standard deviation of the guanidine hydrochloride method is ± 0.15- ± 0.30‰ in water samples and ± 0.61‰ in samples from fluid inclusions.The standard deviation of the zinc method for fluid inclusion samples is ± 4.8‰. The texture of halite and the stable isotopic relationships of primary halite fluid inclusions in a long core show that the history of Charhan Lake (from about 50, 000 years ago to now) can be divided into 3 evolution periods with different salt- forming environments: the relatively stable desiccation period (50, 000-30, 000a.B.P), the fluctuation desiccation period with low temperature (30, 000-15, 000 a.B.P.) and the fluctuation desiccation period with temperture increasing (from 15, 000 a.B.P. to now) For all three periods there appear to have been minor fluctuations between fresh and saline water, the lake never became dried, but there appear to have been sudden changes on salt-florming environment between these periods. Modern Dabusun Lake originated as a dissolution lake, like the newborn (1967) West Dabusun Lake, however, Dabusun Lake is concentrataed enough to form potash salts on its northern shore.The water supply to the modern Charhan saline pan is dominated by meteoric water (mainly from the Kunlun Mountains to the south), however, smaller amounts of oil field brines do flow to the surface along the north edge of the Charhan saline pan.