CHLORINE ISOTOPIC GEOCHEMISTRY OF SALT LAKES IN THE QAIDAM BASIN, CHINA

The isotopic compositions of chlorine in salt lake brine coexisting wi th halite, oil-field water and river water from the Qaidam Basin (Qing hai, China) have been examined using high-precision measurement of chl orine isotopes based on thermal ionization mass spectrometry of the Cs 2Cl+ ion. delta(37)Cl values vary from -2.05 to +2.94 parts per thousa nd, with average values of +2.94 parts per thousand for hot spring wat er, -1.35 parts per thousand for river water, -0.38 parts per thousand for oil-field brine, -0.40 parts per thousand for saline water and -0 .63 parts per thousand for salt lake brine. The preliminary results sh ow isotopic fractionation of chlorine in salt lake brine during salt p recipitation, The variation of the delta(37)Cl value in these salt lak es is mainly associated with brine evaporation and halite precipitatio n and with the hydrochemistry of the input water. The chlorine isotopi c composition may reflect the degree of evaporation of the salt lake b rine and may be used to distinguish brines that are in equilibrium wit h precipitating salts and brines that have dissolved earlier evaporate s.