Chlorine isotopic geochemistry of salt lakes in the Qaidam Basin, China

The isotopic compositions of chlorine in salt lake brine coexisting with halite, oil-field water and river water from the Qaidam Basin (Qinghai, China) have been examined using high-precision measurement of chlorine isotopes based on thermal ionization mass spectrometry of the Cs2Cl+ ion. δ 37Cl values vary from −2.05 to +2.94‰, with average values of +2.94‰ for hot spring water, +1.35‰ for river water, −0.38‰ for oil-field brine, −0.40‰ for saline water and −0.63‰ for salt lake brine. The preliminary results show isotopic fractionation of chlorine in salt lake brine during salt precipitation. The variation of the δ 37Cl value in these salt lakes is mainly associated with brine evaporation and halite precipitation and with the hydrochemistry of the input water. The chlorine isotopic composition may reflect the degree of evaporation of the salt lake brine and may be used to distinguish brines that are in equilibrium with precipitating salts and brines that have dissolved earlier evaporates.