Modern carnallite mineralization and late Pleistocene to Holocene brine evolution in the nonmarine, Qaidam Basin, China

Qarhan salt plain, Qaidam Basin, China is the site of the largest accumulation of modern carnallites (MgKCl;A model for carnallite formation in the shallow subsurface of Qarhan salt plain is proposed, based on evidence from the chemical compositions, densities, and temperatures of surface and groundwater brines. The model relies on the strong temperature control of carnallite solubility as the mechanism by which supersaturation with respect to carnallite is attained. Carnallites are predicted to form when carnallite-saturated surface brines sink and cool. Calculations indicate that cooling of carnallite-saturated surface brines may be very effective in producing the large amount of carnallite present in the subsurface of Qarhan salt plain.;The evolution of surface and groundwater brines, and Quaternary climatic record for the Qaidam Basin, is evaluated based on the major element chemistry of fluid inclusions in halites. Fluid inclusion major element chemistries were determined using fluid inclusion microthermometry. The compositions of primary halite fluid inclusions, representing lake brines, indicate relatively wet conditions throughout most of the late Pleistocene. Desiccation occurred during a short-lived event at about 50,000 yr B.P., from about 17,000 to 8,000 yr B.P., and from about 2,000 yr B.P. to the modern. Modern halites contain the most concentrated fluid inclusion brines, indicating that the modern may be the most desiccated period recorded. Diagenetic halite fluid inclusions, formed from shallow groundwater brines, confirm the primary halite climatic record. The occurrence of carnallite saturated brines in diagenetic halite fluid inclusions in the top 12m of sediments confirms the diagenetic origin of carnallites.