ABSTRACT

Basic technique for the extraction and isotopic analysis of fluid inclusions in halite was investigated using synthetic single crystals of halite and three natural halite samples from China. Vacuum ball-mill, vacuum decrepitation and vacuum melting methods were examined for the extraction of fluid inclusions. Results of analyses on δD and δ¹⁸O of the water in fluid inclusions extracted by the ball-mill method from synthetic single crystals agreed with those of the mother solution from which the single crystals were formed. Although the δ¹⁸O value of the water extracted by the melting method agrees well with that of the mother solution, the δD value was about 7‰ more negative than that of the mother solution. There was a great difference in both δD and δ¹⁸O of the water extracted by the two methods applied to the identical natural halite samples; the melting method gave consistently more negative values for both δD and δ¹⁸O compared with those by the ball-mill method. The difference was interpreted as a result of the hydrolysis of NaCI with H₂O combined with the formation of oxides of alkaline-earth elements in brine inclusions in natural halite samples in the process of the melting method. Although the melting method has an advantage of complete recovery of volatiles in halite samples, the chemical and isotopic compositions of volatiles can not be retained owing to a variety of thermal reactions which occur at high temperatures. It was concluded that the ball-mill method is much superior to the melting method in order to obtain isotopic and chemical information on fluid inclusions in halite, though the recovery of fluid inclusions by the ball-mill method is not 100%, and high concentrations of Mg²⁺ and Ca²⁺ in fluid inclusions require correction for the δD and δ¹⁸O values of the water.