Study on the Separation of Sericite for 40Ar/39Ar Dating
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ABSTRACT
BACKGROUND The 40Ar/39Ar dating of sericites is a very important method for constraining the age of deposits, but the purity of sericite remains a problem. The sericite aggregate acquired by the available separation and purification method commonly contains microline. For example, the content of microline obtained by the conventional magnet-heavy liquid method in this study is 0-28%, whereas the content of microline obtained by the conventional suspension method is 3%-45%. When the content of microline in the sericite aggregate exceeds 10%, it will directly affect the accuracy of dating.OBJECTIVES To find a useful method to increase the content of sericite and decrease the content of microline while the grain size is not too fine to cause 39Ar recoil loss during the irradiation.METHODS The purification effects of conventional magnetic separation-heavy liquid method and conventional suspension method are firstly investigated. The experimental results show that the purity of sericites obtained by the two methods are low, and the particle size of the conventional suspension method is irregular and cannot meet the requirements of dating. The ultrasonic disaggregation-suspension method was used to carry out conditional experiments on the microline-bearing sericite aggregates obtained by the magnetic separation-heavy liquid method.RESULTS Using the ultrasonic disaggregation-suspension method, the content of sericite increased sharply while the content of microline decreased greatly. For example, the content of sericite for one experiment increased from 28% to 77% and the content of microline relative to sericite decreased from 12.5% to 0. The fraction of the grain size < 1μm was above 95% and the smallest grain size was >0.356μm which was larger than 0.08μm estimated for the 39Ar recoil distance. Such grain size cannot obviously cause 39Ar recoil loss and did not have obvious effects on the age of middle to high temperature steps.CONCLUSIONS When the content of microline relative to sericite in the sericite-bearing aggregates concentrated by magnet-heavy liquid method is more than 10%, the ultrasonic disaggregation-suspension method can be used to further increase the content of sericite and decrease the content of microline to improve the accuracy of 40Ar/39Ar dating.
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