Study On The Method Of Re-Os Dating For Black Shale And Oil Shale

Black shale and oil shale are playing important roles in solving the problem of energy source crisis, studying on the geochemical evolution of earth, and tracing the source of the materials because of their characteristics. Among these studies , the precise dating of the black shale and oil shale is one of the key tasks. However, which still existed a great deal of controversies about geochronometry of black shale and oil shale, because it lacked appropriate and accurate methods for a long time or the date lacked evidence from isotopic geochronometer.Many experiment conditions have been reseached, including selecting oxidant, limiting total procedure blanks,sampling, temperature. The following previous works about Re-Os chemical analysis procedure was improved and established in our labs. The chemical analysis procedure consists of the digestion using Carius tube and oxidant, purification of Os by distillation and microdistillation, Re extraction by acetone. Re and Os solutions were determined using ICP-MS after isotopic dilution, and we also researched the different digestion result about CrO3–H2SO4 and aqua regia as digestion medium, respectively. Comparison of the Re–Os dates obtained from CrO3–H2SO4 methods suggests that the aqua regia may contain nonhydrogenous Re and Os, whereas the CrO3–H2SO4 method dominantly liberates hydrogenous Re–Os from black shale and oil shale .This is the first time to offer Re-Os isotopic geochronometer about the black shale from Polkowice mine, Polandand, and the oil shale from Qiangtang basin,northern Tibet ,China. The Polkowice black shale yields a Re-Os isochron age of (226±16)Ma with an initial 187Os/188Os ratio of (1.7±1.3)by using aqua regia methods, and also yields to Re-Os isochron age of (239±6.4) Ma with an initial 187Os/188Os ratio of (1.03±0.33) by using CrO3–H2SO4 methods. The Qiangtang basin oil shale yields to a Re-Os isochron age of (113±29) Ma with an initial 187Os/188Os ratio of (1.80±0.13) by using aqua regia ,and also yields Re-Os isochron age of (115±25) Ma with an initial 187Os/188Os ratio of (1.80±0.11) by using CrO3–H2SO4 methods. The geologists also agreed with the obtained results. Standard reference materials were analysed according to the procedure described above ,in order to check the validity of the method. The results obtained are precise and reasonable within analytical uncertainties.