Rhenium-osmium geochronometry based on the radioactive decay of 187Re to 187Os with a half-life of 4.23 × 1010 years is well suited for dating some sulfide minerals whose formation ages can not easily be determined by other conventional geochronom-eters. An ID-ICPMS technique determining rhenium and osmium concentrations and osmium isotopic composition for the purpose of direcly dating molybdenite is described. Samples were dissolved and equilibrated with 185Re-and 190Os-enriched spikes by alkaline fusion. Separation of rhenium from matrix elements was effected by solvent extraction and cation exchange chromatography while osmium was distilled as OsO4 from H2SO4-Ce(SO4)2 solution. All isotopic measurements were made on a VG Plasma Quad facility. Total chemical recoveries for both elements were greater than 90% and total procedural blanks were 0.07 ng for rhenium and 0.01 ng for 187Os. The Re-Os model ages of molybdenites from Shanxi, Henan and Tibet as well as the Re-Os isochronic age of molybdenite from Liaoning were determined with a precision (2σ) of about ±3%. These results conform satisfactorily with their geological backgrounds and agree reasonably with the previous results obtained by using other conventional geochronometers. Analytical data show that osmium of molybdenites examined is primarilly radiogenic 187Os formed by in-situ decay of 187Re, which is in full agreement with previous observations of other workers.
Acta Geologica Sinica
rhenium-osmium method, molybdenite, dating of ore-forming age, isotope dilution, inductively couppled plasmasource mass spectrometry