Re-Os dating of molybdenite : new anlytical approaches using ICP-MS

Abstract: In recent years, there has been a surge of interest in the direct dating of ore deposits using Re-Os geochronometer for the ore minerals themselves. Molybdenite has been or particular interest because it is a common ore mineral and is suitable for analysis by the Re-Os method due to relatively high concentration of both Re and Os. Until recently, reliable Re-Os dating of molybdenite was only possible using negative thermal ionisation mass spectrometry (NTIMS) involving time- and labour-consuming sample preparation procedures. Growing use of double focusing sector field inductively-coupled plasma mass spectometers, with their inherent capabilities for multielement analyses and large sample throuthput, demands the development of new analytical approaches for precise and accurate determination of Re and Os isotopes and dating of molybdenite. The major goal of this study was to develop a method for Re-Os dating of molybdenite using inductively copuled plasma mass spectrometry (ICP-MS) that can provide routine and precise quantification of Re and Os isotopes. As the method of isotope dilution (ID) is applied for accurate determination of isotope ratios in geochronological studies, the first attempts were made on the optimisation of Re and Os spike addition. In order to make a preliminary assessment of Os and Re concentrations in molybdenite and define the amounts of spikes necessary for precise isotope ratio determinations, the technique with minimum sample preparation - laser ablation ICP-MS (LA- ICP-MS) was used. It was demonstrated that LA-ICP-MS is a useful tool in the rapid, semi-quantitative determination of Re and Os concentrations in molybdenite. To improve the accuracy and precision of the LA-ICP-MS measurements, different calibration strategies were examined. External calibration with correction of instrumental drift by monitoring one of the major isotopes of molybdenite was found to be most effective for the determination of the Re concentration. Simple and robust mass discrimination correction using the 184W/182W isotope ratio performed in this LA study allows mathematical calculation of Os concentrations from measured 187(Re-Os)/185Re and tabulated 187Re185Re ratios with an accuracy sufficient for subsequent optimisation of the spike additions. Following addition of the spikes, the decomposition of molybdenite was studied. A novel two stage decomposition procedure in an autoclave was proved to allow complete spike-sample equilibration as a result of nitric and sulphuric attack under elevated temperature and pressure. The decomposition procedure also avoids the risk of explosion and extends the upper limit of the sample amount, these being the serious disadvantages of previously used techniques. Modification of the destillation procedure adopted for Os separation from the digest ensures high Os recovery and makes this procedure fast and optimally suitable for ICP-MS measurements. Re was separated from the matrix elements by using anion exchange. Minor modifications of this technique were also made that allowed more efficient Re separation from Mo. A significant achievement obtained in this study was the effective elimination of Os memory effects during ICP-MS measurement by using an ammonia solution. Optimisation of both the plasma conditions of the instrument using a shielded torch and the analyser scanning parameters resulted in improved sensitivity, providing the basis for accurate and precise Os isotope ratios measyrements. Advantages of the develpoed method are low cost of the equipment involved and its easy adaptation in chemical laboratories. Accuracy of the Re-Os dating was evaluated by replicate analyses of two moblybdenite reference materials (HLP-f and JDC). The Re-Os ages for these reference materials were quite reproducible (better than 0.92& RSD, 1 s, n=6) and compare well with the previously published data. Reproducibility typically obtained in the age determinations of mineral separates was about 1.5% RSD (1 s, n=5). This level of precision allows confident distinction between many geological events in the Precambrian.