Isotopic analyses and micro-characterization

The objective of this project is the development of analyses on micro-quantities and of in-situ analyses (when necessary, with the use of lasers) and the scientific significance of the results with respect to the technical performances of the apparatus. For stable isotopes, the oxygen extraction technique from silicates and oxides has been developed for 11 different standards. The extraction line and the mode of operation are not new but improvements have allowed to lower the blanks and increase the reproducibility. For some minerals, a grain-size effect on the isotopic composition has been demonstrated. An analytic protocol has been defined and the calibration of new minerals is in progress. In-situ analyses on quartz minerals have been performed using the same method. Thanks to multi-collector mass spectrometers, the internal reproducibility reached on radiogenic lead isotope ratio determinations is below 0.01 %. However, because of the lead isotope ratio instability, the mass discrimination cannot be easily corrected. The external reproducibility has been measured on a large number of natural samples in two independent laboratories. The U/Pb ratios were measured using ICP-MS for a correction of the radiogenic lead contribution to the Pb/Pb ratio. (J.S.). 6 refs., 2 figs