PRECISE MEASUREMENT OF CERIUM ISOTOPE COMPOSITION IN ROCK SAMPLES

A high-precision analytical method for the measurement of Ce-138/Ce-142 ratios is reported using static multicollection mass spectrometry. This technique reduced the data acquisition time for 2 hr. for 400 ratios and improved analytical reproducibility to +/- 0.002% (n = 16) and precision to +/- 0.002-0.003%. The better precision and reproducibility were established collecting a large ion beam [(CeO)-Ce-142-O-16 of (2-7).10(-11) A], short data acquisition time and in situ measurement of O-18/O-16 ratios during the analysis. To reduce the blank effect to the Ce isotope analysis, the chemical procedure for separation of Ce was refined using a small ion-exchange resin bed column (4 cm length x 3 mm diameter) with which the procedural total blank was lowered to 0.04 ng and the recovery yield of Ce from 20 mg BCR-1 was 90%. In order to confirm the reproducibility of this technique including the chemical procedure, six Ce isotope analyses individually separated from the USGS standard BCR-1, were carried out with an analytical reproducibility of +/- 0.002%. With these analytical precision and reproducibility and normalization to BCR-1 in order to eliminate any inter-laboratory biases, it is now possible to apply the La-Ce isotope system to the terrestrial and extraterrestrial samples combined with other isotope systems, such as Sm-Nd and Rb-Sr.