Precise measurement of cerium isotope composition in rock samples

Abstract

A high-precision analytical method for the measurement of ${}^{138}\text{Ce}{}^{142}\text{Ce}$ 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 [¹⁴²Ce¹⁶O of (2–7) · 10⁻¹¹ A], short data acquisition time and in situ measurement of ${}^{18}\text{O}{}^{16}\text{O}$ 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 × 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.