La-Ce isotope measurements by multicollector-ICPMS

Abstract

The ¹³⁸La-¹³⁸Ce decay system (half-life 1.02 × 10¹¹ years) is a potentially highly useful tool to unravel information about the timing of geological processes and about the interaction of geological reservoirs on earth, complementing information from the more popular ¹⁴⁷Sm-¹⁴³Nd and ¹⁷⁶Lu-¹⁷⁶Hf isotope systems. Previously published analytical protocols were limited to TIMS. Here we present for the first time an analytical protocol that employs MC-ICPMS, with an improved precision and sensitivity. To perform sufficiently accurate La-Ce measurements, an efficient ion-chromatographic procedure is required to separate Ce from the other rare earth elements (REE) and Ba quantitatively. This study presents an improved ion-chromatographic procedure that separates La and Ce from rock samples using a three-step column separation. After REE separation by cation exchange, Ce is separated employing an Ln Spec column and selective oxidation. In the last step, a cation clean-up chemistry is performed to remove all remaining interferences. Our MC-ICPMS measurement protocol includes all stable Ce isotopes (¹³⁶Ce, ¹³⁸Ce, ¹⁴⁰Ce and ¹⁴²Ce), by employing a 10¹⁰ ohm amplifier for the most abundant isotope ¹⁴⁰Ce. An external reproducibility of ±0.25ε-units (2 r.s.d) has been routinely achieved for ¹³⁸Ce measurements for as little as 150-600 ng Ce, depending on the sample-skimmer cone combinations being used. Because the traditionally used JMC-304 Ce reference material is not commercially available anymore, a new reference material was prepared from AMES laboratory Ce metal (Cologne-AMES). In order to compare the new material with the previously reported isotopic composition of AMES material prepared at Mainz (Mainz-AMES), Cologne-AMES and JMC-304 were measured relative to each other in the same analytical session, demonstrating isotope heterogeneity between the two AMES and different JMC-304 batches used in the literature. To enable sufficiently precise age correction of radiogenic ¹³⁸Ce and to perform isochron dating, a protocol was developed where La and Ce concentrations are determined by isotope dilution (ID), using an isotope tracer enriched in ¹³⁸La and ¹⁴²Ce. The new protocols were applied to determine the variations of Ce isotope compositions and La-Ce concentrations of certified geochemical reference materials (CRMs): BCR-2, BCR-1, BHVO-2, JR-1, JA-2, JB-3, JG-1, JR-1, JB-1b, AGV-1 and one in-house La Palma standard.

Fig. 1. Sketch illustrating decay of ¹³⁸La to stable ¹³⁸Ba (electron capture (EC), 65.5%) and ¹³⁸Ce (β^– decay, 34.4%, t_1/2 = 1.02 × 10¹¹ years).

Fig. 2. Elution scheme illustrating separation of the REEs from the matrix using cation resin in stage 1.

Fig. 3. Elution scheme illustrating separation of Ce from REEs using Ln Spec (stage 2).

Fig. 4. Elution scheme illustrating separation of La and Ce from REEs using a long Ln Spec column (see also Table 2).

Fig. 5. Measured ε¹³⁸Ce values and ε¹⁴²Ce value plotted against measured ratios of (A) ¹³⁷Ba/¹⁴⁰Ce (B) ¹³⁹La/¹⁴⁰Ce (C) ¹⁴⁴Nd/¹⁴⁰Ce.

Fig. 6. Comparison of ¹³⁸Ce compositions of (i) JMC 304 reference material, (ii) Cologne-AMES and (iii) Mainz-AMES for a single analytical session (March 2015). All values are given relative to JMC-304. The ε¹³⁸Ce value is calculated as ¹³⁸Ce/¹³⁶Ce, normalized to ¹³⁶Ce/¹⁴⁰Ce using a ¹³⁶Ce/¹⁴⁰Ce of 0.002124072 (ref. 18 and 33) and the exponential law. The weighted means of ε¹³⁸Ce are –0.03 ± 0.12 for JMC-304, +0.73 ± 0.11 for Cologne-AMES and +2.44 ± 0.14 for Mainz-AMES (all 2 r.s.d).

Fig. 7. Long term comparison of the ¹³⁸Ce compositions of Cologne-AMES and Mainz-AMES. All values are given relative to JMC-304, measured during 10 analytical sessions. The weighted means are ε¹³⁸Ce +0.81 ± 0.10 for Cologne-AMES and +2.62 ± 0.09 for Mainz-AMES (all 2 r.s.d).

Fig. 8. Absolute ¹³⁸Ce/¹³⁶Ce composition (error bars 2 r.s.d.) obtained for the Mainz AMES standard for 10 different analytical sessions (red/blue dots). The black dots are literature data: (1) Willbold, (2) Doucelance et al., (3) Bellot et al.

Fig. 9. Measured Ce isotope ratios for rock reference samples given in ε-units relative CHUR. Reported uncertainties correspond to 2 r.s.d. Grey dots are literature data from Tanaka (JR-1, JG-1, JB-3 and BCR-1) and Bellot et al. (BHVO-2 and BCR-2).

Fig. 10. Measured Ce isotope ratios for rock reference samples using different interface cone combinations. ¹³⁸Ce/¹³⁶Ce ratios are expressed in ε-units relative to CHUR. Reported uncertainties correspond to 2 s.e.