A new method for TIMS high precision analysis of Ba and Sr isotopes for cosmochemical studies

Abstract

We present a new method for high precision measurement of both strontium and barium isotopes. Our method is primarily designed for extraterrestrial samples but is actually suitable for any sample, particularly for samples with low Sr and Ba concentrations and complex matrices. The main separation of Sr, Ba and matrix elements is done with Eichrom™ Sr resin, which provides a quick and easy way to separate all three fractions with little further purification needed. All isotope data were acquired on a Thermal Ionization Mass Spectrometer (TIMS) Thermo Triton™ plus. Isotopic ratios were calculated using a multidynamic acquisition scheme when possible. The decay of the amplifiers associated with Faraday cups was carefully tuned to ensure that there was no effect in the multidynamic acquisition mode. This technique improves the uncertainty for ⁸⁴Sr/⁸⁶Sr, ⁸⁶Sr/⁸⁶Sr and ¹³⁵Ba/¹³⁶Ba by a factor of two compared with previous data obtained in static collection mode. The uncertainties attainable with our new method were 2 ppm and 19 ppm (2 s.d.) for the ⁸⁶Sr/⁸⁶Sr and ⁸⁴Sr/⁸⁶Sr ratios, respectively and 4 ppm (2 s.d.) for the ¹³⁵Ba/¹³⁶Ba ratio. The low-abundance ¹³²Ba isotope was measured with a Faraday cup equipped with a 10¹² Ω resistor to improve the precision of the ¹³²Ba/¹³⁶Ba ratio. We assessed the robustness of our method by measuring several terrestrial rock standards. The use of the multidynamic calculation shows the small, but resolvable difference of 20–30 ppm previously observed for the ⁸⁴Sr/⁸⁶Sr ratio between Earth and isotopic standard SRM NIST 987 (formerly NBS 987), which rules out the hypothesis of a ⁸⁴Sr contamination or a fractionation of NIST 987 [Moynier et al., Astrophys. J., 2012, 758, 45; Paton et al., Astrophys. J., 2013, 763, L40] and is in agreement with the results of Hans et al., Earth Planet. Sci. Lett., 2013, 374, 204–214.