5:15 PM - 6:30 PM
[MTT42-P01] High-precision La isotope measurements to apply early Solar System chronology and nuclear cosmochronology
Keywords:La, supernova, early Solar System, MC-ICP-MS
In the nature, La consists of two isotopes, 138La and 139La. 138La is expected to be synthesized only by supernovae likewise short-lived radionuclides 92Nb and 98Tc. 92Nb, which decays to 92Zr with a half-life of 37 Myr, is one of the important chronometers for planetary differentiation in the early Solar System. It has been recently shown that 92Nb was heterogeneously distributed in the early Solar System (Hibiya et al., in review). Thus, the use of 92Nb as a chronometer requires the calibration of the heterogeneous distribution. The long-lived nuclide 138La (T1/2=105 Gyr) can be used as an index of the 92Nb heterogeneity because of having the same origin as 92Nb. Moreover, combining 138La with 98Tc, which decays to 98Ru with a half-life of 4.2 Myr, potentially allows us to precisely constrain the timing and location of the supernova explosion which introduced these nuclides into the parental molecular cloud of the Solar System. Thus, 138La analysis can be a strong tool for both the early Solar System chronology and nuclear cosmochronology.
So far, La isotope analysis has been conducted for limited samples such as Ca-Al-rich inclusions (CAIs) and several chondritic bulk samples. The results revealed that CAIs have 138La excesses of up to 60±16ε (2SD) (Shen and Lee, 2003), whereas for bulk meteorite sample 138La anomalies are marginally resolvable only for carbonaceous chondrites (12±8ε (2SD)) (Shen et al., 1994). To apply 138La anomaly for the early Solar System chronology and nuclear cosmochronology, it is necessary to improve the precision of La isotope analysis.
In this study, we tested the precision of La isotope measurements by multiple collector ICP mass spectrometry (MC-ICP-MS). With this instrument, we can adopt the external calibration technique for instrumental mass bias effect for La having only two isotopes. Meanwhile, MC-ICP-MS analysis requires the correction of isobaric interferences on 138La from 138Ba and 138Ce. The very low abundance of 138La (0.1%) also makes it difficult to carry out a high-precision analysis. While 1011 Ω amplifiers are often used in mass spectrometries, we adopted 1013 Ω amplifier on 138 amu and 1010 Ω amplifier on 139 amu to improve the precision.
We measured a La standard solution NIST 3127a. Adopting the external calibration technique has improved the precision 10–20 times better than La standard bracketing method. Moreover, the isobaric interferences of 138Ba and 138Ce were corrected for by monitoring 137Ba and 140Ce, increasing the precision about 2 times compared to when no interference corrections made. As a result, we achieved an internal precision of ±1.4ε (2SE) for a 80 ppb La standard solution.
So far, La isotope analysis has been conducted for limited samples such as Ca-Al-rich inclusions (CAIs) and several chondritic bulk samples. The results revealed that CAIs have 138La excesses of up to 60±16ε (2SD) (Shen and Lee, 2003), whereas for bulk meteorite sample 138La anomalies are marginally resolvable only for carbonaceous chondrites (12±8ε (2SD)) (Shen et al., 1994). To apply 138La anomaly for the early Solar System chronology and nuclear cosmochronology, it is necessary to improve the precision of La isotope analysis.
In this study, we tested the precision of La isotope measurements by multiple collector ICP mass spectrometry (MC-ICP-MS). With this instrument, we can adopt the external calibration technique for instrumental mass bias effect for La having only two isotopes. Meanwhile, MC-ICP-MS analysis requires the correction of isobaric interferences on 138La from 138Ba and 138Ce. The very low abundance of 138La (0.1%) also makes it difficult to carry out a high-precision analysis. While 1011 Ω amplifiers are often used in mass spectrometries, we adopted 1013 Ω amplifier on 138 amu and 1010 Ω amplifier on 139 amu to improve the precision.
We measured a La standard solution NIST 3127a. Adopting the external calibration technique has improved the precision 10–20 times better than La standard bracketing method. Moreover, the isobaric interferences of 138Ba and 138Ce were corrected for by monitoring 137Ba and 140Ce, increasing the precision about 2 times compared to when no interference corrections made. As a result, we achieved an internal precision of ±1.4ε (2SE) for a 80 ppb La standard solution.