5:15 PM - 7:15 PM
[MZZ42-P02] Evaluation of La isotope heterogeneity in the early solar system by high-precision isotope analysis
Keywords:high-precision isotope analysis, MC-ICP-MS, isotopic heterogeneity, early solar system
Recent high-precision isotope analysis has revealed that the isotopic composition of various elements was heterogeneous in the early solar system (e.g., 50Ti, 54Cr; Warren, 2011). The origin of this heterogeneity is suggested to be the heterogeneous distribution of supernova ejecta. In fact, it has been shown that the meteorite NWA 6704, originating from the outer solar system, contains a significantly higher amount of the extinct nuclide 92Nb—only synthesized in supernova explosions—compared to many meteorites from the inner solar system (Hibiya et al., 2023). However, determining the abundance of the extinct nuclide 92Nb is challenging, and analysis of other outer solar system meteorites has not yet been successful. The long-lived nuclide 138La is a good tracer of supernova ejecta, and its abundance can be measured directly. Moreover, it can be used for calibrating the conventional Nb-Zr chronometry, which previously assumed a homogeneous distribution of 92Nb. To do this, La isotope ratios in bulk meteorites from both the inner and outer solar system need to be determined.
Previous studies using Thermal Ionization Mass Spectrometer(TIMS) reported La isotope anomalies up to +60ε in the refractory inclusions (CAIs) of the Allende meteorite (carbonaceous chondrite) (Shen & Lee, 2003). However, due to insufficient analytical precision in bulk meteorites analysis, La isotopic anomalies have not been detected. Therefore, this study aims to establish a high-precision La isotope analysis method using MC-ICP-MS and to evaluate nucleosynthetic La isotope anomalies at the bulk meteorite scale.
The problems in the La isotope analysis are as follows; the very low abundance of 138La, the inability to correct for mass fractionation using only La (since there are only two isotopes), and significant isobaric interference from 138Ba and 138Ce. In our work, sensitive measurement of the 138La became possible by using a 1013Ω amplifier for Faraday cup detector. The mass fractionation effect of La isotopes within the instrument was corrected by monitoring the Nd isotope fractionation effect of sample solution which Nd standard (JNdi-1) solution was spiked(external correction method). Furthermore, we improved the analytical precision by correcting variations in isotope ratios caused by differences in signal responses between the 1013Ω and 1010Ω amplifiers. Additionally, by determining the correction factors through Ba dope tests after each measurement, proper correction even for samples with higher Ba/La ratios were ensured.
To achieve the reduction of isobaric interferences, a three-step column chromatography method to separate La from Ba and Ce was adopted. However, it has been pointed out that isotopic fractionation can occur when the Ln-spec resin used(Ohno & Hirata, 2013). The fractionation could potentially affect La isotopic study. This study revealed that isotopic fractionation during La separation occurs. However, when more than 80% of La is recovered, the isotopic fractionation effect falls within the measurement error range and can be considered negligible.
Because La has only two isotopes (138La and 139La), when analyzing meteorite samples, it is necessary to evaluate non-nucleosynthetic anomaly, such as "isotopic fractionation due to geological processes" and "isotopic variation due to neutron capture reactions." To assess isotopic fractionation during geological processes, La isotope analysis on various igneous and sedimentary rock standard samples from Earth were performed. The results showed that all geological samples were consistent within the error, and no isotopic anomalies due to geological processes were detected. It was found that isotopic variations due to geological processes are within ±2.2ε. The effect of neutron capture reactions was calculated using neutron capture cross-sections and neutron fluence. It was found that the effect is below 0.5ε, which is not problematic within the measurement precision.
Additionally, La isotope analysis was conducted on whole rock meteorites from both the inner solar system (Eucrites) and the outer solar system (Allende carbonaceous meteorite).
Although no La isotope anomaly was observed in the Eucrite samples (ε138La = -1.3 ~ +2.0), a positive La isotope anomaly was detected in the Allende meteorite (ε138La = +10.5 ± 2.5). This result reveals the La isotope heterogeneity in the early solar system.
Previous studies using Thermal Ionization Mass Spectrometer(TIMS) reported La isotope anomalies up to +60ε in the refractory inclusions (CAIs) of the Allende meteorite (carbonaceous chondrite) (Shen & Lee, 2003). However, due to insufficient analytical precision in bulk meteorites analysis, La isotopic anomalies have not been detected. Therefore, this study aims to establish a high-precision La isotope analysis method using MC-ICP-MS and to evaluate nucleosynthetic La isotope anomalies at the bulk meteorite scale.
The problems in the La isotope analysis are as follows; the very low abundance of 138La, the inability to correct for mass fractionation using only La (since there are only two isotopes), and significant isobaric interference from 138Ba and 138Ce. In our work, sensitive measurement of the 138La became possible by using a 1013Ω amplifier for Faraday cup detector. The mass fractionation effect of La isotopes within the instrument was corrected by monitoring the Nd isotope fractionation effect of sample solution which Nd standard (JNdi-1) solution was spiked(external correction method). Furthermore, we improved the analytical precision by correcting variations in isotope ratios caused by differences in signal responses between the 1013Ω and 1010Ω amplifiers. Additionally, by determining the correction factors through Ba dope tests after each measurement, proper correction even for samples with higher Ba/La ratios were ensured.
To achieve the reduction of isobaric interferences, a three-step column chromatography method to separate La from Ba and Ce was adopted. However, it has been pointed out that isotopic fractionation can occur when the Ln-spec resin used(Ohno & Hirata, 2013). The fractionation could potentially affect La isotopic study. This study revealed that isotopic fractionation during La separation occurs. However, when more than 80% of La is recovered, the isotopic fractionation effect falls within the measurement error range and can be considered negligible.
Because La has only two isotopes (138La and 139La), when analyzing meteorite samples, it is necessary to evaluate non-nucleosynthetic anomaly, such as "isotopic fractionation due to geological processes" and "isotopic variation due to neutron capture reactions." To assess isotopic fractionation during geological processes, La isotope analysis on various igneous and sedimentary rock standard samples from Earth were performed. The results showed that all geological samples were consistent within the error, and no isotopic anomalies due to geological processes were detected. It was found that isotopic variations due to geological processes are within ±2.2ε. The effect of neutron capture reactions was calculated using neutron capture cross-sections and neutron fluence. It was found that the effect is below 0.5ε, which is not problematic within the measurement precision.
Additionally, La isotope analysis was conducted on whole rock meteorites from both the inner solar system (Eucrites) and the outer solar system (Allende carbonaceous meteorite).
Although no La isotope anomaly was observed in the Eucrite samples (ε138La = -1.3 ~ +2.0), a positive La isotope anomaly was detected in the Allende meteorite (ε138La = +10.5 ± 2.5). This result reveals the La isotope heterogeneity in the early solar system.