10:15 〜 10:30
[PPS12-06] 炭素質コンドライト・普通コンドライト全岩におけるNd同位体異常
キーワード：同位体異常、プレソーラー粒子
We have performed high precision Nd isotope analysis of chondrites coupled with a new sample digestion technique that confirms complete dissolution of acid resistant presolar grains. We also developed an improved dynamic multicollection method using TIMS to improve the analytical reproducibilities. To test the analytical reproducibility in the dynamic method, we repeatedly analyzed a standard sample (JNdi-1) for eight months. The long-term reproducibilities obtained in the dynamic method were 4.2 ppm, 6.6 ppm and 9.7 ppm for 142Nd/144Nd, 148Nd/144Nd, 150Nd/144Nd (n = 35), which are 2–11 to times superior to the static and multistatic method.
We analyzed eight ordinary chondrites, showing uniform isotope anomalies for μ142Nd (–12 ± 5 ppm), μ148Nd (10 ± 8 ppm) and μ150Nd (20 ± 12 ppm). Although the μ142Nd values for ordinary chondrites obtained in this study are generally consistent with those of previous studies, positive anomalies in μ148Nd and μ150Nd were not recognized in previous studies. In contrast to ordinary chondrites, carbonaceous chondrites show variable Nd isotope anomalies exceeding analytical uncertainties. individual carbonaceous chondrites are categorized into three groups as a function of μ142Nd; NWA 2090 (–5 ppm), Tagish Lake (–20 ppm), and Allende, DaG 190/082, and Dhofar 1432 (–30 ppm).
The data points for ordinary chondrites are generally plotted on mixing line between the terrestrial composition and the putative s-process endmember. This means that the isotope anomalies in ordinary chondrites are induced by the heterogeneous distribution of s-process nuclides in early Solar System. By contrast, most of the carbonaceous chondrites deviate from the mixing line towards the direction with lower μ142Nd values. We presume that the offset from the mixing line is caused by the heterogeneous distribution of p-nuclides in the early Solar System, because a part of 142Nd was produced by the p-process nucleosynthesis and α decay of a pure p-nuclide 146Sm. Although the Earth and parent bodies of chondrites do not share building blocks with a common Nd isotopic composition, the excess 142Nd signature of the Earth would not necessarily require the existence of a hidden reservoir with a subchondritic Sm/Nd ratio deep in the Earth’s mantle as proposed previously.
We analyzed eight ordinary chondrites, showing uniform isotope anomalies for μ142Nd (–12 ± 5 ppm), μ148Nd (10 ± 8 ppm) and μ150Nd (20 ± 12 ppm). Although the μ142Nd values for ordinary chondrites obtained in this study are generally consistent with those of previous studies, positive anomalies in μ148Nd and μ150Nd were not recognized in previous studies. In contrast to ordinary chondrites, carbonaceous chondrites show variable Nd isotope anomalies exceeding analytical uncertainties. individual carbonaceous chondrites are categorized into three groups as a function of μ142Nd; NWA 2090 (–5 ppm), Tagish Lake (–20 ppm), and Allende, DaG 190/082, and Dhofar 1432 (–30 ppm).
The data points for ordinary chondrites are generally plotted on mixing line between the terrestrial composition and the putative s-process endmember. This means that the isotope anomalies in ordinary chondrites are induced by the heterogeneous distribution of s-process nuclides in early Solar System. By contrast, most of the carbonaceous chondrites deviate from the mixing line towards the direction with lower μ142Nd values. We presume that the offset from the mixing line is caused by the heterogeneous distribution of p-nuclides in the early Solar System, because a part of 142Nd was produced by the p-process nucleosynthesis and α decay of a pure p-nuclide 146Sm. Although the Earth and parent bodies of chondrites do not share building blocks with a common Nd isotopic composition, the excess 142Nd signature of the Earth would not necessarily require the existence of a hidden reservoir with a subchondritic Sm/Nd ratio deep in the Earth’s mantle as proposed previously.