12:00 〜 12:15
[PCG23-12] 外側太陽系におけるニオブ-92の初生存在度
キーワード：ニオブ-92、短寿命放射性核種、p-過程、ニュートリノ反応、Type II 超新星、同位体二分性
Introduction: The p-process radionuclide niobium-92 (92Nb) decays to zirconium-92 (92Zr) by electron capture with a half-life of 37 million years (Ma). The system is a promising chronometer for addressing the early solar system evolution and planetary differentiation [1, 2]. Thus, the initial abundance of 92Nb and its distribution in the early solar system provide valuable constraints on the time-scale of our solar system evolution, and on the origin of p-process nuclides. The initial 92Nb abundance at the solar system formation was previously determined to be (92Nb/93Nb)0 = (1.7 ± 0.6) × 10–5, applying the internal isochron approach to the NWA 4590 angrite (U–Pb age: 4557.93 ± 0.36 Ma) [2]. This value is consistent with those obtained from eucrites, ordinary chondrites, and mesosiderites [1, 3], indicating that 92Nb was homogeneously distributed among their source regions. Yet, all samples previously studied for 92Nb are thought to have originated from the inner solar system. Here we report internal Nb–Zr isochron dating of Northwest Africa (NWA) 6704.
NWA 6704 is a primitive achondrite having a fresh igneous texture [4] with a U–Pb age of 4562.76 ± 0.30 Ma [6]. This meteorite underwent melting above liquidus temperature and subsequent rapid cooling (> 10-1 °C/yr; [4]), making the effect of differing closure temperatures between the U–Pb and Nb–Zr systems insignificant. Furthermore, this meteorite has Δ17O, ε50Ti, ε54Cr and ε84Sr values similar to those of carbonaceous chondrites [4-6], indicating that it samples the same reservoirs in the solar nebula as the carbonaceous chondrite parent bodies (i.e., the outer solar system). Thus, NWA 6704 enables us to evaluate the distribution of 92Nb between the inner and outer solar system for the first time.
Results & Discussion: We prepared mineral and whole rock fractions from five fragments of NWA 6704. All Nb–Zr isotopic data were obtained by the ICP mass spectrometry. The isochron defines an initial 92Nb/93Nb of (2.8 ± 0.3) × 10–5 at the time of NWA 6704 formation. By combining this value with the U–Pb age of NWA 6704, an initial 92Nb/93Nb of (3.0 ± 0.3) × 10–5 at the time of solar system formation is derived. The obtained value is distinctly higher than the initial value in the inner solar system of (1.7 ± 0.6) × 10–5 [2]. This indicates that 92Nb was heterogeneously distributed in the protoplanetary disk before the formation of NWA 6704, and was relatively enriched in the outer solar system. The difference between these two initial values causes the apparent Nb–Zr age difference of ~30 Ma, demonstrating that the current canonical value of (92Nb/93Nb)0 = (1.7 ± 0.6) × 10–5 should not be used for the Nb–Zr dating of planetary materials from the outer solar system. The newly obtained initial 92Nb/93Nb value is clearly higher than the expected value in the model of 92Nb synthesis by Type Ia supernova (SNIa) [7]. Thus, our results require another production site to be invoked for selectively producing 92Nb. At the moment, only the ν-process in Type II supernova (SNII) [8] satisfies such requirement. If so, our finding suggests that the time-interval from the last SNII explosion to the formation of our solar system needs be <100 My and that nuclides synthesized by the last SNII were preferentially implanted or preserved in the outer solar system. Such enrichment of the last SNII components in the outer solar system may account for the isotopic dichotomy between carbonaceous and non-carbonaceous meteorites [e.g., 9].
References: [1] Schönbächler et al. (2002), [2] Iizuka et al. (2016), [3] Haba et al. (2017), [4] Hibiya et al. (2019) GCA., [5] Amelin et al. (2019), [6] Hibiya et al. (2019) GGR., [7] Lugaro M. et al. (2016), [8] Hayakawa et al. (2013), [9] Warren (2011).
NWA 6704 is a primitive achondrite having a fresh igneous texture [4] with a U–Pb age of 4562.76 ± 0.30 Ma [6]. This meteorite underwent melting above liquidus temperature and subsequent rapid cooling (> 10-1 °C/yr; [4]), making the effect of differing closure temperatures between the U–Pb and Nb–Zr systems insignificant. Furthermore, this meteorite has Δ17O, ε50Ti, ε54Cr and ε84Sr values similar to those of carbonaceous chondrites [4-6], indicating that it samples the same reservoirs in the solar nebula as the carbonaceous chondrite parent bodies (i.e., the outer solar system). Thus, NWA 6704 enables us to evaluate the distribution of 92Nb between the inner and outer solar system for the first time.
Results & Discussion: We prepared mineral and whole rock fractions from five fragments of NWA 6704. All Nb–Zr isotopic data were obtained by the ICP mass spectrometry. The isochron defines an initial 92Nb/93Nb of (2.8 ± 0.3) × 10–5 at the time of NWA 6704 formation. By combining this value with the U–Pb age of NWA 6704, an initial 92Nb/93Nb of (3.0 ± 0.3) × 10–5 at the time of solar system formation is derived. The obtained value is distinctly higher than the initial value in the inner solar system of (1.7 ± 0.6) × 10–5 [2]. This indicates that 92Nb was heterogeneously distributed in the protoplanetary disk before the formation of NWA 6704, and was relatively enriched in the outer solar system. The difference between these two initial values causes the apparent Nb–Zr age difference of ~30 Ma, demonstrating that the current canonical value of (92Nb/93Nb)0 = (1.7 ± 0.6) × 10–5 should not be used for the Nb–Zr dating of planetary materials from the outer solar system. The newly obtained initial 92Nb/93Nb value is clearly higher than the expected value in the model of 92Nb synthesis by Type Ia supernova (SNIa) [7]. Thus, our results require another production site to be invoked for selectively producing 92Nb. At the moment, only the ν-process in Type II supernova (SNII) [8] satisfies such requirement. If so, our finding suggests that the time-interval from the last SNII explosion to the formation of our solar system needs be <100 My and that nuclides synthesized by the last SNII were preferentially implanted or preserved in the outer solar system. Such enrichment of the last SNII components in the outer solar system may account for the isotopic dichotomy between carbonaceous and non-carbonaceous meteorites [e.g., 9].
References: [1] Schönbächler et al. (2002), [2] Iizuka et al. (2016), [3] Haba et al. (2017), [4] Hibiya et al. (2019) GCA., [5] Amelin et al. (2019), [6] Hibiya et al. (2019) GGR., [7] Lugaro M. et al. (2016), [8] Hayakawa et al. (2013), [9] Warren (2011).