Presentation Information
[PPS12-P04]Progressive reduction of the early solar nebula during CAI crystallization revealed by the evolution of intracrystalline Eu anomalies in fassaite
*Akimasa Suzumura1,2, Norikatsu Akizawa3, Daiki Yamamoto4, satoki okabayashi5, Shoichi Itoh6, Hisashi Asanuma2 (1.Faculty of Science, Shinshu University, 2.Graduate School of Human and Environmental Studies, Kyoto University, 3.Graduate School of Advanced Science and Engineering, Hiroshima University, 4.Faculty of Sciences, Kyushu University, 5.College of Bioresource Sciences, Nihon University, 6.Department of Earth and Planetary Sciences, Kyoto University)
Keywords:
CAI,Eu anomaly,redox condition,crystal growth
The chemical and isotopic compositions of Ca-Al-rich inclusions (CAIs) in chondrites have constrained the ranges of physicochemical conditions, such as temperature and pressure, in the CAI-forming region. In contrast, constituent minerals of igneous CAIs, such as Compact Type A (CTA) and Type B, preserve the temporal evolution of the nebular environment during crystallization. In particular, Al-Ti-rich clinopyroxene, called by fassaite, exhibits oxygen isotopic evolution from 16O-poor to 16O-rich, presumably due to gas-melt interactions (e.g., Kawasaki et al., 2018; Suzumura et al., 2021; Yamamoto et al., 2021). Chemical characteristics are also expected to record the evolution of the nebular environment; for instance, the Ti valence state is inferred to reflect redox conditions. Although previous studies reported a decrease in the Ti3+/Titotal from ~0.7 to 0.1 with decreasing total Ti content (Titotal = Ti3+ + Ti4+) during fassaite growth (e.g., Simon and Grossman, 2006), this trend has been interpreted as the result of fractional crystallization without equilibration with the nebular gas. On the other hand, rare earth elements (REEs) are used to estimate condensation processes and the degree of fractional crystallization, and their anomalies are sensitive to changes in valence states. Therefore, identifying effective REE species as redox indicators along growth profiles will contribute to understanding of solar nebula dynamics.
In this study, we performed petrographic observations using SEM-EDS, Ti spot and mapping analyses using EPMA (JEOL JXA-8900, JXA-iSP100), and trace element analyses using LA-ICP-MS (Raijin α + Agilent 8900) along the fassaite growth profiles. KU-N-04 CTA CAI from the NWA 7865 reduced CV3 chondrite is the counterpart to thin section KU-N-02, which is well-studied their formation history, O-isotope evolution, and Al-Mg systematics (Suzumura et al., 2021, 2024).
Blocky fassaite crystals in KU-N-04 are the final solidification phase from the residual melt, and exhibit concentric zoning, with decreasing Ti contents from the core to the rim (Titotal: 17 ~ 4 wt%). The Ti3+/Titotal remains nearly constant (0.7–0.85) regardless of the Titotal content, and the clear decreasing trend reported in previous studies was not observed. REE patterns exhibit an enrichment from light to heavy REEs with a negative Eu anomaly, consistent with previous studies (e.g., Simon et al., 1999). REE concentrations increase with crystal growth, which was also confirmed two-dimensionally by REE mapping. The degree of the negative Eu anomaly (Eu/Eu*) significantly decreases from 0.2 to 0.05 with crystal growth. Crystallization model calculations under closed- and open-system conditions (constant redox, progressive oxidation, and progressive reduction) indicate that a deepening negative Eu anomaly requires the residual melt to reach equilibrium with a progressively reducing gas. This suggests that the depletion of Ti3+ in the melt due to fractional crystallization was offset by the reduction of Ti4+ to Ti3+ driven by the surrounding reducing gas, thereby maintaining a nearly constant Ti3+/Titotal. This chemical trend is synchronized with the oxygen isotopic evolution from 16O-poor (Δ17O~ −2‰) to 16O-rich (Δ17O~ −24‰) observed in KU-N-02 fassaite. Coupled chemical and isotopic evolution suggests that the depletion of H2O gas (the 16O-poor reservoir) led to a decrease in the H2O/H2 (redox state), causing the nebular gas to become progressively more reducing. These results demonstrate that REE analysis along growth profiles within a single crystal can reveal the dynamic evolution of the redox environment in the early solar nebula.
In this study, we performed petrographic observations using SEM-EDS, Ti spot and mapping analyses using EPMA (JEOL JXA-8900, JXA-iSP100), and trace element analyses using LA-ICP-MS (Raijin α + Agilent 8900) along the fassaite growth profiles. KU-N-04 CTA CAI from the NWA 7865 reduced CV3 chondrite is the counterpart to thin section KU-N-02, which is well-studied their formation history, O-isotope evolution, and Al-Mg systematics (Suzumura et al., 2021, 2024).
Blocky fassaite crystals in KU-N-04 are the final solidification phase from the residual melt, and exhibit concentric zoning, with decreasing Ti contents from the core to the rim (Titotal: 17 ~ 4 wt%). The Ti3+/Titotal remains nearly constant (0.7–0.85) regardless of the Titotal content, and the clear decreasing trend reported in previous studies was not observed. REE patterns exhibit an enrichment from light to heavy REEs with a negative Eu anomaly, consistent with previous studies (e.g., Simon et al., 1999). REE concentrations increase with crystal growth, which was also confirmed two-dimensionally by REE mapping. The degree of the negative Eu anomaly (Eu/Eu*) significantly decreases from 0.2 to 0.05 with crystal growth. Crystallization model calculations under closed- and open-system conditions (constant redox, progressive oxidation, and progressive reduction) indicate that a deepening negative Eu anomaly requires the residual melt to reach equilibrium with a progressively reducing gas. This suggests that the depletion of Ti3+ in the melt due to fractional crystallization was offset by the reduction of Ti4+ to Ti3+ driven by the surrounding reducing gas, thereby maintaining a nearly constant Ti3+/Titotal. This chemical trend is synchronized with the oxygen isotopic evolution from 16O-poor (Δ17O~ −2‰) to 16O-rich (Δ17O~ −24‰) observed in KU-N-02 fassaite. Coupled chemical and isotopic evolution suggests that the depletion of H2O gas (the 16O-poor reservoir) led to a decrease in the H2O/H2 (redox state), causing the nebular gas to become progressively more reducing. These results demonstrate that REE analysis along growth profiles within a single crystal can reveal the dynamic evolution of the redox environment in the early solar nebula.