17:15 〜 18:45
[MZZ45-P05] Nucleosynthesis calcium isotope anomalies in refractory inclusions from the CV chondrites
キーワード：核合成起源の同位体異常、CAI、カルシウム同位体
Nucleosynthetic isotopic anomalies of minor and trace elements (e.g., Ti, Cr, Sr, Mo, Nd) in calcium-aluminum-rich inclusions (CAIs) have been studied intensively over the last two decades [1,2]. Isotopic anomalies of heavy elements in CAIs indicate that the components produced by supernovae were heterogeneously distributed in the early Solar System [2]. Nevertheless, due to the small contribution of heavy elements to the mineral components, the relationship between the isotopic anomalies of heavy elements seen in CAIs and the chemical evolution of nebular gases remains unclear. In contrast, Ca is enriched in CAIs because it is a major element in diopside and melilite. Since the mineral composition of CAIs reflects direct condensation from nebular gases [3], the Ca isotopic composition is important for understanding the relationship between isotopic anomalies and the chemical evolution of the nebular gas. However, the typical Ca isotope measurements with conventional analytical precision have not been sufficient to fully discuss the variation of Ca isotope compositions in CAIs for elucidating the isotopic and chemical evolution of the early Solar System.
Here, we performed high-precision Ca isotopic measurements of refractory materials including CAIs, amoeboid olivine aggregates (AOAs), and Al-rich chondrules (ARCs) found from three CV3ox chondrites (Allende, NWA 2364, and NWA 3118). The CAIs studied include coarse-grained (CGs), which underwent melting, and fine-grained (FGs), which escaped melting after condensation from nebular gases. The Ca isotopic compositions in the samples were measured using an MC-ICP-MS instrument (Neptune-Plus; Thermo Fisher Scientific) installed at the University of Copenhagen. The 43Ca/44Ca and 48Ca/44Ca ratios were reported as µ43Ca and µ48Ca notations, respectively, representing 106 times relative deviation from NIST SRM 915b.
In the refractory inclusions studied here, µ43Ca and µ48Ca were negatively correlated, and no differences were found in the variability trends of isotopic compositions between CGs, FGs, AOAs, and ARCs. Since the Ca abundances in the CAIs are higher than in the matrix, the effect of matrix mixing during the sample collection is expected to be negligible. Additionally, a mixing line with a positive slope should form when terrestrial materials and the Allende matrix are mixed to refractory inclusions. This indicates that the terrestrial contamination and the Allende matrix mixing cannot explain the negative correlation between μ43Ca and μ48Ca.
The FGs examined in this study are mainly composed of Al-diopside and spinel, and the µ43Ca value becomes relatively high and low in spinel-depleted and spinel-rich FGs, respectively, while the opposite is the case for µ48Ca. These observations suggest that the modal abundance of several mineral phases with distinct Ca isotopic compositions controls the µ43Ca and µ48Ca values of each FG. The association between mineral phases and µCa values reflects the evolution of chemical and Ca isotopic compositions in the CAI-forming gas reservoirs. The mineralogy of the FGs indicates that spinel condensed prior to melilite and diopside [4]. Thus, spinel-rich FGs would have condensed preferentially at a location closer to the central star where the gas temperature and μ48Ca were higher and μ43Ca was lower than those in the condensation location of melilite and diopside. The relationship between Ca isotopic abundance and formation location indicates that the isotopic heterogeneity of the CAI formation location extended in the heliocentric direction of the disk.
References: [1] Trinquier et al. (2009). Science, 324, 374. [2] Brennecka et al. (2013). PNAS, 110, 17241. [3] Grossman (1973). GCA, 37, 1119. [4] Krot et al. (2004). MAPS, 39, 1517.
Here, we performed high-precision Ca isotopic measurements of refractory materials including CAIs, amoeboid olivine aggregates (AOAs), and Al-rich chondrules (ARCs) found from three CV3ox chondrites (Allende, NWA 2364, and NWA 3118). The CAIs studied include coarse-grained (CGs), which underwent melting, and fine-grained (FGs), which escaped melting after condensation from nebular gases. The Ca isotopic compositions in the samples were measured using an MC-ICP-MS instrument (Neptune-Plus; Thermo Fisher Scientific) installed at the University of Copenhagen. The 43Ca/44Ca and 48Ca/44Ca ratios were reported as µ43Ca and µ48Ca notations, respectively, representing 106 times relative deviation from NIST SRM 915b.
In the refractory inclusions studied here, µ43Ca and µ48Ca were negatively correlated, and no differences were found in the variability trends of isotopic compositions between CGs, FGs, AOAs, and ARCs. Since the Ca abundances in the CAIs are higher than in the matrix, the effect of matrix mixing during the sample collection is expected to be negligible. Additionally, a mixing line with a positive slope should form when terrestrial materials and the Allende matrix are mixed to refractory inclusions. This indicates that the terrestrial contamination and the Allende matrix mixing cannot explain the negative correlation between μ43Ca and μ48Ca.
The FGs examined in this study are mainly composed of Al-diopside and spinel, and the µ43Ca value becomes relatively high and low in spinel-depleted and spinel-rich FGs, respectively, while the opposite is the case for µ48Ca. These observations suggest that the modal abundance of several mineral phases with distinct Ca isotopic compositions controls the µ43Ca and µ48Ca values of each FG. The association between mineral phases and µCa values reflects the evolution of chemical and Ca isotopic compositions in the CAI-forming gas reservoirs. The mineralogy of the FGs indicates that spinel condensed prior to melilite and diopside [4]. Thus, spinel-rich FGs would have condensed preferentially at a location closer to the central star where the gas temperature and μ48Ca were higher and μ43Ca was lower than those in the condensation location of melilite and diopside. The relationship between Ca isotopic abundance and formation location indicates that the isotopic heterogeneity of the CAI formation location extended in the heliocentric direction of the disk.
References: [1] Trinquier et al. (2009). Science, 324, 374. [2] Brennecka et al. (2013). PNAS, 110, 17241. [3] Grossman (1973). GCA, 37, 1119. [4] Krot et al. (2004). MAPS, 39, 1517.