2:30 PM - 2:45 PM
[PPS08-04] Nucleosynthetic chromium isotope anomalies in Al-rich chondrules
Keywords:Al-rich chondrule, Isotope anomalies, Carbonaceous Chondrites
Chondrites are important research targets for understanding the origin and evolution of the Solar System because they did not experience melting in their parent bodies. Recent high-precision isotopic measurements of O, Ti, and Cr enabled a new classification of chondrites into groups, carbonaceous chondrites (CCs) and non-carbonaceous chondrites (NCs), with no meteorites having intermediate isotopic compositions between the two groups (Warren, 2011; Kleine et al., 2020). This is referred to as the 'isotopic dichotomy' of the meteorites and suggests that the origins of the parent bodies of CCs and NCs are spatiotemporally distinct. In contrast, the isotopic ratios of Cr, Ti and O in chondrules from CCs (hereafter CC chondrules) showed large variations that exceeded the range of those of bulk CCs, whereas chondrules from NCs (hereafter NC chondrules) had isotopic compositions consistent with those of bulk NC chondrites (e.g., Schneider et al., 2020). Therefore, understanding the origin of CC chondrules is essential for understanding material transport and mixing processes in the early Solar System. Among the CC chondrules for which isotopic ratios have been measured in previous studies, Al-rich chondrules (ARCs) are unique in their isotopic composition. ARCs are more Al-rich than Mg-rich chondrules that can be commonly found in CCs, which have characteristics of both Mg-rich chondrules and CAIs in terms of mineral composition and oxygen isotope ratios. The isotopic ratios of ARCs can be explained by the mixing of the isotopic ratios of bulk NCs and CAIs (Schneider et al., 2020; Williams et al., 2020), making it an important material for elucidating the mixing process between the two. However, as ARCs are a rare constituent of meteorites, there are very few examples of isotope ratio measurements in previous studies.
In this study, we found two ARCs in the thick sections of Allende (CVox3) and one in Leoville (CVred3). After a detailed mineral description of the ARCs, we sampled them with a microdrill device to determine the elemental abundances and Cr isotope ratios by using ICP-MS and TIMS, respectively. On the basis of the mineral assemblages and elemental abundances in the ARCs, the three ARCs are found to be classified into two groups. Two ARCs (Allende-ARC-1, Leoville-ARC), classified as porphyritic olivine pyroxene (POP), were characterized by a mottled texture with Group II CAI-like REE abundance patterns showing depletions in HREEs. On the other hand, the other ARC (Allende-ARC-2), classified as BO (Barred Olivine), was characterized by rod-shaped crystals with a relatively flat REE pattern. The results indicate that the ARCs were derived from different types of CAI and formed in different environments. The Cr isotope ratios (ε54Cr values; 104 times deviation from a terrestrial standard) were 1.57 ± 0.45 and 0.42 ± 0.46 for Allende-ARC-1 and Leoville-ARC, respectively. These values were similar to those in Mg-rich chondrules and showed little influence of CAIs. The Cr abundance in CAIs is ~200 μg/g (Trinquier et al., 2009) while that in normal chondrules is ~3000 μg/g (Zhu et al., 2019). Assuming that the CAI and chondrule precursor incorporated into the ARCs had Cr = 200 µg/g and ε54Cr = 10.0 for the CAI and Cr = 3000 µg/g and ε54Cr = 0 for the precursor, the ε54Cr value in the Leoville ARC can be explained by a mixture of the CAI and precursor with a ratio of 42 : 58.
To further discuss the correlation between chondrules and CAIs, we plan to measure Ti isotope ratios in the ARCs examined in this study.
In this study, we found two ARCs in the thick sections of Allende (CVox3) and one in Leoville (CVred3). After a detailed mineral description of the ARCs, we sampled them with a microdrill device to determine the elemental abundances and Cr isotope ratios by using ICP-MS and TIMS, respectively. On the basis of the mineral assemblages and elemental abundances in the ARCs, the three ARCs are found to be classified into two groups. Two ARCs (Allende-ARC-1, Leoville-ARC), classified as porphyritic olivine pyroxene (POP), were characterized by a mottled texture with Group II CAI-like REE abundance patterns showing depletions in HREEs. On the other hand, the other ARC (Allende-ARC-2), classified as BO (Barred Olivine), was characterized by rod-shaped crystals with a relatively flat REE pattern. The results indicate that the ARCs were derived from different types of CAI and formed in different environments. The Cr isotope ratios (ε54Cr values; 104 times deviation from a terrestrial standard) were 1.57 ± 0.45 and 0.42 ± 0.46 for Allende-ARC-1 and Leoville-ARC, respectively. These values were similar to those in Mg-rich chondrules and showed little influence of CAIs. The Cr abundance in CAIs is ~200 μg/g (Trinquier et al., 2009) while that in normal chondrules is ~3000 μg/g (Zhu et al., 2019). Assuming that the CAI and chondrule precursor incorporated into the ARCs had Cr = 200 µg/g and ε54Cr = 10.0 for the CAI and Cr = 3000 µg/g and ε54Cr = 0 for the precursor, the ε54Cr value in the Leoville ARC can be explained by a mixture of the CAI and precursor with a ratio of 42 : 58.
To further discuss the correlation between chondrules and CAIs, we plan to measure Ti isotope ratios in the ARCs examined in this study.