講演情報
[PPS12-P11]CVコンドライトコンドルール酸素同位体比のサイズ依存性
花井 翔1、福田 航平1、金丸 礼2、藤谷 渉3、寺田 健太郎1、*牛久保 孝行4,5 (1.大阪大学、2.宇宙航空研究開発機構、3.茨城大学、4.海洋研究開発機構 高知コア研究所、5.高知大学 海洋コア国際研究所)
キーワード：
コンドルール、酸素同位体比、原始太陽系円盤、二次イオン質量分析計
Revealing the efficiency of material transport in the Solar protoplanetary disk is important for understanding the locality of planetesimal precursors, which in turn constrain the evolution of planetesimals after their accretion. The (Cr-Ti-)O isotope ratios of chondrules in non-carbonaceous (NC) and major carbonaceous (CC) chondrite groups (CV, CM, and CO) typically exhibit an isotope dichotomy as observed for Cr and Ti nucleosynthetic isotope anomalies in bulk chondrites [1,2,3]. The chondrule isotope dichotomy suggests that chondritic parent bodies have been made by distinct precursor materials that locally existed in the accretion regions of each chondrite group. However, some large chondrules in CV chondrites preserved NC-like isotope characteristics, indicating that at least some portions of chondrules were transported outwards from the inner to outer Solar system [4,5]. If correct, the physical barrier between the NC and CC isotope reservoirs is not very efficient and might have allowed some material transport between the two reservoirs. This could allow the reduction of the local dependence of precursor materials for chondrite parent bodies formed in different disk regions.
To constrain the efficiency of material transport in the Solar protoplanetary disk, we investigated the size dependence of oxygen isotope ratios and chemical compositions of chondrules in CV chondrites. We focus on mini chondrules with diameters smaller than 100 µm, as smaller chondrules should have been transported more than larger chondrules by advection and diffusion in the protoplanetary disk [e.g., 6]. We analyzed 32 mini CV chondrules found in NWA 8613 and NWA 5028 that are the least metamorphosed reduced CV chondrites. The Mg# (=MgO/[MgO+FeO] molar %) and Δ17O relationships of the 32 mini CV chondrules are almost consistent with those of typical CV chondrules that are larger than 100 µm in diameter [7,8], suggesting that mini CV chondrules formed in a similar physicochemical environment where typical CV chondrules formed. In contrast to our expectations, we did not find any mini chondrules with NC-like O isotope signatures. The presence of large chondrules with NC-like signatures, but not of smaller chondrules, suggests that oxygen isotope ratios of smaller chondrules are more easily modified by gas-melt interactions than those of larger chondrules, resulting in NC-like mini chondrules being transferred to O-isotopically CC-like chondrules. Alternatively, minor amounts of larger CV chondrules might have transported in an earlier stage of the solar system evolution [5], and the CV chondrite parent body was accreted immediately after the CV chondrule formation, resulting in the majority of chondrules being dominant in the CV chondrule population.
[1] Schneider et al. (2000) EPSL 551, 116585. [2] Tenner et al. (2018) In Chondrules Records of Protoplanetary Disk Processes 196-246. [3] Marrocchi et al. (2024) SSR 220, 69. [4] Williams et al. (2020) PNAS 117, 23426-23435. [5] Fukuda et al. (2024) MAPS 59, 3282-3304. [6] Cuzzi et al. (2010) Icarus 208, 518-538. [7] Hertwig et al. (2018) GCA 224, 116-131. [8] Hertwig et al. (2019) MAPS 54, 2666-2685.
To constrain the efficiency of material transport in the Solar protoplanetary disk, we investigated the size dependence of oxygen isotope ratios and chemical compositions of chondrules in CV chondrites. We focus on mini chondrules with diameters smaller than 100 µm, as smaller chondrules should have been transported more than larger chondrules by advection and diffusion in the protoplanetary disk [e.g., 6]. We analyzed 32 mini CV chondrules found in NWA 8613 and NWA 5028 that are the least metamorphosed reduced CV chondrites. The Mg# (=MgO/[MgO+FeO] molar %) and Δ17O relationships of the 32 mini CV chondrules are almost consistent with those of typical CV chondrules that are larger than 100 µm in diameter [7,8], suggesting that mini CV chondrules formed in a similar physicochemical environment where typical CV chondrules formed. In contrast to our expectations, we did not find any mini chondrules with NC-like O isotope signatures. The presence of large chondrules with NC-like signatures, but not of smaller chondrules, suggests that oxygen isotope ratios of smaller chondrules are more easily modified by gas-melt interactions than those of larger chondrules, resulting in NC-like mini chondrules being transferred to O-isotopically CC-like chondrules. Alternatively, minor amounts of larger CV chondrules might have transported in an earlier stage of the solar system evolution [5], and the CV chondrite parent body was accreted immediately after the CV chondrule formation, resulting in the majority of chondrules being dominant in the CV chondrule population.
[1] Schneider et al. (2000) EPSL 551, 116585. [2] Tenner et al. (2018) In Chondrules Records of Protoplanetary Disk Processes 196-246. [3] Marrocchi et al. (2024) SSR 220, 69. [4] Williams et al. (2020) PNAS 117, 23426-23435. [5] Fukuda et al. (2024) MAPS 59, 3282-3304. [6] Cuzzi et al. (2010) Icarus 208, 518-538. [7] Hertwig et al. (2018) GCA 224, 116-131. [8] Hertwig et al. (2019) MAPS 54, 2666-2685.