Chromium Isotopic Evidence for an Early Formation of Chondrules from the Ornans CO Chondrite
Chromium Isotopic Evidence for an Early Formation of Chondrules from the Ornans CO Chondrite
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DOI：
10.3847/1538-4357/aafe79
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发表时间：
2019-03
期刊：
影响因子：
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通讯作者：
K. Zhu 朱;Jia Liu;F. Moynier;L. Qin;C. Alexander;Yongsheng He
中科院分区：
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作者：
K. Zhu 朱;Jia Liu;F. Moynier;L. Qin;C. Alexander;Yongsheng He
球粒是原始陨石的主要成分，也可能是行星胚胎和类地行星的基石。然而，它们的年龄和形成方式仍然存在很大争议。在这里，我们提出了高精度的铬同位素数据的9个球粒从一个更原始的陨石，CO 3球粒陨石奥尔南斯。这些球粒定义了一个外部的53 Mn-53 Cr等时线，初始53 Mn/55 Mn为（7.1 ± 1.6）× 10−6，对应于4567.6 ± 1.3 Ma（当锚定在角闪石D 'Orbigny（U校正））。该年龄与富钙铝包裹体（CAIs）的形成年龄误差不大。所有球粒的ε 54 Cr值范围很广（+0.20至+1.22），ε 53 Cr和ε 54 Cr值之间呈正相关，表明原行星盘中不同同位素来源的混合。这可以反映来自CAI形成区域的硅酸盐材料（具有与CAI互补的组成，即，低Mn/Cr和ε 54 Cr）迁移到CO球粒陨石母体的增生区，并与高Mn/Cr和ε 54 Cr的类Cl物质混合。这种混合必须在球粒前体形成之前发生。此外，来自具有更多CAIs（CV和CO）的陨石球粒在ε 54 Cr方面表现出比来自具有更少CAIs的稍后形成的陨石球粒更大的变异性（例如，CB和CR），表明前者的吸积区比后者接收更多的来自内太阳系的物质。这种二分法可能表明CB和CR球粒陨石比其他球粒陨石的轨道距离更大。
Chondrules are the main components of primitive meteorites and possibly the building blocks of planetary embryos and terrestrial planets. However, their ages and modes of formation are still highly debated. Here, we present high-precision Cr isotope data of nine chondrules from one of the more primitive chondrites, the CO3 chondrite Ornans. These chondrules define an external 53Mn–53Cr isochron, with an initial 53Mn/55Mn of (7.1 ± 1.6) × 10−6, corresponding to an age of 4567.6 ± 1.3 Ma when anchored to the angrite D’Orbigny (U-corrected). This age is within error of the age of formation of calcium-aluminum-rich inclusions (CAIs). All chondrules show a wide range of ε54Cr values (+0.20 to +1.22) and a positive correlation between ε53Cr and ε54Cr values, suggesting mixing of different isotopic sources in the protoplanetary disk. This could reflect that silicate materials from the CAI-forming region (with complementary compositions to CAIs, i.e., low Mn/Cr and ε54Cr) were transported to the accretion region of the CO chondrite parent body and mixed with CI-like material (high-Mn/Cr and ε54Cr) during chondrule formation. Such mixing must have occurred prior to the formation of chondrule precursors. Furthermore, chondrules from chondrites with more CAIs (CV and CO) exhibit greater variability in ε54Cr than chondrules from chondrites formed later with fewer CAIs (e.g., CB and CR), suggesting that the accretion regions of the former received more material transported from the inner solar system than the latter. This dichotomy may indicate the CB and CR chondrites accreted at greater orbital distances than other chondrites.