Torrano Zachary A, Brennecka Gregory A, Williams Curtis D, Romaniello Stephen J, Rai Vinai K, Wadhwa Meenakshi
Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287, USA.
Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Germany.
Geochim Cosmochim Acta. 2019 Oct 15;263:13-30. doi: 10.1016/j.gca.2019.07.051. Epub 2019 Aug 2.
Calcium-aluminum-rich inclusions (CAIs) are the first solids to form in the early Solar System, and they exhibit nucleosynthetic anomalies in many isotope systems. The overwhelming majority of isotopic data for CAIs has been limited to inclusions from the CV chondrite Allende and a select few other CV, CO, CM, and ordinary chondrites. It is therefore important to ascertain whether previously reported values for CAIs are representative of the broader CAI-forming region and to make a more rigorous assessment of the extent and implications of isotopic heterogeneity in the early Solar System. Here, we report the mass-independent Ti isotopic compositions of a suite of 23 CAIs of diverse petrologic and geochemical types, including 11 from Allende and 12 from seven other CV3 and CK3 chondrites; the data for CAIs from CK chondrites represent the first reported measurements of Ti isotope compositions of refractory inclusions from this meteorite class. The resolved variation in the mass-independent Ti isotopic compositions of these CAIs indicates that the CAI-forming region of the early Solar System preserved isotopic variability at their time of formation. Nevertheless, the range of Ti isotope compositions reported here for CAIs from CV and CK chondrites falls within the range observed in previously analyzed CAIs from CV, CO, CM, and ordinary chondrites. This implies that CAIs from CV, CK, CO, CM, and ordinary chondrites originated from a common nebular source reservoir characterized by mass-independent isotopic variability in Ti (and other select elements). We further interpret these data to indicate that the Ti isotope anomalies in CAIs represent the isotopic signatures of supernova components in presolar grains that were incorporated into the Solar System in an initially poorly mixed reservoir that was progressively homogenized over time. We conclude that the differing degrees of isotopic variability observed for different elements in normal CAIs are the result of distinct carrier phases and that these CAIs were likely formed towards the final stages of homogenization of the large-scale isotopic heterogeneity that initially existed in the solar nebula.
富钙铝包体（CAIs）是早期太阳系中最早形成的固体，并且它们在许多同位素体系中表现出核合成异常。CAIs的绝大多数同位素数据仅限于来自碳质球粒陨石阿伦德的包体以及其他少数几种碳质球粒陨石、氧化型普通球粒陨石、原始型普通球粒陨石和普通球粒陨石。因此，确定之前报道的CAIs值是否代表更广泛的CAI形成区域，并对早期太阳系中同位素非均一性的程度和影响进行更严格的评估非常重要。在此，我们报告了一组23个不同岩石学和地球化学类型的CAIs的质量无关钛同位素组成，其中包括11个来自阿伦德的和12个来自其他七种CV3和CK3球粒陨石的；来自CK球粒陨石的CAIs数据代表了该陨石类难熔包体钛同位素组成的首次报道测量结果。这些CAIs的质量无关钛同位素组成的解析变化表明，早期太阳系的CAI形成区域在其形成时保留了同位素变异性。然而，这里报道的来自CV和CK球粒陨石的CAIs的钛同位素组成范围落在之前分析的来自CV、CO、CM和普通球粒陨石的CAIs所观察到的范围内。这意味着来自CV、CK、CO、CM和普通球粒陨石的CAIs起源于一个共同的星云源储库，其特征是钛（以及其他特定元素）存在质量无关的同位素变异性。我们进一步解释这些数据表明，CAIs中的钛同位素异常代表了前太阳颗粒中超新星成分的同位素特征，这些颗粒在一个最初混合不佳的储库中被纳入太阳系，该储库随着时间的推移逐渐均匀化。我们得出结论，正常CAIs中不同元素观察到的不同程度的同位素变异性是不同载体相的结果，并且这些CAIs可能是在太阳星云最初存在的大规模同位素非均一性的均匀化的最后阶段形成的。
