A multielement isotopic study of refractory FUN and F CAIs: Mass-dependent and mass-independent isotope effects
A multielement isotopic study of refractory FUN and F CAIs: Mass-dependent and mass-independent isotope effects
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DOI：
10.1016/j.gca.2017.04.029
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发表时间：
2018-01-15
影响因子：
5
通讯作者：
Davis, Andrew M.
中科院分区：
文献类型：
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作者：
Koop, Levke;Nakashima, Daisuke;Davis, Andrew M.
富含钙铝的包裹体（CAI）是太阳系内形成的最古老的年代物体。其中包括具有较大质量依赖性分馏效应 (F CAI) 的稀有神秘物体，有时也具有较大的核合成异常和短寿命放射性核素 Al-26 (FUN CAI) 的初始丰度较低。我们研究了来自默奇森 (CM2) 陨石的 7 种富含难熔黑铝矿的 CAI 和一种富含钙镁石的 CAI，了解它们的氧、钙和钛同位素组成。 Al-26-Mg-26 系统也在其中七个 CAI 中进行了研究。我们在所有测量的元素中发现了质量依赖性重同位素富集，但在同一个 CAI 中从未同时富集。这些数据很难与单级熔体蒸发起源相一致，并且对于某些研究的 CAI，蒸发后可能需要重新引入或重新平衡镁、氧和钛。从模型等时线推断的初始 Al-26/Al-27 比率范围从 < 1 x 10(-6) 到规范（类似于 5 x 10(-5)）。 CAI 显示了推断的活性 Al-26 掺入与 Ca-48 和 Ti-50 中可解析异常的存在之间的相互排斥关系。此外，在富含黑铝矿的 CAI 中，Al-26 的掺入与 Delta O-17 之间存在关系（即，不含 Al-26 的 CAI 已解决了 Delta O-17 的变化，而解决了 Mg-26 过量的 CAI 的 Delta O-17 值接近 -23 ppm）。尽管 Al-26/Al-27 接近标准，但只有富含钙镁石的 CAI 具有相对较高的 Delta O-17 值（类似于千分之 17）。我们将这些数据解释为表明，富含分馏黑碳酸钙的 CAI 在很长一段时间内形成，并对太阳星云同位素演化的多个阶段进行了采样，包括：(1) 贫 Al-26 星云，Ca-48 和 Ti-50 以及可变 Delta O-17 中存在较大的正负异常； (2) Al-26 混合阶段，在此期间 Ca-48 和 Ti-50 的异常已被大大稀释，并且在 CAI 形成区域中已实现了类似于千分之 23 的 Delta O-17 值； (3) 在 CAI 形成区域中，Al-26 的星云接近标准水平，Delta O-17 值接近千分之 23。 (C) 2017 Elsevier Ltd. 保留所有权利。
Calcium-aluminum-rich inclusions (CAIs) are the oldest dated objects that formed inside the Solar System. Among these are rare, enigmatic objects with large mass-dependent fractionation effects (F CAIs), which sometimes also have large nucleosynthetic anomalies and a low initial abundance of the short-lived radionuclide Al-26 (FUN CAIs). We have studied seven refractory hibonite-rich CAIs and one grossite-rich CAI from the Murchison (CM2) meteorite for their oxygen, calcium, and titanium isotopic compositions. The Al-26-Mg-26 system was also studied in seven of these CAIs. We found mass-dependent heavy isotope enrichment in all measured elements, but never simultaneously in the same CAI. The data are hard to reconcile with a single-stage melt evaporation origin and may require reintroduction or reequilibration for magnesium, oxygen and titanium after evaporation for some of the studied CAIs.The initial Al-26/Al-27 ratios inferred from model isochrons span a range from < 1 x 10(-6) to canonical (similar to 5 x 10(-5)). The CAIs show a mutual exclusivity relationship between inferred incorporation of live Al-26 and the presence of resolvable anomalies in Ca-48 and Ti-50. Furthermore, a relationship exists between Al-26 incorporation and Delta O-17 in the hibonite-rich CAIs (i.e., Al-26-free CAIs have resolved variations in Delta O-17, while CAIs with resolved Mg-26 excesses have Delta O-17 values close to -23 parts per thousand). Only the grossite-rich CAI has a relatively enhanced Delta O-17 value (similar to-17 parts per thousand) in spite of a near-canonical Al-26/Al-27. We interpret these data as indicating that fractionated hibonite-rich CAIs formed over an extended time period and sampled multiple stages in the isotopic evolution of the solar nebula, including: (1) an Al-26-poor nebula with large positive and negative anomalies in Ca-48 and Ti-50 and variable Delta O-17; (2) a stage of Al-26-admixture, during which anomalies in Ca-48 and Ti-50 had been largely diluted and a Delta O-17 value of similar to-23 parts per thousand had been achieved in the CAI formation region; and (3) a nebula with an approximately canonical level of Al-26 and a Delta O-17 value of similar to-23 parts per thousand in the CAI formation region. (C) 2017 Elsevier Ltd. All rights reserved.