Abstract Magnesium stable isotope ratios and minor element abundances of five olivine particles from comet 81P/Wild 2 were examined by secondary ion mass spectrometry (SIMS). Wild 2 olivine particles exhibit only small variations in δ25MgDSM-3 values from –1.0 +0.4/–0.5 ‰ to 0.6 +0.5/–0.6 ‰ (2σ). This variation can be simply explained by mass-dependent fractionation from Mg isotopic compositions of the Earth and bulk meteorites, suggesting that Wild 2 olivine particles formed in the chondritic reservoir with respect to Mg isotope compositions. We also determined minor element abundances, and O and Mg isotope ratios of olivine grains in amoeboid olivine aggregates (AOAs) from Kaba (CV3.1) and DOM 08006 (CO3.01) carbonaceous chondrites. Our new SIMS minor element data reveal uniform, low FeO contents of ∼0.05 wt% among AOA olivines from DOM 08006, suggesting that AOAs formed at more reducing environments in the solar nebula than previously thought. Furthermore, the SIMS-derived FeO contents of the AOA olivines are consistently lower than those obtained by electron microprobe analyses (∼1 wt% FeO), indicating possible fluorescence from surrounding matrix materials and/or Fe,Ni-metals in AOAs during electron microprobe analyses. For Mg isotopes, AOA olivines show more negative mass-dependent fractionation (–3.8 ± 0.5‰ ≤ δ25MgDSM-3 ≤ –0.2 ± 0.3‰; 2σ) relative to Wild 2 olivines. Further, these Mg isotope variations are correlated with their host AOA textures. Large negative Mg isotope fractionations in olivine are often observed in pore-rich AOAs, while those in compact AOAs tend to have near-chondritic Mg isotopic compositions. These observations indicate that pore-rich AOAs preserved their gas-solid condensation histories, while compact AOAs experienced thermal processing in the solar nebula after their condensation and aggregation. Importantly, one 16O-rich Wild 2 LIME olivine particle (T77/F50) shows negative Mg isotope fractionation (δ25MgDSM-3 = –0.8 ± 0.4‰, δ26MgDSM-3 = –1.4 ± 0.9‰; 2σ) relative to bulk chondrites. Minor element abundances of T77/F50 are in excellent agreement with those of olivines from pore-rich AOAs in DOM 08006. The observed similarity in O and Mg isotopes, and minor element abundances suggest that T77/F50 formed in an environment similar to AOAs, probably near the proto-Sun, and then was transported to the Kuiper belt, where comet 81P/Wild 2 likely accreted.

中文翻译：

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彗星 81P/Wild 2 以及来自 CV 和 CO 球粒陨石的 AOA 中橄榄石凝结起源的相关同位素和化学证据

摘要 采用二次离子质谱（SIMS）检测了来自 81P/Wild 2 彗星的 5 个橄榄石颗粒的镁稳定同位素比和微量元素丰度。野生 2 橄榄石颗粒的 δ25MgDSM-3 值仅表现出很小的变化，从 –1.0 +0.4/–0.5 ‰ 到 0.6 +0.5/–0.6 ‰ (2σ)。这种变化可以通过地球和块状陨石的镁同位素组成的质量依赖性分馏来简单解释，这表明就镁同位素组成而言，Wild 2 橄榄石颗粒是在球粒陨石储层中形成的。我们还测定了来自 Kaba (CV3.1) 和 DOM 08006 (CO3.01) 碳质球粒陨石的变形虫橄榄石聚集体 (AOA) 中的微量元素丰度以及 O 和 Mg 同位素比率。我们新的 SIMS 微量元素数据显示，来自 DOM 08006 的 AOA 橄榄石中 FeO 含量均匀、低，约为 0.05 wt%，这表明 AOA 形成于太阳星云中比之前认为的还原性环境更强的环境中。此外，AOA 橄榄石中 SIMS 衍生的 FeO 含量始终低于通过电子微探针分析获得的含量（~1 wt% FeO），表明在电子微探针过程中可能来自周围基质材料和/或 AOA 中的 Fe、Ni 金属的荧光分析。对于 Mg 同位素，相对于 Wild 2 橄榄石，AOA 橄榄石表现出更多的负质量依赖性分馏 (–3.8 ± 0.5‰ ≤ δ25MgDSM-3 ≤ –0.2 ± 0.3‰; 2σ)。此外，这些镁同位素变化与其宿主 AOA 纹理相关。橄榄石中大的负镁同位素分馏经常在富含孔隙的 AOA 中观察到，而致密 AOA 中的镁同位素组成往往接近球粒状。这些观察结果表明，富含孔隙的 AOA 保留了其气固凝结历史，而致密的 AOA 在凝结和聚集后在太阳星云中经历了热处理。重要的是，一颗富含 16O 的 Wild 2 LIME 橄榄石颗粒 (T77/F50) 相对于块状球粒陨石显示出负的 Mg 同位素分馏 (δ25MgDSM-3 = –0.8 ± 0.4 ‰，δ26MgDSM-3 = –1.4 ± 0.9 ‰；2σ)。T77/F50 的微量元素丰度与 DOM 08006 中富含孔隙的 AOA 中的橄榄石非常一致。观察到的 O 和 Mg 同位素的相似性以及微量元素丰度表明 T77/F50 形成于与 AOA 类似的环境中，可能在原太阳附近，然后被传送到柯伊伯带，彗星 81P/Wild 2 可能在那里吸积。