Abstract It is widely believed that the precursors of coarse-grained CAIs in chondrites are solar nebula condensates that were later reheated and melted to a high degree. Such melting under low-pressure conditions is expected to result in evaporation of moderately volatile magnesium and silicon and their mass-dependent isotopic fractionation. The evaporation of silicate melts has been extensively studied in vacuum laboratory experiments and a large experimental database on chemical and isotopic fractionations now exists. Nevertheless, it remains unclear if vacuum evaporation of CAI-like melts adequately describes the evaporation in the hydrogen-rich gas of the solar nebula. Here we report the results of a detailed experimental study on evaporation of a such melt at 1600 °C in both vacuum and low-pressure hydrogen gas, using 1.5- and 2.5-mm diameter samples. The experiments show that although at 2 × 10−4 bar H2 magnesium and silicon evaporate ∼2.8 times faster than at 2 × 10−5 bar H2 and ∼45 times faster than in vacuum, their relative evaporation rates and isotopic fractionation factors remain the same. This means that the chemical and isotopic evolutions of all evaporation residues plot along a single evaporation trajectory regardless of experimental conditions (vacuum or low-PH2) and sample size. The independence of chemical and isotopic evaporation trajectories on PH2 of the surrounding gas imply that the existing extensive experimental database on vacuum evaporation of CAI-like materials can be safely used to model the evaporation under solar nebula conditions, taking into account the dependence of evaporation kinetics on PH2. The experimental data suggest that it would take less than 25 min at 1600 °C to evaporate 15–50% of magnesium and 5–20% of silicon from a 2.5-mm diameter sample in a solar nebula with PH2 ∼2 × 10−4 bar and to enrich the residual melt in heavy magnesium and silicon isotopes up to δ25Mg ∼5–10‰ and δ29Si ∼2–4‰. The expected chemical and isotopic features are compatible to those typically observed in coarse-grained Type A and B CAIs. Evaporation for ∼1 h will produce δ25Mg ∼30–35‰ and δ29Si ∼10–15‰, close to the values in highly fractionated Type F and FUN CAIs. These very short timescales suggest melting and evaporation of CAI precursors in very short dynamic heating events. The experimental results reported here provide a stringent test of proposed astrophysical models for the origin and evolution of CAIs.

中文翻译：

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在低压氢气和真空中对类 CAI 熔体蒸发过程中元素和同位素分馏进行量化的实验：对原行星盘中 CAI 热处理的限制

摘要 人们普遍认为，球粒陨石中粗粒CAI的前体是太阳星云凝聚物，后来重新加热并高度熔化。这种在低压条件下的熔化预计会导致中等挥发性的镁和硅的蒸发及其质量相关的同位素分馏。硅酸盐熔体的蒸发已在真空实验室实验中得到广泛研究，并且现在存在关于化学和同位素分馏的大型实验数据库。然而，CAI 类熔体的真空蒸发是否充分描述了太阳星云富氢气体中的蒸发，目前尚不清楚。在这里，我们报告了使用 1.5 和 2 在真空和低压氢气中在 1600°C 下蒸发这种熔体的详细实验研究结果。5 毫米直径的样品。实验表明，虽然在 2 × 10−4 bar H2 条件下镁和硅的蒸发速度比在 2 × 10−5 bar H2 条件下快 2.8 倍，比真空条件下快 45 倍，但它们的相对蒸发速率和同位素分馏因子保持不变. 这意味着无论实验条件（真空或低 PH2）和样品大小如何，所有蒸发残留物的化学和同位素演化都沿着单一蒸发轨迹绘制。化学和同位素蒸发轨迹对周围气体 PH2 的独立性意味着，考虑到蒸发动力学的依赖性，可以安全地使用现有的广泛的 CAI 类材料真空蒸发实验数据库来模拟太阳星云条件下的蒸发在 PH2 上。实验数据表明，在 PH2 ∼2 × 10−4 的太阳星云中，直径 2.5 毫米的样品在 1600 °C 下蒸发 15-50% 的镁和 5-20% 的硅需要不到 25 分钟的时间。 bar 并富集残余熔体中的重镁和硅同位素，最高可达 δ25Mg ∼5–10‰ 和 δ29Si ∼2–4‰。预期的化学和同位素特征与通常在粗粒 A 型和 B 型 CAIs 中观察到的特征相兼容。蒸发约 1 小时将产生 δ25Mg 约 30-35‰ 和 δ29Si 约 10-15‰，接近高度分馏的 F 型和 FUN CAI 的值。这些非常短的时间尺度表明 CAI 前体在非常短的动态加热事件中熔化和蒸发。这里报告的实验结果为 CAI 的起源和演化提出的天体物理模型提供了严格的测试。