Experiments quantifying elemental and isotopic fractionations during evaporation of CAI-like melts in low-pressure hydrogen and in vacuum: Constraints on thermal processing of CAIs in the protoplanetary disk
Experiments quantifying elemental and isotopic fractionations during evaporation of CAI-like melts in low-pressure hydrogen and in vacuum: Constraints on thermal processing of CAIs in the protoplanetary disk
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量化低压氢气和真空中 CAI 类熔体蒸发过程中元素和同位素分馏的实验：原行星盘中 CAI 热处理的限制
DOI：
10.1016/j.gca.2020.09.005
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发表时间：
2021
影响因子：
5
通讯作者：
Mendybaev R
中科院分区：
文献类型：
--
作者：
Mendybaev R
人们普遍认为球粒陨石中粗粒cai的前体是太阳星云凝聚体，这些凝聚体后来被再加热并熔化到很高的程度。这种在低压条件下的熔融预计会导致中等挥发性镁和硅的蒸发以及它们的质量依赖的同位素分馏。硅酸盐熔体的蒸发在真空实验室实验中得到了广泛的研究，目前存在一个关于化学和同位素分馏的大型实验数据库。然而，目前尚不清楚的是，类cai熔体的真空蒸发是否能充分描述太阳星云中富氢气体的蒸发。在这里，我们报告了一项详细的实验研究的结果，在1600 °C的真空和低压氢气中蒸发这种熔体，使用直径1.5和2.5 mm的样品。实验表明，虽然在2 × 10−4bar条件下，h2镁和硅的蒸发速度比在2 × 10−5bar条件下快~ 2.8倍，比在真空条件下快~ 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）和样本量如何，所有蒸发残留物的化学和同位素演变都沿着单一蒸发轨迹绘制。化学和同位素蒸发轨迹与周围气体PH2的独立性意味着，考虑到蒸发动力学对PH2的依赖性，现有的广泛的cai类材料真空蒸发实验数据库可以安全地用于模拟太阳星云条件下的蒸发。实验数据表明，在1600 °C下，在PH2 ~ 2 × 10−4bar的太阳星云中，从直径2.5 mm的样品中蒸发15-50%的镁和5-20%的硅只需要不到25分钟，并使重镁和重硅同位素的残余熔体富集到δ25Mg ~ 5-10‰和δ29Si ~ 2 - 4‰。预期的化学和同位素特征与在粗粒度A型和B型cai中典型观察到的特征相一致。蒸发1 h将产生δ25Mg ~ 30 ~ 35‰和δ29Si ~ 10 ~ 15‰，接近高分馏F型和FUN型CAIs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提出的cai起源和演化的天体物理模型提供了严格的测试。
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 × 10−4bar H2magnesium and silicon evaporate ∼2.8 times faster than at 2 × 10−5bar H2and ∼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 PH2of 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 × 10−4bar 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.
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影响因子：
5
作者：
Grossman, L.;Simon, S. B.;Mayeda, T. K.
通讯作者：
Mayeda, T. K.
C3球粒陨石中难熔包裹体的主量元素化学和同位素组成：凝结和蒸发的单独作用☆
影响因子：
5
作者：
L. Grossman;D. Ebel;S. Simon;A. Davis;F. Richter;N. Parsad
通讯作者：
N. Parsad
DOI：
--
发表时间：
2003
期刊：
影响因子：
--
作者：
G. MacPherson
通讯作者：
G. MacPherson
关于球粒的起源
影响因子：
5.2
作者：
C. Sonett
通讯作者：
C. Sonett
镁橄榄石的蒸发动力学及其对早期太阳星云的影响
影响因子：
64.8
作者：
A. Hashimoto
通讯作者：
A. Hashimoto