近日，成都理工大学刘耘团队最新的研究重建了真空条件下硅酸盐熔体蒸发的同位素分馏理论。这一研究成果发表在2024年6月14日出版的国际学术期刊《地球化学学报》上。

据介绍，同位素效应是理解硅酸盐熔体蒸发和行星吸积过程的关键。目前的理论通常基于Hertz-Knudsen方程，由于其过于简化的假设，常常无法预测实验室实验中观察到的同位素分值。研究人员指出，基于Hertz-Knudsen方程的理论对于硅酸盐熔体蒸发情况是不完整的，并且只能用于蒸发的物质与熔体中的物质相同的情况。

研究人员提出了一种新的真空条件下硅酸盐熔体蒸发模型。该模型考虑了多个步骤，包括传质、化学反应和成核。模型的推导揭示了动力学同位素分馏因子(KIFF或α) α_{our model}=[m(¹species)/m(²species)]^0.5，其中m(species)为反应/成核限制步骤的反应物或扩散限制步骤的反应物质量，上标1和2分别表示轻同位素和重同位素。

该模型可以有效地再现大多数实验室实验已报道的各种元素KIFFs，即Mg、Si、K、Rb、Fe、Ca和Ti。KIFF混合模型，即通过两个步骤共同确定总蒸发速率，该模型可以解释低P_H2压力、成分和温度的影响。此外，该研究发现，通过使用ln(-lnf)对ln(t)的拟合斜率，化学反应、扩散和成核可以控制硅酸盐熔体的总蒸发速率。值得注意的是，研究模型允许对活化能(E_a)等参数进行理论计算，为研究成分和环境对蒸发过程的影响提供了一种新方法，并揭示了原始太阳和地月系统的形成和演化。

附：英文原文

Title: Rebuilding the theory of isotope fractionation for evaporation of silicate melts under vacuum condition

Author: Wang, Jie, Liu, Yun

Issue&Volume: 2024-06-14

Abstract: Isotope effects are pivotal in understanding silicate melt evaporation and planetary accretion processes. Based on the Hertz–Knudsen equation, the current theory often fails to predict observed isotope fractionations of laboratory experiments due to its oversimplified assumptions. Here, we point out that the Hertz-Knudsen-equation-based theory is incomplete for silicate melt evaporation cases and can only be used for situations where the vaporized species is identical to the one in the melt. We propose a new model designed for silicate melt evaporation under vacuum. Our model considers multiple steps including mass transfer, chemical reaction, and nucleation. Our derivations reveal a kinetic isotopic fractionation factor (KIFF or α)  α_{our model}=[m(¹species)/m(²species)]^0.5, where m(species) is the mass of the reactant of reaction/nucleation-limiting step or species of diffusion-limiting step and superscript 1 and 2 represent light and heavy isotopes, respectively. This model can effectively reproduce most reported KIFFs of laboratory experiments for various elements, i.e., Mg, Si, K, Rb, Fe, Ca, and Ti. And, the KIFF-mixing model referring that an overall rate of evaporation can be determined by two steps jointly can account for the effects of low P_H2 pressure, composition, and temperature. In addition, we find that chemical reactions, diffusion, and nucleation can control the overall rate of evaporation of silicate melts by using the fitting slope in ln(-lnf) versus ln(t). Notably, our model allows for the theoretical calculations of parameters like activation energy (E_a), providing a novel approach to studying compositional and environmental effects on evaporation processes, and shedding light on the formation and evolution of the proto-solar and Earth-Moon systems.

DOI: 10.1007/s11631-024-00709-y

Source: https://link.springer.com/article/10.1007/s11631-024-00709-y