Evaporation Experiments on Planetary Materials.

通过蒸发实验测定了形成类地行星的重要矿物(镁橄榄石、金属铁和硫铁矿)的蒸发速率及其蒸发系数。镁橄榄石的蒸发规律一致，遵循线性速率规律。其蒸发系数表明，镁橄榄石在富氢气氛下的蒸发机理与自由蒸发相似。根据这一结果，可以估计原始太阳星云中镁橄榄石的蒸发速率。金属铁的蒸发也服从线性速率定律，其蒸发系数接近于1。硫铁矿与金属铁渣的不一致蒸发符合线性速率定律，这是由于硫铁矿-金属铁界面上的反应是一个速率控制过程。根据实验结果和前人的数据，得到了形成类地行星的重要矿物的蒸发速率。根据汇编的数据，讨论了原始太阳星云中矿物尘埃蒸发引起的元素分馏(Mg/Si和Fe/S)。结果表明，顽火辉石没有发生不一致蒸发，但发生了明显的一致蒸发，因此不存在镁硅分馏。然而，只要粉尘是无定形的硅酸镁，就有可能进行镁硅分馏。铁/S分馏是由于硫铁矿的不一致蒸发所致。根据矿物的蒸发速率，建立了固体蒸发引起的同位素质量分馏模型。分馏取决于蒸发率、固体中的扩散系数、固体的大小以及由蒸发分子的质量决定的同位素分馏系数。根据该模型，成功地估算了陨石中发现的同位素质量分馏条件。

Evaporation rates and their evaporation coefficients of important minerals forming terrestrial planets (forsterite, metallic iron, and troilite) were determined by evaporation experiments. Forsterite evaporates congruently, and the evaporation obeys a linear rate law. Its evaporation coefficient showed that the evaporation of forsterite under a H_2-rich atmosphere occurs by mechanism similar to that of free evaporation. From the results, evaporation rates of forsterite in a primordial solar nebula can be estimated. Evaporation of metallic iron also obeys a linear rate law, and its evaporation coefficient is close to unity. Incongruent evaporation of troilite with metallic iron residue obeys a linear rate law due to reaction at the troilite-metallic iron interface as a rate-determining process. From the experimental results together with previous data, evaporation rates of important minerals forming terrestrial planets were complied. Elemental fractionation (Mg/Si and Fe/S) due to evaporation of mineral dust in the primordial solar nebula was discussed based on the complied data. It was suggested that incongruent evaporation of enstatite did not occur but apparent congruent evaporation of enstatite occurred, and thus no Mg/Si fractionation is expected. However, Mg/Si fractionation is possible as long as the dust was amorphous Mg-silicate. Fe/S fractionation was expected due to incongruent evaporation of troilite. A model for isotopic mass fractionation due to evaporation of solids was also constructed based on the evaporation rates of the minerals. The fractionation depends on the evaporation rate, diffusion coefficient in the solid, the size of the solid, and isotopic fractionation factor which is determined from the mass of evaporated molecules. Based on the model, conditions for isotopic mass fractionation found in meteorites were estimated successfully.