Evaporation kinetic and isotopic fractionation of minerals in the early solar nebula

为了描述早期太阳星云中矿物的化学和同位素分馏随温度、压力和时间的变化，以橄榄石和橄榄石为原料进行了蒸发实验。晶体的蒸发速率和微观结构表现出很强的蒸发各向异性。显微结构受位错密度和性质的控制，导致镁橄榄石和天然橄榄石表面形成不同的显微结构。镁橄榄岩的蒸发速率是温度的函数，这一结果可以用来研究镁、硅和氧同位素分馏的可能性。凝聚相的化学和同位素分馏程度受元素扩散控制，并成功地用蒸发速率、扩散速率和分馏因子作为独立参数的方程表示。如果蒸发达到稳定状态，就可以在蒸发实验中获得元素的扩散系数。当其他常规实验难以获得理想的条件时，这种技术在高温下是有效的。得到了反映地球化学过程的重要参数之一--镁-铁互扩散系数。这一数值与Meisner(1974)的低温实验外推值接近，但略高于后者。较高值的原因是：(1)由于高温下扩散机制的改变，现值是正确的；(2)现值是由于蒸发表面产生空位而正确的；或者(3)现值是不正确的，是由于高估了蒸发系数。在本研究中，化学和同位素分馏被模拟为温度和时间的函数，这使我们能够估计太阳星云中的化学分馏。

In order to describe the chemical and isotopic fractionation due to eveporation of minerals in the early solar nebula as a function of temperature, pressure, and time, evaporation experiments were carried out by using forsteirte and olivine as starting materials.Crystals show strong evaporation anisotropy for the evaporation rate and microstructures. The microstructures are controlled by density and nature of dislocation, which results in different microstructures developing on the surface of forsterite and natural olivine. Evaporation rate of forsterite was obtained as a function of temperature, and thus the results can be used to investigate the possilility of isotopic fractionation of Mg, Si, and O.The degree of chemical and isotopic fractionantion from a condensed phases is controlled by elemental diffusion, which is successfully shown by an equation including evaporation rate, diffusion rate, and fractionation factor as independent parameters.The diffusion coefficient of an element can be obtained in the evaporation experiments if a steady state for evaporation is achieved. This technique is powerful at high temperatures when other conventional experiments have difficulties in making c desirable conditions. The Mg-Fe inter-diffusion coefficient was obtained in the present study, which is one of the most important parameter for geochemical processes. The value is close to that extrapolated from lower temperature experiments by Meisner (1974) , but is a little higher than that. The reason for a higher value is either (1) the present value is correct becaouse diffusion mechanism changes at high temperatures, (2) the present value is correct due to generation of vacancies at the evaporating surface, or (3) the value is incorrect due to overestimation of evaporation coefficient. In the present study, the chemical and isotopic fractionation was modeled as a function of temperature and time, which enables us to estimate the chemical fractionation in the solar nebula.