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蒸发引起的化学分馏和同位素分馏随温度、压力和时间的变化规律，以橄榄石和橄榄石为起始材料进行了蒸发实验。晶体在蒸发速率和微观结构上表现出较强的蒸发各向异性。其显微组织受位错密度和性质的控制，导致橄榄石和橄榄石表面形成不同的显微组织。该结果可用于探讨Mg、Si和o的同位素分馏的可能性。缩合相的化学分馏和同位素分馏的程度受元素扩散的控制，并成功地用一个包含蒸发速率、扩散速率和分馏因子为独立参数的方程来表示。在蒸发实验中，只要达到一个稳定的蒸发状态，就可以得到元素的扩散系数。这项技术在高温下是强大的，而其他传统的实验很难创造理想的条件。本文得到了Mg-Fe相互扩散系数，这是地球化学过程的重要参数之一。该值接近Meisner（1974）从低温实验中推断出的值，但略高于此值。数值较高的原因可能是：(1)由于高温下扩散机制发生变化，现值是正确的；(2)由于蒸发面产生空位，现值是正确的；(3)由于高估蒸发系数，值是不正确的。在本研究中，化学和同位素分馏被建模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.