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

早期太阳星云中矿物质的蒸发动力学和同位素分馏

基本信息

批准号：
07454139
负责人：
NAGAHARA Hiroko
金额：
$ 4.93万
依托单位：
University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
1995
资助国家：
日本
起止时间：
1995 至 1997
项目状态：
已结题

项目摘要

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.

为了描述早期太阳星云中矿物蒸发的化学分馏和同位素分馏随温度、压力和时间的变化，以橄榄石和镁橄榄石为原料进行了蒸发实验。晶体的蒸发速率和显微结构表现出强烈的蒸发各向异性。位错密度和位错性质控制了镁橄榄石和天然橄榄石的显微结构，使其表面形成了不同的显微结构。镁橄榄石的蒸发速率与温度的关系可用于研究Mg、Si和O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的控制，可用蒸发速率、扩散速率和分馏因子作为独立参数，在蒸发实验中，如果达到蒸发的稳定状态，则可以获得元素的扩散系数。这种技术在高温下是强大的，当其他常规实验在使c理想的条件有困难。在本研究中获得的镁铁互扩散系数，这是地球化学过程的最重要的参数之一。该值接近于Meisner（1974）从低温实验中推断出的值，但略高于该值。较高值的原因是（1）由于高温下扩散机制发生变化，当前值是正确的，（2）由于在蒸发表面处产生空位，当前值是正确的，或者（3）由于过高估计蒸发系数，该值是不正确的。在目前的研究中，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。