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引起的化学和同位素分馏随温度、压力和时间的变化，以镁橄榄石和橄榄石为起始材料进行了蒸发实验。晶体表现出强烈的蒸发各向异性蒸发速率和微观结构。微观结构受位错密度和性质控制，导致镁橄榄石和天然橄榄石表面形成不同的微观结构。镁橄榄石的蒸发速率是温度的函数，因此该结果可用于研究 Mg、Si 和 O 的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，成功地由包含蒸发速率、扩散速率和分馏因子作为独立参数的方程表示。如果蒸发达到稳态，则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创造理想的条件时，该技术在高温下非常有效。本研究获得了Mg-Fe互扩散系数，它是地球化学过程最重要的参数之一。该值接近 Meisner (1974) 根据低温实验推断的值，但略高一些。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) 由于蒸发表面处产生空位，当前值是正确的，或 (3) 该值不正确由于高估了蒸发系数。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。