Evaporation kinetics of minerals and isotopic fractination ; application to the solar nebula

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Evaporation rate of olivine in vacuum and hydrogen gas flow of various pressures were experimentally investigated. The evaporation rate has large dependence on the hydrogen gas pressures. The evaporation rate is equal to that in vacuum at P_<H2> below 10^<-6> bar, is dependent on P_<H2> with power of unity at P_<H2> between 10^<-6> and 10^<-4> bar, and has small dependence on P_<H2> above 10^<-4> bar. The relationship between evaporation rate and hydrogen pressure at P_<H2> below 10^<-4> bar is shown by the equation j=0.33P_<H2>+8x10^<-7>, where j is the evaporation flux from a unit surface area in a unit time. The power of unity suggests that the elementary reaction is between one mole of forsterite at the surface and one mole of adsorbed hydrogen molecule.By using above results, the life-time of forsterite grains in the solar nebula was estimated. The grains are assumed to be spherical with a uniform size, the nebula is assumed to consist of H_2, O_2 and C,and the system was assumed … More to go up to a high temperature instantaneously and kept at that temperature for a long time. The evaporation rate from the grains are the same as that in vacuum at the beginning of heating and gets smaller with time due to increase of partial pressure of Mg-or Si-bearing gas species. The change of grain size and evaporation flux are obtained as a function of a dimensionless parameter eta. Ethe represents the degree of enrichment of dust against hydrogen gas normalized to the solar system elemental abundance. At etasmaller than 3, forsterite grains evaporate totally and the evaporation rate is equal to that in vacuum. On the contrary at eta larger than 3, the system gets equilibrium between solid and gas resulted in cease of evaporation. The time required for getting equilibrium becomes longer with decreasing eta. It requires infinite time at eta around 3. In the solar nebula, is estimated to be between 0.1 to 100, and therefore, interstellar dust could survive from nebular heating when the degree of enrichment was high, and that they totally evaporated when the degree was small which resulted in homogenization of the system. Less