Shogo Tachibana

Evaporation of forsterite in the primordial solar nebula; rates and accompanied isotopic fractionation

Abstract Evaporation rates of forsterite in the primordial solar nebula were modeled. There are 3 evaporation regimes expected: 1. free evaporation-dominated (FED) regime, where forsterite evaporates as free evaporation, 2. hydrogen reaction-dominated (HRD) regime, where the evaporation is affected by H 2 gas, and 3. H 2 O/H 2 buffer-dominated (HBD) regime, where the evaporation is controlled by redox states buffered by the H 2 O/H 2 ratio in the nebula. The FED, HRD, and HBD regimes appear in high- T /low- p total , low- T /low- p total to high- T /high- p total , and low- T /high- p total regions, respectively ( T is temperature, and p total is total pressure). The evaporation rate, j Fo , is only a function of T in the FED and HBD regimes, while j Fo increases with increasing H 2 pressure (≈ p total ) in the HRD regime. Evaporation behaviors of forsterite dust in the primordial solar nebula and possible isotopic fractionation accompanied with the evaporation were discussed by using the evaporation rate model with estimated evaporation coefficient of 0.1. Under nebula T - p total conditions, the HRD and HBD regimes are expected in inner and outer regions of the nebula, respectively, and the FED regime is expected only by local heating in a very outer region at low pressures. Kinetic effects of the evaporation by infall of forsterite dust along the nebula midplane should be small, while those by vertical movement in a turbulent flow and local heating should be important. Numerical calculations show that isotopic fractionation by evaporation is determined by the Peclet number, P e ≡ Rr 0 / D ( R is normal evaporation rate of forsterite, r 0 , initial radius of forsterite particles, and D , diffusion coefficient of element having isotopes); little, partial and Rayleigh fractionations are expected for P e > 10 2 , 10 2 > P e > 10 −1 , and P e −1 , respectively. The evaporation rates showed that isotope fractionation of only Mg was possible in the nebula especially for small particles (typically less than 10 μm). Isotopic fractionation is suppressed by evaporation in a closed system, and this can be one of the candidates to explain issue on elemental fractionation without isotopic fractionation in meteorites and planetary materials.

Incongruent evaporation of troilite (FeS) in the primordial solar nebula: an experimental study

Abstract Incongruent evaporation experiments on troilite (FeS) were carried out under H2-rich conditions at total pressure 1.0–10−6 atm (800–970°C) to elucidate the kinetics of incongruent evaporation of troilite in the primordial solar nebula. Sulfur evaporates from troilite linearly with time, and a porous residual layer of metallic iron is formed. It was concluded from the results and consideration on individual possible processes during the evaporation that the evaporation rate of sulfur is controlled by the surface chemical reaction. The evaporation rate at 1 atm total pressure depends largely on hydrogen pressure, p(H2), while that under low p(H2) conditions has a little dependence on p(H2). These results indicate that sulfur evaporates from troilite mainly as H2S under high p(H2) conditions, while mainly as S2 (and HS) under low p(H2) conditions. The evaporation coefficients, α, which represent the degree of kinetic constrains of evaporation, were obtained from the experimental results and thermodynamic calculations: αH2S = 2.03 × 10−3 p(H2)0.106 exp(−940/T), αHS = 1.94 × 10−2 p(H2)−0.136 exp(−2040/T), and αS2 = 0.922 exp(−2220/T). Small values of α for evaporation as H2S ( Taking the present results and p-T conditions in the primordial solar nebula into consideration, it was concluded that troilite would evaporate incongruently as H2S in low temperature regions of the outer primordial solar nebula and as S2 (and HS) in high temperature regions. Since residual metallic iron evaporates little, the incongruent evaporation of troilite can cause the Fe/S fractionation in the primordial solar nebula.

Experimental study of incongruent evaporation kinetics of enstatite in vacuum and in hydrogen gas

Variations in bulk Mg/Si ratios in the various groups of chondritic meteorites indicate that Mg/Si fractionation occurred in the primitive solar nebula. Enstatite (MgSiO3) evaporates incongruently forming forsterite (Mg2SiO4) as an evaporation residue; therefore, evaporation of enstatite produces Mg/Si variations in solid (Mg-rich) and gas (Si-rich) and must be considered as a probable process responsible for Mg/Si fractionation recorded in chondrites. To understand the evaporation kinetics of enstatite, incongruent evaporation experiments on enstatite single crystals have been carried out in vacuum and in hydrogen gas at temperatures of 1300 to 1500°C. A polycrystalline forsterite layer is formed on the surface of enstatite by preferential evaporation of the SiO2 component, both in vacuum and in hydrogen gas. The thickness of the forsterite layer in vacuum increases with time in the early stage of evaporation and later the thickness of the forsterite layer remains constant (several microns). This is due to the change in the rate limiting process from surface reaction plus nucleation and growth to diffusion in the surface forsterite layer. The activation energy of the diffusion-controlled evaporation rate constant of enstatite is 457 (±58) kJ/mol. A thinner forsterite layer is formed on the surface of enstatite in hydrogen gas than in vacuum. Evaporation of enstatite in hydrogen gas is also considered to be controlled by diffusion of ions through the forsterite layer. The thin forsterite layer formed in hydrogen gas is ascribed to the enhanced evaporation rate of forsterite in the presence of hydrogen gas. n nThe results are applied to incongruent evaporation under the solar nebular conditions. The steady thickness of the forsterite of nebular pressure-temperature conditions is estimated to be submicron because of the enhanced evaporation rate of forsterite under hydrogen-rich nebular conditions if evaporated gases are taken away immediately and no back reaction occurs (an open system). Because enstatite grains in the solar nebula would be comparable to the estimated steady thickness of forsterite, evaporation of such enstatite grains under kinetic conditions could play an important role in producing variations in Mg/Si ratios between solid and gas in the solar nebula.