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 H2 gas, and 3. H2O/H2 buffer-dominated (HBD) regime, where the evaporation is controlled by redox states buffered by the H2O/H2 ratio in the nebula.The FED, HRD, and HBD regimes appear in high-T/low-ptotal, low-T/low-ptotal to high-T/high-ptotal, and low-T/high-ptotal regions, respectively (T is temperature, and ptotal is total pressure). The evaporation rate, jFo, is only a function of T in the FED and HBD regimes, while jFo increases with increasing H2 pressure (≈ptotal) 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-ptotal 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 Péclet number, Pe ≡ Rr0/D (R is normal evaporation rate of forsterite, r0, initial radius of forsterite particles, and D, diffusion coefficient of element having isotopes); little, partial and Rayleigh fractionations are expected for Pe > 102, 102 > Pe > 10−1, and Pe < 10−1, 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.