Evaporation of olivine: Low pressure phase relations of the olivine system and its implication for the origin of chondritic components in the solar nebula

Low-pressure phase relations and vapor pressures of forsterite and fayalite were studied with the Knudsen method in the temperature range of 1400-1860°C for forsterite and 1100-1160°C for fayalite. The triple points are 5.2 × 10 ^-5 bar (1890°C) and 6.3 × 10 ^-8 bar (1217°C) for forsterite and fayalite, respectively. The enthalpy and entropy of evaporation of forsterite are ∆H_v = 543 ± 33 kJ/mol and ∆S_v = 169 ± 21 J/(mol·K), and those for fayalite are 502 ± 9 kJ/mol and 199 ± 6 J/(mol·K), respectively. By assuming that gas and olivine are ideal solutions, vaporous and solidus curves of the olivine solid solution system in the pressure range from 10 ^-10 to 10 ^-3 bar were calculated from the thermochemical data. At pressures above the triple point of forsterite, the phase diagram is the same as that at 1 bar, because the vapor field is at higher temperatures than those of liquid-bearing fields. At pressures between the triple points of forsterite and fayalite, liquid-bearing stability fields appear below the gas-solid stability field in the Fe-rich portion of the system. The compositional range of liquid-bearing fields expand with increasing pressure; at 10 ^-4 bar, the liquid-bearing fields cover most of the olivine system ( X_Mg < 0.9). Liquid is absent at pressures below the triple point of fayalite, regardless of the composition. Large Mg/Fe fractionation between gas and solid is more extensive, compared to the solid and liquid relationship. When heated in equilibrium, olivine melts before evaporation in the pressure range of 10 ^-7-10 ^-3 bar. In a solar nebula with total pressure of 10 ^-3 bar, olivine melts when the nebula is enriched in dust by more than an order of magnitude over the solar nebular value and when the (Mg + Si + Fe)/H ratio increased. The origin of type IA and II chondrules and matrix olivine in ordinary and carbonaceous chondrites can be explained in terms of the pressure of olivine gas. Type IA chondrules were formed in the gas-solid phase field where volatiles (FeO, Na ₂O, and others) were lost during heating events responsible for chondrule formation. The residual materials became enriched in refractory elements. Type II chondrules were formed in the temperature range of the liquid-solid phase field. Under these conditions, the original compositions were retained after heating of chondrule formation. Provided that type IA and II chondrules were formed at a similar temperature, type IA chondrules were formed at an olivine pressure one order of magnitude lower than that of type II chondrules. In this model, matrix olivine is a condensate from gas which formed by partial evaporation of chondritic material during type IA chondrule formation.