Condensation, evaporation, melting, and crystallization in the primitive solar nebula; experimental data in the system MgO-SiO₂-H₂ to 1.0X10_-9 bar and 1870 degrees C with variable oxygen fugacity

Crystal-vapor, melt-vapor, and crystal-melt-vapor phase relations in the system MgO-SiO₂-H₂ have been determined at low pressures (10⁻⁴ to 10⁻⁹ bar) and high temperature (1400 to 1870 °C) from vapor-pressure determinations of periclase, forsterite, silica minerals, enstatite, and liquid compositions in the system and from conventional phase-equilibrium measurement techniques. In addition to pressure, temperature, and bulk-composition variables, the oxygen fugacity was varied from 1.5 orders of magnitude above that of the iron-wüstite oxygen buffer to about 2.5 orders of magnitude below.

In this system, periclase, forsterite, and silica (tridymite and cristobalite) evaporate congruently to a gas phase consisting of MgO, SiO₂, SiO, Si, and oxygen species, whereas enstatite (most likely protoenstatite) evaporates incongruently to forsterite plus a silica-rich vapor. Liquid is unstable at silicate vapor pressures below 10⁻⁵ to 10⁻⁶ bar. The temperatures of the triple points where crystals, liquid, and gas coexist range from 1550 °C for enstatite, to 1600 °C for silica and 1700 °C for forsterite. The temperature of the triple point for periclase was not determined, but is likely to be near 2800 °C. The pressures of the triple points increase by several orders of magnitude upon dilution of the vapor phase to hydrogen/silicate molar ratios relevant to condensation in the solar nebula (e.g., 10⁴), and the temperatures decrease by several hundred degrees.

Vaporous diagrams in the systems MgO-SiO₂ and MgO-SiO₂-Fe show that at pressures and hydrogen/silicate ratios appropriate for the solar nebula, the vaporous phase is forsterite with a vaporous field extending from about 95-99 mol% MgO (forsterite-periclase vaporous boundary) to less than 10 mol%, where the enstatite and forsterite coexist. These relations are consistent with extensive Mg-Si fractionation by fractional condensation of forsterite during cooling in the early solar nebula. In the Fe-bearing, ternary system the forsterite vaporous extends to nearly 100% Fe, thus suggesting that very extensive magnesiosilicate fractional condensation (and temperature reduction—at least 400 °C) is necessary before Fe metal will condense from the early solar nebula.