Phase diagrams describing solid-gas equilibria in the system Fe-Mg-Si-O-C-H, and its bearing on redox states of chondrites
Phase diagrams describing solid-gas equilibria in the system Fe-Mg-Si-O-C-H under H-rich conditions (∼700-2000 K and 10-2-10-20 atm of PH2), including solar nebula conditions, were constructed based on thermochemical calculations. Boundaries of vaporous phases, which are the first phases to condense from a gas, can be obtained without calculating condensation temperatures of individual gas compositions because the numbers of major gaseous species are the same as those of components in the concerned systems. Fractionations by condensation and/or evaporation can be discussed easily in such phase diagrams. A thermal divide, which is a barrier that vapors cannot cross by a single cooling process, was recognized in the phase diagrams. This is present on the Fe-MgO-SiO2-CO-H plane at high temperatures (>500-700 K) and plays an important role in fractionations.Oxidizing states of ordinary chondrites and carbonaceous chondrites before aqueous alteration are located at the 0-rich side of the thermal divide. Such oxidizing states can be formed from the solar gas by fractionation in the primordial solar nebula because the solar composition is located on the 0-rich side. On the other hand, the reducing states of enstatite chondrites, located at the 0-poor side, cannot be formed as long as the thermal divide is present. The reducing states can be obtained by CO to CH4 molecular reaction at low temperatures (≤500-700 K), where the high-temperature- thermal divide is absent. Addition of H2O-rich and CH4-rich ice can explain establishment of the redox states of ordinary and enstatite chondrites, respectively.