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Phase diagrams describing solid‐gas equilibria in the system Fe‐Mg‐Si‐O‐C‐H, and its bearing on redox states of chondrites
Author(s) -
Tsuchiyama Akira,
Kitamura Masao
Publication year - 1995
Publication title -
meteoritics
Language(s) - English
Resource type - Journals
eISSN - 1945-5100
pISSN - 0026-1114
DOI - 10.1111/j.1945-5100.1995.tb01147.x
Subject(s) - chondrite , enstatite , oxidizing agent , formation and evolution of the solar system , condensation , phase diagram , volatiles , evaporation , chondrule , chemistry , phase (matter) , mineralogy , meteorite , analytical chemistry (journal) , astrobiology , thermodynamics , physics , organic chemistry , chromatography
— 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 P h 2 ), 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‐SiO 2 ‐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 O‐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 O‐rich side. On the other hand, the reducing states of enstatite chondrites, located at the O‐poor side, cannot be formed as long as the thermal divide is present. The reducing states can be obtained by CO to CH 4 molecular reaction at low temperatures (≤500–700 K), where the high‐temperature thermal divide is absent. Addition of H 2 O‐rich and CH 4 ‐rich ice can explain establishment of the redox states of ordinary and enstatite chondrites, respectively.