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, and its bearing on redox states of chondrites
复制标题
描述 Fe-Mg-Si-O-C-H 系统中固气平衡的相图及其对球粒陨石氧化还原态的影响
DOI：
10.1111/j.1945-5100.1995.tb01147.x
复制
发表时间：
1995
期刊：
影响因子：
--
通讯作者：
M. Kitamura
中科院分区：
文献类型：
--
作者：
A. Tsuchiyama;M. Kitamura
摘要：描述富氢条件下（10 - 700-2000 K和10 - 2-10 - 20 atm的Ph 2），包括太阳星云条件下Fe-Mg-Si-O-C-H系统中固-气平衡的相图是基于热化学计算构建的。由于主要气体种类的数量与有关系统中组分的数量相同，因此可以在不计算单个气体成分的冷凝温度的情况下获得作为从气体冷凝的第一相的汽相的边界。通过冷凝和/或蒸发的分馏可以在这样的相图中容易地讨论。在相图中发现了一个热分水岭，这是一个蒸汽不能通过单一冷却过程穿过的屏障。这在高温（≥500-700 K）下存在于Fe-MgO-SiO2-CO-H平面上，并在分馏中起重要作用。
水蚀变前的普通碳酸盐岩和碳质碳酸盐岩的氧化状态位于热分水岭富氧一侧。这种氧化态可以在原始太阳星云中通过分馏从太阳气体中形成，因为太阳的成分位于富含O的一侧。另一方面，位于贫氧侧的顽火辉石的还原态不能形成，只要存在热分界。在低温（≤500-700 K）下，CO分子反应生成CH 4分子，可以得到还原态，而高温热区不存在。富H2O和富CH 4冰的加入可以分别解释普通和顽火辉石氧化还原态的建立。
Abstract— 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 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 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.