Experimental control of oxygen fugacities by graphite-gas equilibriums
Experimental control of oxygen fugacities by graphite-gas equilibriums
复制标题
通过石墨-气体平衡控制氧逸度的实验
DOI：
10.1029/jz070i006p01529
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发表时间：
1965
影响因子：
--
通讯作者：
H. Eugster
中科院分区：
文献类型：
--
作者：
B. French;H. Eugster
用固相缓冲技术在实验上建立了CO+CO气氛中氧逸度的控制。缓冲液和封闭样品之间的平衡是通过通过缓冲液扩散气体大气来建立的。在菱铁矿FeCO3分解关系的研究中，实验结果的可逆性表明该方法建立了平衡。在还原条件下从CO+CO气氛中析出固体石墨提供了一种通过晶态石墨和气相中的碳氧化物之间的平衡来控制ƒ值的方法。根据热力学数据计算了ƒ=Pco+Pco的几个平衡值。对于大于100巴的PGA值，可使用以下近似值，而不会产生严重误差：
LOGƒ=−(20,586/T)−0.044+LOG PGAS−0.028(PGAS−1)/T
这些结果表明，在火成岩和变质岩中存在石墨的两个重要后果。首先，石墨和气体之间的平衡将缓冲ƒ的值，该值在大多数地质环境下处于磁铁矿的稳定范围内。其次，如果系统的变化不因它们的引入而改变，则这种缓冲也将发生在具有额外挥发性组件的系统中。在C-H-O体系的气相中，通过与石墨的平衡，可以近似计算出CO2/CO、H2O/H2和CH4/H2等比率。一个值得注意的结果是，在广泛的地质条件下，与石墨平衡的CH4/H2的值将很大；在适度还原的条件下，甲烷将成为气相中重要的主导成分。
Control of oxygen fugacities in a CO + CO atmosphere has been established experimentally by using a solid-phase buffer technique. Equilibration between the buffer and the enclosed sample is established by diffusion of the gas atmosphere through the buffer. The establishment of equilibrium by this method is indicated by the reversibility of experimental results in a study of the decomposition relations of siderite, FeCO3. Precipitation of solid graphite from a CO + CO atmosphere under reducing conditions provides a method for control of ƒ values through equilibrium between crystalline graphite and the oxides of carbon in the gas phase. Equilibrium values of ƒ are calculated from thermodynamic data for several values of Pgas = Pco + Pco. For values of Pgas greater than 100 bars, the following approximation may be used without serious error:
log ƒ = −(20,586/T) − 0.044 + log Pgas − 0.028(Pgas−1)/T
These results indicate two important consequences of the presence of graphite in igneous and metamorphic rocks. First, equilibrium between graphite and gas will buffer a value of ƒ which is within the stability field of magnetite for most geological environments. Second, such buffering will also occur in systems with additional volatile components if the variance of the system is not changed by their introduction. In gas phases in the system C-H-O, equilibration with graphite allows approximate calculation of such ratios as CO2/CO, H2O/H2, and CH4/H2. A noteworthy result is that the value of CH4/H2 in equilibrium with graphite will be large over a wide range of geological conditions; methane will become a significant-to-dominant component of the gas phase under moderately reducing conditions.