Equilibrium thermodynamics of multiply substituted isotopologues of molecular gases
Equilibrium thermodynamics of multiply substituted isotopologues of molecular gases
复制标题
DOI：
10.1016/j.gca.2004.05.039
复制
发表时间：
2004-12
影响因子：
5
通讯作者：
Zhengrong Wang;E. Schauble;J. Eiler
中科院分区：
文献类型：
--
作者：
Zhengrong Wang;E. Schauble;J. Eiler
含有一种以上稀有同位素的分子气体的同位素(多取代同位素)可以高精度地分析(1σ<；0.1‰)，尽管它们的自然丰度很低(空气中的∼ppm到ppt)，并可能限制自然系统的地球化学收支。我们推导了一种计算热力学平衡的同位素的丰度的方法，并给出了在1000K和193K到77K之间的O2、CO、N2、NO、CO2和N2O的计算结果。在大多数情况下，在地球表面温度下，多重取代的同位素相对于随机(随机)分布的丰度被预测为大约1到2‰。这种偏差被定义为ΔI，在≤500K温度下，这种偏差一般以1/T线性增加，但N2O例外，它显示出复杂的温度依赖关系和某些同位素的10‘S单磨丰度或贫化。这些计算为区分由平衡热力学控制的分馏和大气气体预算中的其他类型的同位素分馏提供了基础。此外，由于在热力学平衡的分子群中，多取代同位素的丰度随温度系统地变化，它们可以用作地温计。这种温度计的不同寻常之处在于，它们涉及的是均相平衡，而不是非均相平衡(例如，仅在气态二氧化碳中的同位素分布，而不是二氧化碳和共存水之间的同位素组成的差异)。此外，对于具有一个以上多重取代同位素的所有分子，存在多个独立的温度计(例如，基于18O13C16O和18O12C18O丰度的温度计是独立的)；因此，通过这种方法估计的温度可以进行内部一致性测试。
Isotopologues of molecular gases containing more than one rare isotope (multiply substituted isotopologues) can be analyzed with high precision (1σ <0.1‰), despite their low natural abundances (∼ ppm to ppt in air), and can constrain geochemical budgets of natural systems. We derive a method for calculating abundances of all such species in a thermodynamically equilibrated population of isotopologues, and present results of these calculations for O2, CO, N2, NO, CO2, and N2O between 1000 and 193 to 77 K. In most cases, multiply substituted isotopologues are predicted to be enriched relative to stochastic (random) distributions by ca. 1 to 2‰ at earth-surface temperatures. This deviation, defined as Δifor isotopologue i, generally increases linearly with 1/T at temperatures ≤ 500 K. An exception is N2O, which shows complex temperature dependences and 10’s of per-mill enrichments or depletions of abundances for some isotopologues. These calculations provide a basis for discriminating between fractionations controlled by equilibrium thermodynamics and other sorts of isotopic fractionations in the budgets of atmospheric gases. Moreover, because abundances of multiply substituted isotopologues in thermodynamically equilibrated populations of molecules vary systematically with temperature, they can be used as geothermometers. Such thermometers are unusual in that they involve homogeneous rather than heterogeneous equilibria (e.g., isotopic distribution in gaseous CO2alone, rather than difference in isotopic composition between CO2and coexisting water). Also, multiple independent thermometers exist for all molecules having more than one multiply substituted isotopologue (e.g., thermometers based on abundances of18O13C16O and18O12C18O are independent); thus, temperatures estimated by this method can be tested for internal consistency.