岩浆氧化还原地球化学

M. Blanchard, N. Dauphas
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引用次数: 5

摘要

同位素为铁复杂的地球化学行为提供了有价值的见解。为了将在自然样品中测量到的丰富的铁同位素数据纳入定量框架,重要的是要知道铁同位素是如何在共存的含铁相或种类之间的平衡中分馏的。这些同位素平衡分馏因子可以从同位素交换实验和自然样品的研究中得到,但也可以从配分函数中计算出来,配分函数的主要贡献是振动能。这种方法依赖于第一性原理计算(基于量子力学的原子建模)和振动光谱(Mössbauer和核共振非弹性x射线散射- NRIXS)。通过对这些技术所得结果的比较,使我们对它们的可靠性有了信心,并提高了我们对控制共存相之间铁同位素分馏的参数的理解。在介绍了该领域应用的理论和方法之后,本章将回顾NRIXS和第一性原理计算如何帮助解释天然岩石和矿物中的铁同位素变化。在平衡状态下,铁的重同位素会集中在原子间作用力常数最大的相中,也就是铁键最坚硬的相中。较高的氧化态、较高的共价和较低的配位(较短的键长)往往与较强的键和重铁同位素富集有关。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Magma Redox Geochemistry
Isotopes provide valuable insights into the complex geochemical behavior of iron. To put the wealth of Fe isotopic data measured in natural samples into a quantitative framework, it is important to know how iron isotopes are fractionated at equilibrium between co-existing iron-bearing phases or species. These isotopic equilibrium fractionation factors can be derived from isotopic exchange experiments and the study of natural samples, but can also be calculated from partition functions, whose main contribution is the vibrational energy. This approach relies on first-principles calculations (atomistic modeling based on quantum mechanics) and vibrational spectroscopies (Mössbauer and Nuclear Resonant Inelastic X-ray Scattering – NRIXS). Comparison of the results obtained from these techniques provides confidence in their reliability and improves our understanding of the parameters controlling iron isotopic fractionation among coexisting phases. After an introduction to the theory and methods applied in this field, the chapter will review how NRIXS and first-principles calculations help interpret iron isotopic variations in natural rocks and minerals. At equilibrium, the heavy isotopes of iron will concentrate in the phases where the interatomic force constants are the greatest, meaning in the phases where iron bonds are the stiffest. Higher oxidation state, higher covalency, and lower coordination (shorter bond length) tend to be associated with stronger bonds and heavy iron isotope enrichments.
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