Fe-Mg interdiffusion in orthopyroxene: Complex interdependencies of temperature, composition and oxygen fugacity

IF 4.5 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Geochimica et Cosmochimica Acta Pub Date : 2025-03-04 DOI:10.1016/j.gca.2025.03.002

Maria A. Dias , Ralf Dohmen , Harald Behrens

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Abstract

Knowledge of Fe-Mg interdiffusion coefficients (D_Fe-Mg) in orthopyroxene (opx) is relevant for diffusion chronometry as well as for thermometers based on Fe-Mg exchange between opx and other common mafic minerals. We extended the existing data set for D_Fe-Mg to quantify the effect of the molar fraction of Fe, X_Fe = Fe/(Fe + Mg) = 0.1 to 0.4, and oxygen fugacity, fO₂ = 10^-7 to 10^-11 Pa, in the temperature range, T = 950–1100 °C, where we build up on our recently developed experimental and analytical approach. Thin film diffusion couples using different natural opx with different trace element contents were annealed in vertical gas mixing furnaces. The diffusion profiles were extracted by acquiring backscattered electron images on foils cut from the samples using a focused ion beam-scanning electron microscope. We found complex interplays between the effect of T, X_Fe and fO₂ on D_Fe-Mg. The effect of X_Fe increases with T for fO₂ = 10^-7 to 10^-9 Pa but decreases with T for more reducing conditions. For T > 1000 °C and fO₂ > 10^-10 Pa, D_Fe-Mg depends on fO₂ and X_Fe. For these conditions D_Fe-Mg is described by an activation energy of 284 ± 19 kJ/mol. For T = 950 °C to 1000 °C and for fO₂ < 10^-10 Pa, D_Fe-Mg seemingly ceases to depend on fO₂, indicating a change of the diffusion mechanism, and is described by an activation energy of 246 ± 78 kJ/mol. Based on these diffusion data, we propose a qualitative point defect model for opx where the majority point defects changes over the explored Fe content, fO₂ and T conditions from vacancies on the metal site/electron holes to vacancies on the metal site/Fe³⁺ to Mg interstitials/electrons, each one responsible for a different effect of fO₂ on D_Fe-Mg. Based on the re-evaluation of kinetic data of order–disorder in orthopyroxene we propose that for T < 950 °C D_Fe-Mg becomes insensitive to fO₂ and the effect of Fe is relatively constant and smaller than at higher T. We derived two parameterizations valid for temperatures between 950 and 1100 °C and fO₂ > 10^-10 Pa and for the same temperature range but for log fO₂ ≤ 10^-10 Pa. In addition, we explore how the new diffusion data would affect the re-equilibration of Fe-Mg during cooling between opx and spinel as well as opx and clinopyroxene. Based on our results, it is likely that the peak compositions at the core of opx are easily modified for peak T > 1100 °C, more oxidizing conditions, higher Fe-contents and slower cooling rates (e.g. 10 °C/Myr), which has implications for the evaluation of geothermometric data using the distribution of Fe-Mg between opx and the exchange partner.

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铁-镁在正硝石中的相互扩散：温度、组成和氧逸度的复杂相互关系

了解正辉石（opx）中的铁镁相互扩散系数（DFe-Mg）与扩散时间测定法以及基于opx和其他常见岩浆矿物之间的铁镁交换的温度计有关。我们扩展了现有的 DFe-Mg 数据集，以量化铁摩尔分数 XFe = Fe/(Fe + Mg) = 0.1 至 0.4 和氧富集度 fO2 = 10-7 至 10-11 Pa 在温度范围 T = 950 至 1100 °C（我们最近开发的实验和分析方法）中的影响。使用不同痕量元素含量的不同天然氧化物的薄膜扩散偶在垂直气体混合炉中退火。通过使用聚焦离子束扫描电子显微镜在从样品上切割下来的箔片上获取反向散射电子图像来提取扩散曲线。我们发现 T、XFe 和 fO2 对 DFe-Mg 的影响之间存在复杂的相互作用。在 fO2 = 10-7 到 10-9 Pa 的条件下，XFe 的影响随温度的升高而增大，但在更具还原性的条件下，XFe 的影响随温度的升高而减小。在 T > 1000 °C 和 fO2 > 10-10 Pa 条件下，DFe-Mg 取决于 fO2 和 XFe。在这些条件下，DFe-Mg 的活化能为 284 ± 19 kJ/mol。在 T = 950 °C 至 1000 °C 和 fO2 < 10-10 Pa 条件下，DFe-Mg 似乎不再取决于 fO2，这表明扩散机制发生了变化，其活化能为 246 ± 78 kJ/mol。根据这些扩散数据，我们提出了 opx 的定性点缺陷模型，在该模型中，大多数点缺陷会随着所探讨的铁含量、fO2 和 T 条件的变化而变化，从金属位上的空位/电子空穴到金属位上的空位/Fe3+，再到 Mg 间隙/电子，每一种点缺陷都会造成 fO2 对 DFe-Mg 的不同影响。基于对正长石中有序-无序动力学数据的重新评估，我们提出在温度为 950 °C时，DFe-Mg 对 fO2 不敏感，而 Fe 的影响相对恒定，且小于较高温度时的影响。此外，我们还探讨了新的扩散数据将如何影响蛋白石与尖晶石以及蛋白石与霞石在冷却过程中的铁镁再平衡。根据我们的研究结果，在峰值 T > 1100 °C、更氧化的条件、更高的铁含量和更慢的冷却速率（例如 10 °C/Myr）下，opx 核心的峰值成分很可能很容易发生改变，这对利用opx 与交换伙伴之间的铁镁分布来评估地温数据具有影响。

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来源期刊

Geochimica et Cosmochimica Acta 地学-地球化学与地球物理

CiteScore

9.60

自引率

14.00%

发文量

437

审稿时长

6 months

期刊介绍： Geochimica et Cosmochimica Acta publishes research papers in a wide range of subjects in terrestrial geochemistry, meteoritics, and planetary geochemistry. The scope of the journal includes: 1). Physical chemistry of gases, aqueous solutions, glasses, and crystalline solids 2). Igneous and metamorphic petrology 3). Chemical processes in the atmosphere, hydrosphere, biosphere, and lithosphere of the Earth 4). Organic geochemistry 5). Isotope geochemistry 6). Meteoritics and meteorite impacts 7). Lunar science; and 8). Planetary geochemistry.