Trace-element heterogeneity in rutile linked to dislocation structures: Implications for Zr-in-rutile geothermometry

IF 3.5 2区 地球科学 Q1 GEOLOGY
Rick Verberne, Hugo W. van Schrojenstein Lantman, Steven M. Reddy, Matteo Alvaro, David Wallis, Denis Fougerouse, Antonio Langone, David W. Saxey, William D. A. Rickard
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引用次数: 2

Abstract

The trace-element composition of rutile is commonly used to constrain PTt conditions for a wide range of metamorphic systems. However, recent studies have demonstrated the redistribution of trace elements in rutile via high-diffusivity pathways and dislocation-impurity associations related to the formation and evolution of microstructures. Here, we investigate trace-element migration in low-angle boundaries formed by dislocation creep in rutile within an omphacite vein of the Lago di Cignana unit (Western Alps, Italy). Zr-in-rutile thermometry and inclusions of quartz in rutile and of coesite in omphacite constrain the conditions of rutile deformation to around the prograde boundary from high pressure to ultra-high pressure (~2.7 GPa) at temperatures of 500–565°C. Crystal-plastic deformation of a large rutile grain results in low-angle boundaries that generate a total misorientation of ~25°. Dislocations constituting one of these low-angle boundaries are enriched in common and uncommon trace elements, including Fe and Ca, providing evidence for the diffusion and trapping of trace elements along the dislocation cores. The role of dislocation microstructures as fast-diffusion pathways must be evaluated when applying high-resolution analytical procedures as compositional disturbances might lead to erroneous interpretations for Ca and Fe. In contrast, our results indicate a trapping mechanism for Zr.

金红石中与位错结构相关的微量元素非均质性:对Zr - in -金红石地温测量的影响
金红石的微量元素组成通常用于限制各种变质系统的P–T–T条件。然而,最近的研究表明,金红石中微量元素通过高扩散率途径和位错-杂质缔合物的重新分布与微观结构的形成和演化有关。在这里,我们研究了微量元素在低角度边界中的迁移,该边界是由Lago di Cignana单元(意大利西阿尔卑斯)的绿辉石脉内金红石的位错蠕变形成的。Zr在金红石中的测温以及金红石中石英和绿辉石中柯石英的包裹体将金红石变形的条件限制在500–565°C温度下从高压到超高压(~2.7GPa)的前进边界附近。大金红石晶粒的晶体塑性变形导致低角度边界,产生约25°的总取向差。构成这些低角度边界之一的位错富含常见和不常见的微量元素,包括Fe和Ca,为微量元素沿位错核的扩散和捕获提供了证据。在应用高分辨率分析程序时,必须评估位错微观结构作为快速扩散途径的作用,因为成分扰动可能导致对Ca和Fe的错误解释。相反,我们的结果表明了Zr的俘获机制。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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