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首页> 外文期刊>Contributions to Mineralogy and Petrology >Deformation-related microstructures in magmatic zircon and implications for diffusion
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Deformation-related microstructures in magmatic zircon and implications for diffusion

机译：岩浆锆石中与变形有关的微观结构及其对扩散的影响

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An undeformed glomeroporphyritic andesite from the Sunda Arc of Java, Indonesia, contains zoned plagioclase and amphibole glomerocrysts in a fine-grained groundmass and records a complex history of adcumulate formation and subsequent magmatic disaggregation. A suite of xenocrystic zircon records Proterozoic and Archaean dates whilst a discrete population of zoned, euhedral, igneous zircon yields a SHRIMP U-Pb crystallisation age of 9.3 ± 0.2 Ma. Quantitative microstructural analysis of zircon by electron backscatter diffraction (EBSD) shows no deformation in the inherited xenocrysts, but intragrain orientation variations of up to 30° in 80% of the young zircon population. These variations are typically accommodated by both progressive crystallographic bending and discrete low angle boundaries that overprint compositional growth zoning. Dispersion of crystallographic orientations are dominantly by rotation about an axis parallel to the zircon c-axis [001], which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in EBSD maps. Less common misorientation axes account for minor components of crystallographic dispersion. These observations are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with the operation of {100} and {010} slip systems. The restriction of deformation microstructures to large glomerocrysts and the young magmatic zircon population, and the absence of deformation within the host igneous rock and inherited zircon grains, indicate that zircon deformation took place within a low-melt fraction (<5% melt), mid-lower crustal cumulate prior to fragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmatic decompression. Tectonic stresses within the compressional Sunda Arc at the time of magmatism are considered to be the probable driver for low-strain deformation of the cumulate in the late stages of initial crystallisation. These results provide the first evidence of crystal plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal-plastic microstructures in zircon is not restricted to high-strain rocks. Such microstructures have previously been shown to enhance bulk diffusion of trace elements (U, Th and REE) in zircon. The development of deformation microstructures, and therefore multiple diffusion pathways in zircon in the magmatic environment, has significant implications for the interpretation of geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion and dissolution rates. Keywords Arc magmatism - Cumulate - Electron backscatter diffraction (EBSD) - Cathodoluminescence - Crystal plasticity - Dislocation creep Communicated by B. Collins.
机译：来自印度尼西亚爪哇省da他州弧的未变形的肾盂卟啉岩安山岩，在细粒状地层中含有成片的斜长石和闪石类的肾小球晶，并记录了复杂的堆积形成和随后的岩浆分解的历史。一组异晶锆石记录了元古代和古生界的日期，而离散的，分区的，正体的，火成的锆石群体产生的SHRIMP U-Pb结晶年龄为9.3±0.2 Ma。通过电子反向散射衍射（EBSD）对锆石进行的定量微观结构分析显示，在继承的异晶体中没有变形，但是在80％的年轻锆石种群中，晶粒内取向变化高达30°。这些变化通常通过渐进的晶体学弯曲和覆盖成分生长区的离散低角度边界来解决。绕平行于锆石c轴[001]的轴旋转，主要是晶体学方向的分散，这与EBSD图中相邻分析点的未取向轴的主要方向一致。不太常见的取向错误轴占了晶体学分散的次要成分。这些观察结果与位错蠕变以及{100}和{010}滑移系统的运行相关的倾斜和扭曲边界的形成与锆石变形是一致的。变形显微组织仅限于大的球状晶和年轻的岩浆锆石种群，并且在主火成岩和继承的锆石晶粒内没有变形，这表明锆石变形发生在低熔点（<5％熔体）中， -下部地壳在岩浆解体碎裂之前聚集，在岩浆减压过程中夹带异晶锆石。岩浆作用时，在压缩的Sunda弧内的构造应力被认为是在初始结晶后期使堆积物发生低应变变形的可能驱动力。这些结果提供了与岩浆结晶有关的锆石中晶体塑性位错蠕变的第一个证据，并表明锆石中的晶体塑性微观结构的发展不仅限于高应变岩石。先前已经显示出这种微结构增强了锆石中痕量元素（U，Th和REE）的整体扩散。变形微结构的发展以及岩浆环境中锆石中多种扩散途径的发展，对火成锆石的地球化学数据和熔体的痕量元素预算的解释具有重要意义，这是由于整体扩散和溶解速率的潜在增强。弧岩浆-累积-电子背散射衍射（EBSD）-阴极发光-晶体可塑性-位错蠕变由B. Collins进行通信。

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