Nanoscale processes of trace element mobility in metamorphosed zircon
Nanoscale processes of trace element mobility in metamorphosed zircon
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DOI：
10.1007/s00410-019-1631-1
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发表时间：
2019-11-01
影响因子：
3.5
通讯作者：
Rickard, W. D. A.
中科院分区：
文献类型：
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作者：
Peterman, E. M.;Reddy, S. M.;Rickard, W. D. A.
在阴极发光（CL）成像中观察到的几个来自高压-超高压（UHP）和超高温（UHT）变岩的锆石颗粒的例子显示出特征性的，但非典型的，小于5 μ m宽的核-边缘界面域。界面域位于紧靠岩浆核的位置，由一个不规则的0-2-μ m宽的CL-暗域组成，其边缘是一个复杂的0-5-μ m宽的CL-亮域，具有尖状边缘。界面区外缘为中CL锆石镶边，具有低对比度分带。为了表征界面域的性质，并确定机制的微量元素的流动性在变质锆石，我们分析了几个样品制备的锆石从罗多彼变质杂岩（希腊东部）和歌珊穹（西部马萨诸塞州，美国）通过原子探针层析成像（APT）。这些数据揭示了三种类型的地球化学异常，每一种都具有独特的形态。(1)高浓度的Pb（+ Y，Al）的环形集群被发现专门内的Rhodope颗粒的核心。这些簇被解释为装饰位错环，在变质和退火过程中形成的辐射损伤的晶格。地质年代学和地球化学数据支持这一解释。(2)复杂的，横切面和线性功能与异常浓度的Y + P + Yb或U在空间上限于核心边缘接口域，这些功能不相关的遗传地球化学变化（振荡分区）或变形引起的显微结构。相反，平面特征可能是在变质作用期间传播到晶体中的界面耦合溶解-再沉淀反应中形成的。所观察到的交叉关系是多个事件或最初形成领域的过程的复杂性的产物。(3)具有高浓度Y + P + Yb（+ Al）的椭球特征仅在高Y + P + Yb平面特征内发现。这些特征被解释为当锆石通过固溶线时，在折返过程中发生的旋节分解的产物，其中局部平衡有利于纳米尺度的出溶，以使吉布斯自由能最小化。这些实例中多种类型地球化学特征的存在表明，锆石中微量元素的活动性在造山过程中受到多个过程的驱动。鉴于这些非典型域显然仅限于在超高温和（U）高压条件下变质的锆石，它们的存在可能代表了在非常高级的条件下变质的标志。
Several examples of zircon grains from high- to ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) metapelites exhibit a characteristic, yet atypical, core-rim interface domain < 5-mu m wide observed in cathodoluminescence (CL) imaging. The interface domain is located immediately against the magmatic core and is comprised of an irregular, 0-2-mu m wide, CL-dark domain that is rimmed by a complex, 0-5-mu m wide, CL-bright domain with cuspate margins. The outer margin of the interface domain is rimmed by intermediate-CL zircon with low contrast zoning. To characterize the nature of the interface domain and to identify mechanisms of trace element mobility in metamorphosed zircon, we analyzed several specimens prepared from zircon from the Rhodope Metamorphic Complex (eastern Greece) and the Goshen Dome (western Massachusetts, USA) via atom probe tomography (APT). The data reveal three types of geochemical anomalies, each with a unique morphology. (1) Toroidal clusters with high concentrations of Pb (+ Y, Al) are found exclusively within the core of the Rhodope grain. These clusters are interpreted as decorated dislocation loops that formed during metamorphism and annealing of radiation damage to the lattice. Geochronological and geochemical data support this interpretation. (2) Complex, cross-cutting planar and linear features with anomalous concentrations of Y + P + Yb or U are spatially restricted to the core-rim interface domain; these features do not correlate with inherited geochemical variation (oscillatory zoning) or deformation-induced microstructures. Instead, the planar features likely formed in response to an interface-coupled dissolution-reprecipitation reaction that propagated into the crystal during metamorphism. The observed cross-cutting relationships are the product of either multiple events or complexity of the process that originally formed the domains. (3) Ellipsoidal features with high concentrations of Y + P + Yb (+ Al) are found exclusively within the high-Y + P + Yb planar features. These features are interpreted as the product of spinodal decomposition that occurred during exhumation as the zircon passed the solvus where local equilibria favored nm-scale exsolution to minimize the Gibbs free energy. The presence of multiple types of geochemical features in these examples indicates that trace element mobility in zircon is driven by multiple processes over the course of orogenesis. Given that these atypical domains are apparently restricted to zircon metamorphosed at UHT and (U)HP conditions, their presence may represent a marker of metamorphism at very high-grade conditions.