Disturbance to the 40Ar/39Ar system in feldspars by electron and ion beam irradiation
Disturbance to the 40Ar/39Ar system in feldspars by electron and ion beam irradiation
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DOI：
10.1016/j.chemgeo.2013.07.003
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发表时间：
2013-09-26
期刊：
影响因子：
3.9
通讯作者：
Kelley, S. P.
中科院分区：
文献类型：
--
作者：
Flude, S.;Sherlock, S. C.;Kelley, S. P.
晶内显微结构对缓慢冷却的火成岩中放射成因Ar在碱性长石中扩散的影响程度是热年代学中一个长期存在的问题。通过结合高分辨率电子显微镜与原位紫外激光烧蚀微探针40 Ar/39 Ar分析的相互作用的微结构与同位素年龄可以直接研究，使一些假设的基础40 Ar/39 Ar热年代学技术进行测试，并允许推导热年代学和地质历史。然而，有许多潜在的机制，样品可能会被损坏，其Ar-同位素系统被这种显微镜技术干扰。为了验证这一假设，从270-280马达特穆尔花岗岩，英国，宝石品质的正长石从Itrongay，马达加斯加（类似于470马）的条纹长石碱性长石抛光与胶体二氧化硅或氢氟酸蚀刻和电子和Ga+离子辐照。所使用的加速电压和电流是典型的那些用于通过扫描电子显微镜、X射线分析和通过电子探针绘图的电子束成像，以及用于使用聚焦离子束技术提取箔。没有干扰的Ar-同位素系统，观察Ga+离子照射，或低分辨率的SEM成像，但电子照射的小面积长时间，发生在扩展的高放大倍数的SEM成像，被发现干扰的Ar-同位素系统超过数百微米大小的区域，通过添加K和大气Ar，产生异常年轻的明显的40 Ar/39 Ar年龄。最好的解释，这种年龄的干扰是电迁移的K和注入大气Ar在样品充电。(c)2013作者Elsevier B. V.出版，保留所有权利。
The extent to which intracrystalline microtextures influence the diffusion of radiogenic Ar within alkali feldspars from slowly cooled igneous rocks is a long standing question in thermochronology. By combining high-resolution electron microscopy with in-situ UV-laser ablation microprobe 40Ar/39Ar analysis the interplay of microtextures with isotope ages can be studied directly, enabling some of the assumptions underlying 40Ar/39Ar thermochronological techniques to be tested and allowing deduction of thermochronological and geological histories. However, there are numerous potential mechanisms by which a sample can be damaged and its Ar-isotope system disturbed by such microscopy techniques. To test this hypothesis, perthitic alkali feldspars from the 270-280 Ma Dartmoor Granite, UK, and gem-quality orthoclase from Itrongay, Madagascar (similar to 470 Ma) were polished with colloidal silica or etched with hydrofluoric acid and irradiated with electrons and Ga+ ions. The accelerating voltages and currents used were typical of those for electron beam imaging by scanning electron microscopy, X-ray analysis and mapping by electron probe, and for extraction of foils using the focused ion beam technique. No disturbance to the Ar-isotope system was observed for Ga+ ion irradiation, or for low-resolution SEM imaging, but electron irradiation of small areas for long durations, as occurs during extended high-magnification SEM imaging, was found to disturb the Ar-isotope system over hundreds of micrometre sized areas by the addition of K and atmospheric Ar, producing anomalously young apparent 40Ar/39Ar ages. The best explanation for this age disturbance is electromigration of K and implantation of atmospheric Ar during sample charging. (c) 2013 The Authors. Published by Elsevier B.V. All rights reserved.