Charge contrast imaging of geological materials in the environmental scanning electron microscope
Charge contrast imaging of geological materials in the environmental scanning electron microscope
复制标题
环境扫描电子显微镜中地质材料的电荷对比成像
DOI：
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发表时间：
2000
期刊：
影响因子：
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通讯作者：
P. Kinny
中科院分区：
文献类型：
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作者：
G. Watt;B. Griffin;P. Kinny
环境扫描电子显微镜（ESEM）允许高分辨率，高倍成像的电导率差异在未涂层的地质样品。在正常的ESEM操作条件下，样品室中存在气体（通常是水蒸气）可以防止样品表面的负电荷积聚（来自电子束的轰击）。来自样品的背散射电子和二次电子使室内气体电离，产生的带正电的气体离子向带负电的样品迁移。然而，当室内气体压力低于约250pa时，样品的充电可能发生，因为产生的带正电荷的气体离子不足以平衡电荷。样品中的电荷注入改变了二次电子发射，并且由于晶内电导率的变化是对缺陷密度变化的响应，因此二次电子图像反映了成分变化和/或微观结构特征。这些二次电子图像被称为电荷对比图像（CCI）。为了展示CCI的潜在地质应用，我们展示了锆石（强发光）、石英（弱发光）、黑云母和堇青石（非发光）的生长带、微裂缝、差异扩散域、多色晕和残余流体路径的图像。CCI以类似于阴极发光（CL）的方式检测缺陷，但具有更高的分辨率，因为CCI信号由次级电子组成，次级电子由比CL中使用的光子更小的相互作用体积产生。CCI成像也可以应用于比CL更广泛的地质样品，因为电子电荷捕获不限于宽带隙电子结构。CCI技术最重要的潜在应用之一可能是直接成像经历过交代或蚀变的岩石中的残余流体通道
Abstract The environmental scanning electron microscope (ESEM) allows high-resolution, high-magnification imaging of conductivity differences in uncoated geological samples. Under normal ESEM operating conditions, negative charge buildup at the sample surface (from bombardment by the electron beam) is prevented by the presence of a gas (usually water vapor) in the sample chamber. Backscattered and secondary electrons from the sample ionize this chamber gas, and the resultant positively charged gaseous ions migrate toward the negatively charged sample. When chamber gas pressures lower than approximately 250 Pa are used, however, charging of the sample can occur because insufficient charge balancing positively charged gaseous ions are produced. Charge implantation in the sample alters secondary electron emission, and, because intracrystalline conductivity contrasts occur in response to variations in defect density, secondary electron images reflect compositional variations and/or microstructural features. These secondary electron images are referred to as charge contrast images (CCI). To demonstrate potential geological applications of CCI, we present images of growth zones, microfractures, differential diffusion domains, pleochroic haloes, and relict fluid pathways from zircon (strongly luminescent), quartz (weakly luminescent), and biotite and cordierite (non-luminescent). CCI detect defects in a similar way to cathodoluminescence (CL), but have a higher resolution because the CCI signal is composed of secondary electrons that are generated from a much smaller interaction volume than photons utilized in CL. CCI imaging also can be applied to a wider variety of geological samples than CL, because electronic charge trapping is not restricted to wide-band gap electronic configurations. One of the most important potential applications of the CCI technique may lie in the direct imaging of relict fluid pathways in rocks that have experienced metasomatism or alteration