变质矿物内部结构研究——彩色阴极发光研究的地质意义

Internal structures of metamorphic minerals: The geological significane of cathodoluminescence studies

阴极发光(CL)方法主要应用于原位离子探针和激光剥蚀技术进行年龄测定之前的锆石研究,以及碳酸盐、长石和石英等矿物结构的分析。本研究介绍了采用热阴极发光方法分析变质矿物内部结构的优势,并结合不同岩类各变质相的常见代表性变质矿物,介绍了利用彩色阴极发光研究矿物结构的实例。一般来说,CL的产生是由激发元素如Mn、稀土元素(REE)和晶格缺陷引起的。像Fe这样的CL抑制元素的存在可以减少发光甚至完全不发光。变质矿物内部的CL彩色环带生长特征,在偏光显微镜的薄片研究中无法看到,但用彩色阴极发光方法在几秒钟内就可以识别出来。稀土元素的含量往往低于电子探针的检测限,对此,配之以分光计,阴极发光显微镜甚至可以提供化学成分的数据信息。本研究精选典型的具有彩色阴极发光结构的变质矿物,如大理岩中的橄榄石、镁铝榴石、钙铝榴石、镁铝榴石和钙铝榴石固溶体、锆石、矽线石、红柱石、蓝晶石、灰硅钙石、粒硅钙石、硬柱石、黝帘石/斜黝帘石、透辉石、硬玉/绿辉石、硅灰石、透闪石、葡萄石、绿泥石、钾长石、斜长石、石英、柯石英、玉髓等,展示了其CL显微镜下的结构特征。上述变质矿物来自不同地质环境的岩石中,如Margarita岛的高压(HP)变质岩,Dora-Maira地块(意大利)、苏鲁(中国)、Kokchetav地块(哈萨克斯坦)、西部片麻岩地区(挪威)和Pohorje(斯洛文尼亚)的超高压(UHP)变质岩,罗马尼亚的矽卡岩矿床和接触变质岩,美国内华达州的Twin Lakes和Bushveld杂岩,多米尼加共和国里约热内卢圣胡安杂岩,中国日喀则的交代岩,南非的Namaqualand、中国内蒙古的高温(HT)-超高温(UHT)变质岩,意大利Monzoni、坦桑尼亚Merelani的亚绿片岩和宝石级的黝帘石样品以及来自瑞士Campolungo的钙硅酸盐样品。矿物的内部特征主要指具有不同颜色分布的均匀无带状结构、同心环状分带结构(反映从核到边缘的元素逐渐增加或减少)、振荡环带结构、不均匀生长结构和生长过程中晶体形态变化的结构。进一步研究表明,变质结构还包括矿物的出溶结构、放射性损伤结构、变形结构、不同晶粒域出现的流体通道闭合结构以及细粒矿物的分布结构。CL方法用于薄片研究的优点之一是可以在较低放大倍数下较全面地观察各种矿物之间及其内部的结构。此外,CL记录的是清晰的彩色图像,而不像常规使用的U/Pb年龄测定前的电子探针CL系统,只能记录锆石的黑白图像。尽管上述讨论的CL信息主要是描述性的,但在使用电子探针、离子探针、激光剥蚀以及其他光谱学或矿物流体包裹体等研究之前,可作为重要的辅助研究方法。

The cathodoluminescence(CL) method is mainly applied in zircon studies prior to age dating using in-situ ionprobe and laser ablation techniques and studies on the structure of traditional minerals like carbonates, feldspars and quartz. The paper documents the advantages of the hot cathode CL method visualizing internal structures of metamorphic minerals and presents a selection of examples from different rock types and metamorphic facies.The CL is generally induced by activator-elements such as Mn, rare earth elements(REE) and lattice defects. The existence of such quencher-elements as Fe can reduce or even eliminate luminescence. The internal coloured growth CL features of metamorphic minerals, invisible from thin section studies using the polarizing microscope, can be easily identified with the coloured CL method within a few seconds. Considering that the REE content is usually below the detection limit of the electron microprobe, a spectrometer unit is adopted so that the CL microscope can even provide data on the chemical composition. This study selected typical luminescent metamorphic minerals such as olivine in marble, pyrope, grossular, pyrope-grossular solid solutions, zircon, sillimanite, andalusite, kyanite, spurrite, tilleyite, lawsonite, zoisite/clinozoisite, diopside, jadeite and omphacite, wollastonite, tremolite, prehnite, chlorite, K-feldspar, plagioclase, quartz, coesite, and chalcedony to display their structural characteristics under the CL microscope. The metamorphic minerals selected in this study come from different geological environments, including the high-pressure(HP) metamorphic rocks from Isla Margarita, ultrahigh-pressure(UHP) metamorphic rocks from the Dora-Maira Massif(Italy), Sulu(China), the Kokchetav Massif(Kazakhstan), the Western Gneiss Region(Norway), and Pohorje(Slovenia), skarn deposits and contact metamorphic rocks from Romania, Twin Lakes and Bushveld Complex(Nevada, USA), metasomatic rocks from the Rio San Juan Complex(Dominican Republic) and Xigaze(China), high-temperature(HT) and ultrahigh temperature(UHT) metamorphic rocks from Namaqualand(South Africa) and from Inner Mongolia(China), sub-greenschist facies rocks and gem quality zoisite samples from Monzoni(Italy) and Merelani(Tansania), and calcsilicate from Campolungo(Switzerland). Documented internal features of minerals essentially refer to homogeneously coloured unzoned crystals, concentric zoning(reflecting e.g., progressive increase or decrease of elements from core to rim), oscillatory zoning, inhomogeneous growth and the change of crystal morphology during growth. Other structures displayed and discussed include exsolution textures, the influence of radioactive damage, deformation and absorption features, occurrence of different crystal grain domains, fluid-channelized annealing structures, as well as the distribution of fine-grained minerals. One of the advantages of the CL method applied to thin section studies is the observation of various structures under relatively low magnification. The images recorded by the CL method are in true colour and not in black and white as those for routine zircon studies by using the CL-system of the electron microprobe prior to U/Pb dating. Although the CL information discussed above is mainly descriptive, the CL-microscopy can be used as a significant supplementary studies, using for instance electron microprobe, ion probe, laser ablation, CL-spectroscopy or mineral for fluid inclusion studies.