场发射扫描电镜在现代河流沉积石英颗粒表面形态特征研究中的应用

The application of field emission scanning electron microscopy to the study of surface textures of quartz grains from modern fluvial deposits

石英具有稳定的物理化学性能,能够较好地保存各种地质营力在搬运和沉积过程中形成的微观形貌特征。这些表面形态特征可以用来反映沉积物的源区、搬运动力、沉积环境与演化历史等信息。利用场发射扫描电镜对黄河下游边滩沉积物中石英颗粒表面形态特征进行了观察研究,并统计计算了35种表面形态特征出现的频率分布。结果表明,其发育机械成因、化学成因及机械+化学成因3大类共22种表面形态特征。其中V型撞击坑大量出现;棱角—半棱角轮廓、中贝壳状断口、直形阶梯、溶蚀坑、中浮雕和高浮雕一般发育;圆形轮廓、小贝壳状断口、大贝壳状断口、弓形阶梯、曲型脊线、平行节理面、翻板、新月型撞击坑、定向溶蚀坑、溶蚀裂缝、低浮雕和弓形曲型多边形裂纹少量出现;碟形坑和黏附颗粒极少量出现。其总体表面形态特征表明其搬运时间和长距较短,在流水中并未经过长期磨蚀,棱角轮廓被保留,表面形态改造程度较弱,表面起伏程度较高。其机械形态组合说明石英颗粒处于高能水动力条件下搬运,发生过强烈地机械碰撞与磨损。发育的溶蚀坑证明搬运水体化学性质活泼,石英颗粒在受到强烈机械作用的同时,化学作用亦十分明显。获得了典型河流沉积的标志性形态特征:颗粒一般均有磨圆,以半棱角轮廓为主,随搬运距离越远磨圆度越高;表面起伏程度较高,以高、中浮雕为主;最重要的特征是机械成因形成的V型撞击坑大量发育;各种大小的贝壳状断口及阶梯共生在一起,较发育;水下化学作用明显,溶蚀现象大量出现,溶蚀坑较发育。该研究弥补了国内现代河流沉积物石英颗粒表面研究空白,虽然这些微观形貌的深层意义还有待进一步研究,但这将为沉积过程及其环境意义的研究提供新的思路和证据。

The surfaces of quartz grains maintain numerous micro textures formed during transport and deposition by various geological forces,due to their stable physical and chemical properties.The surface textures include the information about source area,transporting force,sedimentary environment and evolution history of sediment.Surface texture characteristic of quartz grains from modern point bar deposits in the lower reaches of the Yellow River is observed and studied by scanning electron microscopy(SEM).Results indicate that there are twenty-two kinds of surface texture characteristics which caused by mechanical action,chemical action,and mechanical&chemical action.The well-developed surface texture is V-shaped percussion cracks.The common-developed surface textures are angular-subangular outline,medium conchoidal fractures,straight steps,solution crevasses,medium relief and high relief.The sparse-developed surface textures are rounded outline,small and large conchoidal fractures,arcuate steps,meandering ridges,flat cleavage surfaces,upturned plates,crescentic percussion marks,oriented etch pits,solution crevasses and straight/curved grooves and scratches.There are very few dish shaped depression and adhering particles.The overall surface morphology of quartz grains shows its short transporting time and distance.The subangular outline is retained due to through ashort-term abrasion in the water.The weaker transformation of their surface morphology causes a higher surface fluctuation degree.The combined surface textures caused by mechanical action indicate that quartz grains are transporting in a high-energy hydrodynamic condition and suffer a strong mechanical impact and abrasion.The rich solution crevasses prove the chemical property of transportation medium is very active that quartz grains receive an obvious chemical action,as well as a strong mechanical action at the same time.The typical river deposit is characteristic of these surface textures:quartz grains are main subangular outline,whose roundness is higher with the farther motion distance;Surface fluctuation degree of quartz grains is higher,and gives priority to with high and medium relief;The most important features is that the V-shaped percussion cracks are very abundant caused by mechanical action;The conchoidal of different sizes and steps are commondeveloped and with paragenesis relationship;Quartz grains develop much solution phenomenon by chemical action underwater.The study makes up for the blank of surface textures analysis of quartz grains from modern fluvial deposits in China.It provides new ideas and evidence for studies of the sedimentary process and environmental significance,although the deep meanings of these micro textures remain to be further research.