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全球地球化学基准:了解过去,预测未来

     

摘要

In order to recognize the evolution of the Earth and global changes,it is necessary to set up consistent Global-scale Geochemical Baselines as a reference."Geochemical Baseline" is known as the database and the accompanying element distribution maps by systematically documenting the concentration and spatial distribution of chemical elements and compounds on the Earth's surface,which can be applied to quantified the future human-induced or natural changes in the chemistry of the present Earth and the past chemical evolution induced by geological processes.China Geochemical Baselines(CGB),as a part of the Global Geochemical Baselines,is to document both concentrations and spatial distribution of nearly-all chemical elements across whole China.China Geochemical Baseline Networks are designed based on 1∶200000 map sheets of 1°(lng.)×40′(lat.),approximately 1500 grids covering all over China.Approximately 10000 rock samples for primary lithosphere baselines and 6000 soil samples for secondary pedosphere had been collected across the China land surface.Eighty-one geochemical indicators including 78 chemical elements are determined by ICP-MS/AES following 4-acid digestion and by XRF following fusion as backbone methods combined with other 10 analytical methods.Analytical quality is under strict control by using standard reference samples to obtain harmonious high-quality data.In the present paper the author shows some of the examples for the use of preliminary results of the spatial distribution of elements to understand the past and to predict the future.Ir geochemical anomalies have been discovered in the suspected Cretaceous and Tertiary(K-T) boundary ranging from 0.2×10-9 to 0.8×10-9 whereas Ir concentration baselines in Cretaceous and Tertiary rocks are less than 0.02×10-9.It is likely that the Iridium in the suspected boundary is of extraterrestrial origin,providing us with understanding the extinction of dinosaurs due to the impact of an earth-crossing asteroid.It has been found that the dramatic changes of CaO distribution occurred in the past 15 years from 1994 to 2009.The average concentration decreases from 0.53% to 0.41%,the area of CaO1% extends from 872000 km2 to 1073000 km2,increased 23% as compared to that in 1994,due to the leaching of CaCO3 by acid rains caused by human activities.%要认识地球的演化和全球变化,就必须有一致性的全球尺度地球化学基准或基线作为参照标尺。文中阐述了地球化学基准概念、建立方法和应用实例。地球化学基准是指系统记录元素及其化合物在地球表层的含量和空间分布,作为了解过去地球化学演化和预测未来地球化学变化的定量参照标尺。要建立全球地球化学基准,首先必须建立一个覆盖全球的地球化学基准网,在中国以1∶20万图幅为基准网格单元,在全国系统采集有代表性岩石组合样品约10 000件,疏松沉积物样品约6 000件,精确分析涵盖元素周期表上除惰性气体以外几乎所有元素的含量,制作化学元素时空分布基准地球化学图,提供衡量中国大陆化学元素演化和未来变化的标尺。以不同时代岩石代表性样品建立原生岩石圈地球化学基准,作为了解过去地球化学演化参照标尺,如云南禄丰龙产地楚雄—兰坪盆地白垩系-古近系界面(K-T界面)的凝灰质灰岩和凝灰质泥岩中发现Ir的含量为0.25~0.8ng/g,是白垩系和古近系地层基准含量的10~20倍。以疏松沉积物样品建立次生土壤圈地球化学基准,用于作为预测未来变化的参照标尺,如CaO含量分布与气候和酸雨分布有关,CaO在南方酸雨作用下淋失贫化,在西北碱性条件下富集。在过去15年里,由于酸雨的持续作用,CaO平均含量下降了22.6%,CaO含量〈1%的分布面积扩大了23.0%。同时,这种原生和次生地球化学基准的建立,提供了具有时间和空间分布的成矿物质背景,对矿产资源评价和预测具有重要意义。

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