菲律宾南部苏里高地区红土型风化剖面Fe同位素研究

发布时间：2018-10-08 07:21

【摘要】：风化作用是发生在近地表环境下,使岩石原有结构和组成发生变化转变为碎屑颗粒、可溶组分和不溶残余相的一系列过程,其对于连接地球表生系统各圈层—岩石圈、水圈、生物圈具有重要意义。红土以其富Fe所具有的红色为特征,覆盖了陆地面积的近三分之一,占总土壤的85%。而随着雨水等外来流体加入红土风化壳,Fe可以被淋滤进入到水圈中,进而显著改变着河流和海洋的化学组成和同位素组成。Fe又是生命活动所必需的营养元素,如:植物会从风化形成的土壤中吸收生命活动所需的Fe元素,而植物对于Fe的利用过程也会导致Fe同位素发生分馏。由于Fe同位素在示踪地球表面的物质循环、环境演化等过程中的独特优势,使其成为探索地球表生环境中Fe元素循环的重要的研究手段。另一方面,红土风化壳又可以形成镍矿等具有重要的经济价值的矿床,研究其主微量元素在剖面中的分布又对找矿具有极大的指导意义。为了研究热带强风化条件下的红土形成过程和Fe的地球化学循环,本文选取菲律宾南部苏里高地区,一处发育在热带雨林气候条件下的典型红土风化剖面进行研究。整个剖面高约7m,原岩为橄榄岩。由于所受风化程度的变化,整个剖面由下到上可分为:最底部基本未受风化的基岩、上部风化逐渐增强的过渡层、红土层、砾石层和最顶部的表土层。本研究系统采集了不同深度的风化样品,分别对样品进行了 Fe同位素测试、主微量元素及X射线衍射(XRD)等实验分析。通过XRD实验对剖面中矿物的分析显示,剖面中主要含铁矿物为含Fe3+的赤铁矿和针铁矿。主微量元素分析结果表明,主量元素如:钙、钠、钾、镁由于水溶性强,在剖面中明显亏损。而主要的成矿微量元素如Ni、Cr,分别在半风化层上部和红土层底部先后富集。Fe在红土层显著富集。为了衡量剖面中Fe元素的迁移情况,我们选取Ti作为不迁移元素来校正体积变化带来的影响,计算得出Fe的迁移率,τTi.Fe变化范围很大且基本为负值(τTi,Fe=～-50%—～-90%),说明Fe在剖面中存在显著的迁移。而与铁含量(Fe203=32.1-73.3wt%)和铁迁移率的变化范围形成对比的是,Fe同位素的测试结果则显示,来自于不同层位样品的Fe同位素组成整体变化不大,在误差范围内基本一致(δ56Fe=-0.03‰—+0.10‰)。从以上结果,我们可以发现:红土在演化和形成过程中伴随着明显的Fe元素丢失,但与此形成对比的是剖面中Fe同位素不发生显著的分馏。结合本文的观测结果与前人对于热带强风化条件样品的Fe同位素研究结果进行分析得出,由于热带环境下高氧逸度、干湿交替频繁的特点,导致红土风化壳在形成过程中Fe发生迁移前,存在一个完全的、原位的氧化过程,这使得岩石风化释放的Fe在红土中主要以三价铁矿物存在。而考虑到三价铁的溶解度有限,水的淋滤作用对其影响有限,并且生物作用产生的迁移会导致三价铁被还原从而产生分馏,但Fe3+具有带正电易与带负电的粘土矿物结合形成胶体,进而被淋滤迁移出剖面的特点。因此导致三价铁在剖面中发生显著迁移的最可能机制便是三价铁以胶体形式迁移。总结以上可以得出,在红土形成和演化的历史中发生的铁迁移,对于生态系统的Fe同位素组成不产生显著的影响。
[Abstract]:Weathering is a series of processes that change the original structure and composition of rocks into clastic particles, soluble components and insoluble residual phases in the near-surface environment. It is of great significance to connect the sphere, the water circle and the biosphere of each circle layer of the earth table generation system. Red soil, which is characterized by its rich Fe, covers nearly one third of the land area, accounting for 85% of the total soil. However, as rainwater and other foreign fluids are added to the laterite weathering crust, Fe can be leached into the water circle, so that the chemical composition and isotope composition of rivers and ocean can be changed significantly. Fe is also essential to the activity of life, such as the Fe element needed to absorb the life activity from the weathered soil, and the plant's utilization of Fe will also cause fractionation of the Fe isotope. Due to the unique advantages of Fe isotope in the process of tracing the material circulation and environmental evolution of the earth's surface, it becomes an important research method to explore the circulation of Fe in the earth's surface environment. On the other hand, the laterite weathering crust can also form a deposit with important economic value, such as nickel ore, and the distribution of the main trace elements in the section is of great guiding significance to the prospecting. In order to study the process of lateritic formation and the geochemical cycle of Fe under tropical strong weathering conditions, a typical laterite weathering profile developed under the climate of tropical rain forests is selected in this paper. The whole section is about 7m high and the original rock is peridotite. Due to the varying degree of weathering, the whole profile can be divided into: the bottommost basic unweathered bed rock, the transition layer with gradually enhanced upper weathering, the red soil layer, the gravel layer and the top surface layer. Different depth of weathered samples were collected in this research system. The samples were analyzed by Fe isotope test, main trace element and Xray diffraction (XRD). By means of XRD, the analysis of minerals in section shows that the main iron-bearing minerals in the section are hematite and goethite containing Fe3 +. The results of main trace element analysis showed that the main content elements, such as calcium, sodium, potassium and magnesium, had a significant loss in cross section due to strong water solubility. The main ore-forming trace elements such as Ni and Cr are respectively enriched at the upper part of the semi-weathered layer and the bottom of the red soil layer. Fe was significantly enriched in red soil layer. In order to measure the migration of Fe element in the section, Ti is chosen as the non-migration element to correct the influence of volume change, and the mobility of Fe is calculated. Note that there is a significant migration of Fe in the profile. Compared with Fe content (Fe 203 = 32. 1-73. 3wt%) and iron mobility, the results of Fe isotope show that the overall change of Fe isotope composition from different horizon samples is not large, which is basically the same in the error range (0.56Fe =-0.03 Mt. 0. 10 ppmw). From the above results, we can find that the laterite in the evolution and formation process is accompanied by a significant loss of Fe element, but in contrast to this, there is no significant fractionation of the Fe isotope in the section. The results of this paper are compared with the results of the study of Fe isotope in the samples of tropical strong weathering conditions. The results show that due to the characteristics of high oxygen fugacity and alternation of wet and wet conditions in the tropical environment, there is a complete existence of the laterite weathering crust prior to the migration of Fe during the formation. The in-situ oxidation process, which causes the weathering of the rocks to be released, is mainly present in the laterite in the presence of trivalent iron minerals. Considering that the solubility of ferric iron is limited, the leaching effect of water is limited, and the migration of biological action can cause ferric iron to be reduced to produce fractional distillation, but Fe3 + has positively charged and negatively charged clay minerals to form colloid, and then the characteristic of the cross section is removed by the leaching. the most likely mechanism that results in significant migration of trivalent iron in the profile is the migration of ferric iron in the form of a colloid. It is concluded that iron migration in the history of lateritic formation and evolution does not have a significant effect on the Fe isotope composition of the ecosystem.
【学位授予单位】：中国科学技术大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P597;P512.1

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