【摘要】：風化作用是發(fā)生在近地表環(huán)境下,使巖石原有結構和組成發(fā)生變化轉變?yōu)樗樾碱w粒、可溶組分和不溶殘余相的一系列過程,其對于連接地球表生系統(tǒng)各圈層—巖石圈、水圈、生物圈具有重要意義。紅土以其富Fe所具有的紅色為特征,覆蓋了陸地面積的近三分之一,占總土壤的85%。而隨著雨水等外來流體加入紅土風化殼,Fe可以被淋濾進入到水圈中,進而顯著改變著河流和海洋的化學組成和同位素組成。Fe又是生命活動所必需的營養(yǎng)元素,如:植物會從風化形成的土壤中吸收生命活動所需的Fe元素,而植物對于Fe的利用過程也會導致Fe同位素發(fā)生分餾。由于Fe同位素在示蹤地球表面的物質循環(huán)、環(huán)境演化等過程中的獨特優(yōu)勢,使其成為探索地球表生環(huán)境中Fe元素循環(huán)的重要的研究手段。另一方面,紅土風化殼又可以形成鎳礦等具有重要的經(jīng)濟價值的礦床,研究其主微量元素在剖面中的分布又對找礦具有極大的指導意義。為了研究熱帶強風化條件下的紅土形成過程和Fe的地球化學循環(huán),本文選取菲律賓南部蘇里高地區(qū),一處發(fā)育在熱帶雨林氣候條件下的典型紅土風化剖面進行研究。整個剖面高約7m,原巖為橄欖巖。由于所受風化程度的變化,整個剖面由下到上可分為:最底部基本未受風化的基巖、上部風化逐漸增強的過渡層、紅土層、礫石層和最頂部的表土層。本研究系統(tǒng)采集了不同深度的風化樣品,分別對樣品進行了 Fe同位素測試、主微量元素及X射線衍射(XRD)等實驗分析。通過XRD實驗對剖面中礦物的分析顯示,剖面中主要含鐵礦物為含F(xiàn)e3+的赤鐵礦和針鐵礦。主微量元素分析結果表明,主量元素如:鈣、鈉、鉀、鎂由于水溶性強,在剖面中明顯虧損。而主要的成礦微量元素如Ni、Cr,分別在半風化層上部和紅土層底部先后富集。Fe在紅土層顯著富集。為了衡量剖面中Fe元素的遷移情況,我們選取Ti作為不遷移元素來校正體積變化帶來的影響,計算得出Fe的遷移率,τTi.Fe變化范圍很大且基本為負值(τTi,Fe=～-50%—～-90%),說明Fe在剖面中存在顯著的遷移。而與鐵含量(Fe203=32.1-73.3wt%)和鐵遷移率的變化范圍形成對比的是,Fe同位素的測試結果則顯示,來自于不同層位樣品的Fe同位素組成整體變化不大,在誤差范圍內基本一致(δ56Fe=-0.03‰—+0.10‰)。從以上結果,我們可以發(fā)現(xiàn):紅土在演化和形成過程中伴隨著明顯的Fe元素丟失,但與此形成對比的是剖面中Fe同位素不發(fā)生顯著的分餾。結合本文的觀測結果與前人對于熱帶強風化條件樣品的Fe同位素研究結果進行分析得出,由于熱帶環(huán)境下高氧逸度、干濕交替頻繁的特點,導致紅土風化殼在形成過程中Fe發(fā)生遷移前,存在一個完全的、原位的氧化過程,這使得巖石風化釋放的Fe在紅土中主要以三價鐵礦物存在。而考慮到三價鐵的溶解度有限,水的淋濾作用對其影響有限,并且生物作用產生的遷移會導致三價鐵被還原從而產生分餾,但Fe3+具有帶正電易與帶負電的粘土礦物結合形成膠體,進而被淋濾遷移出剖面的特點。因此導致三價鐵在剖面中發(fā)生顯著遷移的最可能機制便是三價鐵以膠體形式遷移。總結以上可以得出,在紅土形成和演化的歷史中發(fā)生的鐵遷移,對于生態(tài)系統(tǒng)的Fe同位素組成不產生顯著的影響。
[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.
【學位授予單位】：中國科學技術大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：P597;P512.1

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