玄武岩发育土壤稀土元素地球化学特征及其指示意义

Geochemical Characteristics and Indicative Significance of Soil Rare Earth Elements Developed from Basalt

稀土元素被广泛应用于岩石风化过程、土壤的物质来源和成因示踪研究,玄武岩发育土壤具有独特的地球化学优势,可以长期记录风化过程中元素地球化学变化的证据,也是一种重要土壤资源。研究玄武岩及其发育土壤的稀土元素特征对了解岩石风化、土壤发育及演化和生物地球化学循环具有重要意义。本文采用土壤地理学和元素地球化学分析方法,探究海南岛典型玄武岩发育土壤的稀土元素地球化学特征及其对母岩和气候的指示意义。研究结果表明:(1)海南玄武岩土壤轻重稀土分馏度(LREE/HREE)较高,呈极端的Ce正异常,但土壤Eu异常(δEu)的变化并不明显,主要是继承了母岩Eu无异常的特征,稀土元素特征参数能有效反映土壤发育程度。(2)土壤剖面垂直方向的稀土元素总量(ΣREE)、LREE/HREE和Ce异常(δCe)呈现相似的趋势,剖面自上而下先增大后减小,并在土壤淀积层(B层)和淀积层与母质层的过渡层(BC层)出现最大值。(3)相同气候带下,玄武岩发育土壤与花岗岩、砂页岩、片麻岩发育土壤的Ce/Eu和Eu/Sm差异较大,表明Ce/Eu和Eu/Sm图解可以较好地区分土壤母岩类型。(4)研究区土壤Ce呈显著的正异常,δCe值介于1.3~3.5之间,LREE/HREE值介于6~13之间,与亚热带、温带玄武岩发育土壤差异明显,土壤Ce显著正异常能指示土壤经历了高温高湿的热带气候。开展土壤稀土元素地球化学特征研究,对理解风化成土过程和古气候重建具有重要的指示意义。

Rare earth elements(REE)are widely used to study rock weathering processes,soil material origin and soil genesis.Basaltic soils,an important soil source,have unique geochemical advantages to record long-term geochemical changes of elements in the weathering process.The characterization of rare earth elements in basaltic rocks and their developing soils is important for the comprehension of rock weathering,soil development and evolution as well as biogeochemical cycles.Here,the geochemical characteristics of rare earth elements in typical basaltic soils on the Hainan Island were studied via soil geography and elemental geochemical analysis.The results show that:(1)the light and heavy rare earth fractionation(LREE/HREE)of basaltic soils in Hainan is high,showing extreme positive Ce anomalies.However,the variation of soil Eu anomaly(SEu)is not remarkable,mainly inherited from the lack of parent rock Eu anomalies.Thus,the characteristic parameters of rare earth elements can effectively reflect the degree of soil development.(2)The vertical distribution of the total amount of rare earth elements(REE),LREE/HREE and Ce anomaly(SCe)were all similar along the soil profile,which first increased and then decreased with the depth.The maximum values appeared in the soil deposition layer(B horizon),the transition layer between the deposition layer(B horizon)and the parent material layer(C horizon).(3)The large differences in Ce/Eu and Eu/Sm between basalt-developed soils as well as granite,sand shale and gneiss-developed soils in the same climatic zone reveal the usefulness of Ce/Eu and Eu/Sm diagrams to distinguish the soil parent rock.(4)Soil Ce in the study area shows significant positive anomalies,with Ce values ranging from 1.3 to 3.5 and LREE/HREE values ranging from 6 to 13,which are significantly different from subtropical and temperate basaltic soils.The significant positive anomalies in soil Ce indicate that the soil has experienced high temperatures and humidity in the tropics.The study of the geochemical characteristics of soil rare earth elements is of great significance for understanding the weathering processes and the palaeoclimate reconstruction.