The Element Geochemistry Characteristic Of Weathering And Soil Forming Processes In Tropical Soil Profiles

There is mutual difference and relationship between soil-forming and weather-ing, the former is the premise and foundation of the latter and the latter is the further part of the former. Weathering and soil forming processes, which is the main form of the interaction among continental surface spheres, is an important process of element geochemistry circulation. The elements recombination and migration is the essence of weathering and soil forming processes. Element geochemistry behavior shows kinds of distribution and evolution characteristics affected by soil forming factors. It is very useful for researching tropical soils forming mechanism and quaternary environmen-tal evolution to explore element fractionation, concentration and migration of the rock weathering and soil forming in hypergenesisn.The tropical soil profiles have experienced weathering and soil forming fully and deeply. The soils, which developed on the basalts in the northern Hainan Island and Lei Zhou peninsula(Lei-Qiong area) erupted in different age, formed relatively completed chronosequential soils, that all the soil forming factors except soil ages are similar. It provides good materials to explore element geochemistry behavior in weathering and soil forming processes. We analyzed the distribution and evolution of major elements, trace elements and rare earth elements(REE) in chronosequential soils to discuss their correlation and evolution, then to explore elements geochemical evolution and influ-ence factors in tropical soil profiles during weathering and soil forming processes.We chose 6 soil profiles developed from basalts erupted in different age in Lei-Qiong area, collected 53 soil samples, and 6 basalts in different profiles. Elements of all 59 samples were measured, and the grain size of 53 soil samples was also mea-sured. Based on this, the elements geochemical properties and evolution features were analyzed. Results are as follows.(1)Parent rocks are the basis of soil profiles, elements contents and fractionation in soil profiles were determined by the parent rocks element geochemical background. The major elements, trace elements and REE showed inheritance character to basalts, which lead to the similarity of elements distribution to the basalts; basalts affected more on the internal fractionation of HREE, less on that of LREE.(2)The adsorption of clay components, formed by weathering and soil forming, led to the enrichment of REEs and trace elements and the separation of LREEs and HREEs. The preferential adsorption of clay components to LREE led to the increase of REE fractionation degree and it affected significantly on the REEs fractionation.(3)The content of E REE and Fe2O3 had highly significant negative correlation, which was related to the increase of iron oxides crystallinity degree and the decrease of amorphous iron content rich in REE; The content of E REE and P2O5,MnO had highly significant positive correlation, which is resulted by phosphate and manganese oxide minerals rich in REE. Sr/Ba reduced during weathering and soil forming pro-cesses, Sr/Ba is a good indicator to weathering intensity.(4)The soil age affected by the weathering processes showed some stage char-acteristics, that significantly on the early stage, when the weathering to a certain de-gree, the effect weakened obviously. The first stage (soil age<1.33Ma), CaO,MgO,Na2O,K2O,SiO2 leaches intensively, and their leaching loss rate increased with soil age, Fe2O3 TiO2 enriched significantly, REE from enrichment to leach, which reflected that stable element may leach on the basis of enrichment during weathering; the second stage (soil age>1.33Ma), CaO,MgO,Na2O,K2O,SiO2 leached al-most completely, Fe2O3,TiO2 accumulation rate and REE mobilization rate tended to be stable.