Study On The Characteristics And Mechanism Of Supergene Migration And Accumulation Of Soil Thallium In The Dadongshan Granite Geological Background Area In South China

Thallium(Tl)enrichment in soils and associated environmental effects are one of the hot topics in supergene geochemistry.Although many researches have been conducted on the source,mechanisms of migration and enrichment and environmental effect of Tl pollution in soils,our understanding for those of geogenic Tl is insufficient.Previous studies have revealed that Tl enrichment is common in soils developed on the Dadongshan(DDS)granite pluton,South China,which might be caused by the processes of granite weathering and soil formation.However,the formation mechanism,e.g.,the sources,migration and enrichment,of Tl in soils in granite geological background areas is still unclear.Based on field investigations,the parent rock-granite and its weathered soil are sampled from the DDS area and chosen for detailed studies in mineralogy and chemical analyses of major and trace element,p H,soil organic matter(SOM)and Tl chemical speciation.Then the index of chemical alteration(CIA),mass transfer coefficient and correlation coefficient of elements are calculated.And the characteristics of spatial distribution in Tl concentration,the Tl transfer activity affected by values of p H and SOM,transformation of Tl chemical speciation and Tl-bearing mineral phases are discussed.The main conclusions are summarized as below:The distribution of Tl in the weathering profile of DDS are obviously different from that of the soil profile in the anthropogenically polluted area,which is mainly distributed in the top layer of the profile.The mass transfer coefficient in the DDS weathering profile shows that Tl is leached from the upper layer,migrated downwards and then precipitated to display enrichment in the illuvial horizon.The Nb/Ta and Zr/Hf ratios of the granite samples vary within a small range,indicating that the parent rock-granite composition is relatively homogeneous.Nb/Ta and Zr/Hf ratios,REE distribution patterns and negative Eu anomalies of the weathered soil are similar to the parent rock,all of which indicate that the compositions of granite weathering crust show clear geochemical inheritance from the granites that are geochemically homogeneous,pointing to its derivation from granite weathering.Tl is mainly present in the residual fraction(>73%),which shows strong positive correlations with Rb and K,and the migration characteristics of elements on the profile are also similar to Rb and K,indicating that Tl exhibits similar geochemical behavior to Rb and mainly hosted in crystallographic sites of K-bearing silicate minerals(K-feldspar)in granites and clay minerals(illite)in soils,respectively,and the release and enrichment of Tl may be related to the transformation and enrichment of k-bearing silicate mineral phase during granite weathering.Although having similar total Tl content,the C-horizon has a significantly lower proportion of residual Tl fraction than the parent granite.This study demonstrates that Tl can be released into and then enriched in the earth surface environment following the decomposition of Kbearing primary minerals during granite weathering,it then migrates and transforms in the surface environment,and eventually most of it enters into mineral lattices like illite,where it exists in a relatively stable residual fraction.As such produces Tl enrichment in soils developed on granitic plutons,South China.