Behaviors of Major and Trace Elements in Soils Developed from Weathering Basalt in Western Guizhou, China

Abstract

Weathering is a crucial geochemical process for basalt and soil formation, which plays a vital role in restoring the karst region in Guizhou, southwest China. Two regolith profiles (Dougu and Shazi) were selected to investigate the behaviors of major and trace elements developed from advanced to extreme weathering of the Emeishan basalt in the western Guizhou province. These profiles are composed of surficial clay-humus, laterite, loess, and weathered basalt. The two profiles contain very high amounts of silica-alumina ratio (Sa), silica sesquioxide ratio (Saf), and chemical index alteration (CIA), with deficient concentrations of CaO, Na2O, MgO, and K2O compared to that of the fresh basalt. They are also characterized by notable depletion of Sr, K, Ba, P, and enrichment of La and Ce. In general, The two profiles showed enrichment in light rare earth elements. Ce is enriched in the laterite profile of Shazi. The variances in the mobilization of elements in these two profiles and their similar protolith nature indicate that they underwent various supergene geological processes. The Shazi and Dougu profiles have experienced a severe action of alkali dissolution, desilication, and enrichment of iron, aluminum, titanium. The variation characteristics of magnesium and calcium indicate more montmorillonite in the Dougu profile formed in the middle and late stages of weathering. The CIA index also shows that the weathering effect of the Shazi profile is more potent than that of the Dougu profile. The rare earth elements in the Shazi profile have apparent positive δCe anomalies, indicating that the Shazi profile is formed in an intense oxidation environment. The content of scandium (Sc) in the parent rock and its weathering degree limits its amount in weathering crusts. The Sc in the Shazi and the Dougu profiles is rich than the parent rock in the weathering crust, and the enrichment is most visible in the red soil layer. The degree of enrichment of Sc is prominent in the Shazi profile with higher Sc content in the parent rock and even reaches the level of mineralization.