Migration and Leaching of Phosphorus and Aluminum in Basalt Weathering Profile:Implications for Late Permian Weathering-Depositional Mineralization in Yunnan-Guizhou-Guangxi Region

Chen Bo; Yang Jianghai; Ren Juntong; Cheng Liang; Liu Ao; Zhang Xiaorong; Ge Haili; Wang Jingfu; Huang Qing; Wang Biao

doi:10.3799/dqkx.2025.019

Abstract：Phosphorus(P)and aluminum(Al)are respectively recognized as a critical nutrient limiting element for biological processes and a significant metallic element.Within the near-surface environment,rock weathering is the predominant mechanism facilitating the release of these elements.Basalt,featuring abundance of phosphate mineral minerals(including basaltic glass and apatite)and aluminosilicate minerals(such as feldspar and pyroxene),constitutes a substantial rock reservoir for both P and Al elements.Despite the importance of these elements,a comprehensive understanding of the behavior of P and Al during basalt weathering remains an area ripe for further investigation.To enhance the understanding of the chemical weathering process of basalt,this study undertook a detailed examination of the weathering profile within the Emeishan Large Igneous Province(ELIP),employing mineralogical,geochemical,and phosphorus form analyses.Additionally,by integrating published data on basalt weathering profiles,the study explores the control mechanisms of physical erosion on the weathering leaching of P and Al elements and analyzes the weathering-deposition mineralization effects related to the Late Permian period.The basal section of Heishi weathering profile is characterized by a semi-weathered layer that is rich in primary minerals,including a substantial quantity of feldspar.Relative to the basaltic parent rock,this layer exhibits a markedly enhanced degree of chemical weathering,as indicated by Chemical Index of Alteration(CIA)values of 40 for the parent rock and a range of 72 to 83 for the semi-weathered layer.The upper section of the profile comprises a soil layer that incorporates quartz,hematite,and various clay minerals,reflecting an advanced stage of weathering,with CIA values that extend from 90 to 92.Utilizing the stability of Ti and normalization to the parent rock for calculating the mobility of elements,the results indicate that there is a varying degree of loss of Na,Ca,Mg,P,and Eu from the bottom up.Fe,K,and Ce show significant depletion in the semi-weathered layer but are relatively enriched in the soil layer.Al is relatively depleted in the soil layer,while Zr is relatively enriched.The soil layer,characterized by high quartz content and a low Ti/Zr ratio,likely indicates the influence of aeolian input from feldspathic dust.A binary mixing curve was constructed based on the composition of the weathered black stone protolith and aeolian dust,revealing that the weathering profiles have relatively low P/Ti ratios,while the Al/Ti ratio only shows a decreasing trend in the soil layer.This pattern suggests that P is significantly(>50%)leached during the early stages of weathering,and the state of phosphorus in the residual soil undergoes a transition from dissolved phosphorus in the protolith to weakly adsorbed phosphorus and then to strongly adsorbed phosphorus.Under extreme weathering conditions,Al can be partially(>20%)mobilized and lost through the percolation and leaching of acidic fluids,Al-rich clay minerals,or complexes.The degree of weathering at the surface is contingent upon the relative rates of physical erosion and chemical weathering.When erosion rates are high,a greater exposure of weakly weathered rocks occurs,which favors the weathering and leaching of P.Conversely,when erosion rates are low,a more extensive development of intensely weathered soil layers takes place,which favors the weathering and leaching of Al.Integrating the weathering trends of Late Permian basaltic mudstones in the western South China,it is posited that the weathering and erosion conditions of the ELIP are significant factors controlling the weathering-deposition enrichment of P and Al.