Geochemical Characteristics of Elements in Laterite Weathering Crust of Basalt in Northwest Hainan Island
Basalt laterite weathering profile is very suitable for studying the geochemical behavior of elements under extreme weathering.A laterite weathering profile developed on the Middle Pleistocene Duowen Formation basalt in Lingao County,northwestern Hainan Island was reported.Detailed analysis of main-trace elements,pH,Eh and cation exchange capacity(CEC)were carried out on 84 profile samples.The migration and redistribution behavior of elements in the profile was studied by mass balance calculation.The laterite weathering profile of Lingao in Hainan Island has high Fe2 O3(17.0%～41.6%)and Al2 O3(15.3%～28.4%),low SiO2(10.6%～43.6%),and very high chemical index of alteration(CIA)(average 99.3).It reflects that the weathering profile has experienced strong chemical weathering with Fe and Al enrichment,and desiliconization under extreme weathering conditions.The mass balance calculation results show that alkali metals and alkaline earth metals are mostly lost along the whole pofile with a high degree.Among the transition metals,Sc,Cu and Zn are leached to a high degree in the section,V and Ni are enriched in the top andⅣ layer of the saprolite,respectively,and high field strength element(HFSE)are leached with different degrees in the profile.Among the redox sensitive elements,Fe mainly precipitates and accumulates in the form of Fe(OH)3 at the top of the saprolite.Cr exists as water-insoluble Cr2 O3 in the profile and is enriched at the top of the saprolite.Mn and Co exist in the form of soluble Mn2+and Co2+,and their enrichment is caused by the dissolution of oxides containing Mn2+and Co2+during weathering.U precipitates and accumulates in the form of UO2 at the bottom of the saprolite,while U in other layers exists in the form of soluble UO2 CO3 and UO2+2.The enrichment behavior is related to the adsorption of iron hydroxide in the profile.The slight enrichment of uranium throughout the profile may be due to groundwater introduction.It is found that the formation of ferrite laterite in Lingao section should be caused by the obvious leaching of Al and the enrichment of Fe at the top of the saprolite,while the ferrite laterite in Wenchang section is the product of both Fe and Al enrichment,which is helpful to understand the difference between the laterite weathering products of basalt in northeast and northwest Hainan Island,and has certain indicative significance for the development and utilization of mineral resources in the future.
northwest of Hainan Islandlaterite weathering profilebasaltgeochemical behavior of elements
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