黔中白云岩风化剖面的铅同位素组成及物源的指示

Pb Isotopic Compositions and Material Sources for Dolomite Weathering Profile in Centrial Guizhou and Indication

碳酸盐岩上覆红土的形成模式是一个争议的问题,尤其是形成该红土的物质来源争议最大。本文对贵州中部平坝剖面进行了铅同位素的综合分析,结果显示全岩样品的Pb同位素组成显示是以长石矿物为主的"普通铅",下伏三叠纪白云岩样品的n(206Pb)/n(204Pb)为19.29,风化剖面全土样品的n(206Pb)/n(204Pb)约为19.04～20.56,它们与平均陆壳、区域人为背景和气溶胶有较大的差别。Pb同位素在n(208Pb)—n(206Pb)与n(207Pb)—n(206Pb)空间上均落在一条较好的混合组合线上,显示为碳酸盐岩和硅质组分的混合,指示剖面为原位化学风化的结果,同时研究表明含磷矿物风化剖面是剖面铅同位素演化的一个重要载体。另外土壤样品的铅浓度和同位素比值与本区潜在的物源中的浓度相差较大,特别是远大于同时代的黄土和现代沉积物,也表明这些潜在的物源可能并不是形成该土壤剖面的真正物源。

It is a disputed question for forming and evolution of red residua on the surface of carbonate rocks,particular for their material sources. The authors have conducted a comprehensive analysis of Pb isotopes for the Pingba profile in Centrial Guizhou. The Pb isotopic composition of the whole-rock samples showed that the lead is dominated by feldspar mineral-derived ＂common lead＂,with n（^206Pb）/n（^204Pb） being about 19.29 and 19.04-20.56 for dolomite and soil samples,respectively,greatly different from the Pb isotopic composition of the average continent,China＇s regional anthrogenic background,and aerosols. The data points of samples for n（^208Pb）—n（^206Pb） and n（^207Pb）—n（^206Pb） isotopic spaces analysis all fall on a better mixing array,showing the mixing of carbonate rocks and siliceous components and indicating that the Pingba profile is the result of chemical weathering. While,the weathering of Pb-rich minerals is an important factor for Pb isotope evolution in the profile.In addition,there is a significant difference in Pb concentrations and Pb isotopic composition between the soil samples and the potential material sources in the studied. The Pb concentrations are far greater than those of contemporaneous loess and modern sediments,indicating that these potential material sources would not the true materials sources responsible for this soil profile.