2006年4月17日北京特大降尘的地球化学特征

2006年4月17日北京遭遇了近年来最为严重的一次降尘天气。分析了降尘的主量、微量和稀土元素地球化学特征,并与洛川马兰黄土和几次北京降尘进行对比,结果显示本次北京降尘与洛川马兰黄土具有相似的元素分布模式。表明与黄土具有相似的物质组成,并在搬运过程中经历了充分的混合,同时,北京降尘与黄土相比具有一定程度的差异性,表现为北京降尘中Mg、Na相对富集,Ca明显亏损并含有较高的烧失量,降尘CIA值较低,表明降尘源区化学风化程度较弱,处于化学风化的初始阶段,降尘∑REE显著高于黄土,同时轻、重稀土分馏程度也较黄土高,4次北京降尘间稀土元素分布模式对比表明,粒度是影响重稀土元素间分馏程度不同的主要因素。

GEOCHEMISTRY OF THE HEAVY DUST FALL ON 17 APRIL 2006 IN BEIJING

There was an extraordinary dust event on April 17,2006 in Beijing.Major element,trace ele- ment and REE were analyzed to examine the geochemical characteristics of this dust fall.The result shows that dust fall has a high similar geochemical element distribution pattern to the last glacial loess at Luo- chuan.It shows that this dust fall came from wide source and was mixed well during transportation.How- ever,there is obvious difference between dust fall and loess.The concentrations of major elements show that dust fall has higher Mg,Na and lower Ca content than Luochuan loess.LOI is also relatively higher in dust fall and may correlate with clay minerals content.CIA value(chemical index of alternation)in dust fall is lower than that of the last glacial loess at Luochuan,suggesting that dust source has undergone weak chemical weathering.The REE patterns between this dust fall and the Luochuan loess are similar in shape.The clear distinction between dust fall and loess is the distinct REE abundances and LREE/HREE fractionation.Several dust falls in Beijing all have higher REE abundances and LREE/HREE fractionation than the loess and are rich in light rare earth elements(LREE).Grain size has great effect on the dust geo- chemical composition and elemental ratio.Coarse grain size has higher Zr,Hf concentrations and other ele- ments than fine grain size in this dust fall.In four dust falls in Beijing,finer grain size dust fails have lower heavy rare earth elements(HREE)content and higher fractionation in HREE.Grain size also makes the trace elements and elemental ratios more complexity.