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 element 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 Luochuan. It shows that this dust fall came from wide source and was mixed well during transportation. However, 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. I.OI 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 I.uochuan, 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 geochemical composition and elemental ratio. Coarse grain size has higher Zr, Hf concentrations and other elements than fine grain size in this dust fall. In four dust falls in Beijing, finer grain size dust falls 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.