Abstract: The loess samples from the Neogene/Quaternary boundary in the Duanjiapo Loess Section, Shaanxi Province, China, were separated into four size fractions: sand (>0.000105m); course silt (0.000105-0.00003m); fine silt (0.00003-0.000002m) and clay (<0.000002m) using a gravity separation procedure, and the contents of Ir and some other elements in each size fraction were determined by RNAA and INAA. The analytical results indicate that the distribution of Ir in different size fractions is rather heterogeneous. The Ir content in each size fraction decreases in the following order: clay>fine silt>sand>course silt. The majority of excess Ir (55%-91%) is concentrated in the fine particle materials. Except for gold, other elements including chalcophile elements such as As, Sb, Bi, show significantly different distribution patterns from Ir. It is obvious that the sources of Ir were different from other elements, and the enrichment of Ir near the boundary was originated neither from volcanic activities nor from supergene geochemistry, but might be related to the extraterrestrial impact event of 2.4Ma ago occurred at the Southeast Pacific. This suggests that the explosive dustsproduced by that impact might be dispersed into north hemisphere. As a rough approximation, it can be estimated from the excess fluency of Ir in loess that the total amount of Ir associated with this impact was about 17.2kt and the mass of impact projectile was 3.6Gt, corresponding to a diameter of 2.6km(for a composition of C1 chondrite). Such a hugeimpact event was quite efficient to induce global climatic catastrophes.

Key words: N/Q boundary, NAA, Iridium distribution, meteoritic impact

摘要： 采用悬浮沉降法对陕西段家坡黄土剖面第三系—第四系界线样品进行了粒级分选，然后用中子活化分析方法测定了铱和其他微量元素在不同粒级中的含量。实验结果显示，铱在不同粒级物质中的含量按以下顺序增加：粗粉砂(30～105μm)＜砂粒(＞105μm)＜细粉砂(2～30μm)＜粘土(＜2μm)。在粘土颗粒物质中，铱的最高含量达856pg/g，较全岩样品富集约15倍。除了金之外，其他元素(包括As、Sb、Bi等亲硫元素)在不同粒级中的分布形式与铱显著不同。很显然，铱和其他元素有着不同的来源，界线黄土中铱的异常富集不可能由火山活动或者表生地球化学作用所形成，而极有可能和上新世末(2.4MaBP)发生在南太平洋的地外撞击事件有关。这一实验结果表明，南太平洋N/Q地外物体撞击事件产生的爆炸尘埃可能扩散到了北半球。作为一种近似，以黄土剖面N/Q界线附近铱的剩余通量作为全球平均，可以估计该撞击物体相当于一直径为2.6km的C1型碳质球粒陨石球体。如此巨大的陨石与地球相撞足以诱发全球性的气候灾变效应。

关键词: N/Q黄土界线, 中子活化分析, 铱的赋存状态, 地外撞击事件