Distribution of Iridium in Different Size Fractions at the N/Q Boundary in Loess:Implications for an Asteroid Impact 2.4 Ma Ago

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 dusts produced 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 huge impact event was quite efficient to induce global climatic catastrophes.

Key words:

N/Q boundary; NAA; Iridium distribution; meteoritic impact

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