Hydrodynamic fractionation of zircon age populations

Abstract

Zircons in transport in the modern Ama- zon River range from coarse silt to medium sand. Older grains are smaller on average: Mesozoic and Cenozoic grains have aver- age equivalent spherical diameter (ESD) 122 ± 42 µm (lower fi ne sand), whereas grains >2000 Ma have average ESD 67 ± 14 µm (up- per coarse silt). As a full Wentworth size class separates the two values, zircons in these age populations are hydraulically distinct. Host sand size is correlated with average size of co-transported zircons, implying hydrodynamic fractionation. Zircon size is positively correlated with percent medium sand, and inversely correlated with percent very fi ne sand (p 50% medium sand, average zircon size is 100 µm, compared with 80 µm in samples with >50% very fi ne sand. We infer from these data that zircon deposition is not size-blind, and that zircons track with hydraulically comparable sand grains. As different aged grains tend to have different characteristic sizes, this indicates the pos- sibility of hydrodynamic fractionation of age populations. Five samples representing different hydro- dynamic microenvironments of a single dune present signifi cantly different detrital zircon age spectra, apparently the result of hydrau- lic processes. Peak mismatch (age peaks fail- ing to overlap at 2σ level) is the most com- mon disparity; but age populations present in some samples are missing from other samples. The lack of correspondence among the samples appears to exceed that attribut- able to random sampling. We conclude that hydrodynamic fractionation of zircons and zircon-age populations does occur. Zircon size should therefore be taken into consider- ation in detrital zircon provenance analysis.