HYDRODYNAMIC FRACTIONATION OF ZIRCON AGE POPULATIONS

Zircons in transport in the Amazon River vary in grain size by an order of magnitude (30-300 µm equivalent spherical diameter (ESD): coarse silt to medium sand), and range in age from a few Ma to 2.8 Ga. Age and size are not independent of one another. There is an overall trend toward decreasing average grain size with increasing age, but superimposed on this trend are two average-size maxima, at 100-300 Ma and 1000-1100 Ma. Mesozoic and Cenozoic grains, for example, have average ESD 122 µm (with standard error 42 µm); whereas grains older than 2000 Ma have average ESD about half that: 67 µm (± 14 µm). A full Wentworth size class (lower fine sand-upper coarse silt) separates the two average values, meaning that zircons in these age populations are hydraulically distinct.

Evidence for hydrodynamic fractionation of zircons comes from comparison of sand size with the sizes of co-transported zircons. Average grain size of host sands and incorporated zircons are correlated, but the best correlations are with specific sand-size fractions. Zircon size is positively correlated with percent medium sand, and inversely correlated with percent very fine sand (p<0.0001 in both cases). In samples with >50% medium sand, average zircon size is 100 µm, compared with 80 µm in samples with >50% very fine sand. This indicates that zircon deposition is not size-blind, and that zircons are tracking with hydraulically comparable sand grains.

Comparison of five samples taken from a single Amazon River dune reveals significant differences among age spectra obtained from different hydrodynamic microenvironments. Samples from a single locality, with identical provenance, would be expected to have zircon age populations statistically indistinguishable from one another; but these samples show differences at the 2σ level, with several age populations occurring in only a subset of the samples.

We conclude that hydrodynamic fractionation of zircons and zircon age populations does occur. Zircon size should therefore be taken into consideration in detrital zircon provenance analysis.