Hydrodynamic fractionation of zircon age populations in fluvial transport
Abstract
Because it is generally assumed that zircon age populations are not significantly fractionated during transport, detrital zircon provenance studies usually characterize locations or units with a single sample. But zircons vary in size by an order of magnitude, ranging from 10s of μm to a few hundred μm in length (i.e. coarse silt to medium sand). Hydrodynamic fractionation of grain sizes is therefore likely; and if different age populations have different characteristic sizes, then the population proportions in a given sample are likely to be influenced by local depositional mechanics. Thus, a single sample may not fully characterize zircon populations for a given location or stratigraphic interval. We examined size and age of zircons from twelve locations along the Amazon River, and compared the data with grain size distributions of the host sands. We found that old zircons in the Amazon tend to be smaller (significant at > 2σ level), and that there is a significant relationship between median grain size of concordant zircons in a sample and median age of the sample. Furthermore, sands with a larger coarse fraction tend to have larger zircons. The inference is that zircons are fractionated by size; and if size and age are related (as in this example), age populations are likely to be affected. To test whether zircon age populations are affected by local hydrodynamics, we analyzed five sand samples from different sub-environments of a single Amazon River dune (28 m x 1 m). We sampled the upstream and downstream troughs, the stoss, and the upper and lower lee slope. From each sample we obtained 51-71 concordant (100 ±10%) zircon ages (using LA-ICPMS). Using probability density diagrams and mixture modeling to define ages and proportions of population components, we found significant differences (2σ level) between samples. In the most extreme case, one sample had an age population not seen in any other, and lacked a population that was present in all other samples. As all samples came from the same site, with hydrodynamic microenvironment the only variable, our results suggest that there may be significant sedimentological effects on zircon age populations. Zircon age populations are not independent of grain size, and hydrodynamic fractionation can affect the number and relative proportions of age populations in a given sample. Although further analyses are necessary to confirm our interpretations, these initial results suggest that single samples may not reliably characterise zircon populations. Comparative provenance analysis may require samples from multiple micro- environments to produce a representative picture of zircon age distributions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H51J..07L
- Keywords:
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- 1115 Radioisotope geochronology;
- 1165 Sedimentary geochronology;
- 5419 Hydrology and fluvial processes