Hydrodynamic fractionation of zircon age populations

Abstract

Zircons in transport in the modern Amazon River range from coarse silt to medium sand. Older grains are smaller on average: Mesozoic and Cenozoic grains have average equivalent spherical diameter (ESD) 122 ± 42 µm (lower fi ne sand), whereas grains >2000 Ma have average ESD 67 ± 14 µm (upper 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 <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 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 possibility of hydrodynamic fractionation of age populations.Five samples representing different hydrodynamic microenvironments of a single dune present signifi cantly different detrital zircon age spectra, apparently the result of hydraulic processes. Peak mismatch (age peaks failing to overlap at 2σ level) is the most common disparity; but age populations present in some samples are missing from other samples. The lack of correspondence among the samples appears to exceed that attributable to random sampling. 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.

Unmixing detrital geochronology age distributions

“…For the Saylor et al . [] study, as well as other studies in both the modern and ancient, uncertainty can be mitigated in a number of ways: (1) by increasing the sample size ( n ) for all samples, both for source and mixed samples, (2) by eliminating proxy source samples and only sampling within the drainage basins (specific only to modern river studies), (3) by considering differences in zircon abundance within different source units to avoid bias based on zircon size due to hydrodynamic sorting [ Morton and Hallsworth , ; Gehrels , ; Garzanti et al ., ; Lawrence et al ., ], or differential zircon fertility [e.g., Moecher and Samson , ; Dickinson , ], and (4) by analyzing multiple samples from the same interval or broadening the number of source intervals considered to better characterize potential sources. This aspect of the research provides important feedbacks to experiment design and execution.…”

“…Third, the units have well‐defined detrital zircon U‐Pb age distributions from sedimentary rocks [ Horton et al ., ; Saylor et al ., ], each with a characteristic age distribution presumably combined into a single mixture at the trunk streams of the drainage networks where modern sands were sampled (Figure a). Finally, because only sedimentary rocks are sourced, zircon fertility [e.g., Moecher and Samson , ; Dickinson , ] is a nonissue; however, differential zircon contribution based on source grain size or hydrodynamic sorting [e.g., Garzanti et al ., ; Lawrence et al ., ] could be an issue, as units comprised of dominantly mudrocks likely contribute little to the river sand detrital age distribution.…”

The zircon story of the Nile: Time‐structure maps of source rocks and discontinuous propagation of detrital signals

“…Zircon, an ultradense mineral (density ~ 4.65 g/cm 3 ), is expected to be systematically concentrated in the fine tail of the size distribution of any sorted sediment deposited by a tractive current (Garzanti, Andò, & Vezzoli, ; Rubey, ). Different source rocks may shed populations of zircon grains characterised by different age and different size, as found for instance in Amazon River sediments (Lawrence, Cox, Mapes, & Coleman, ). Measuring the size of dated zircon grains, looking for size‐age relationships, and checking for hydrodynamic fractionation may thus provide fundamental clues for provenance diagnoses based on detrital‐zircon geochronological data.…”

From static to dynamic provenance analysis—Sedimentary petrology upgraded

“…Because of zircon durability, age spectra may remain unchanged through successive recycling episodes and consequently homogeneous in time and space, ceasing to be useful provenance tracers (Garzanti et al, 2013c). And even zircon populations may be fractionated by hydrodynamic processes (Lawrence et al, 2011).…”

“…Singlemineral techniques are applied to a more limited part of the grain-size spectrum (typically very coarse silt to lower medium sand; Lawrence et al, 2011;von Eynatten and Dunkl, 2012). Focusing on one single detrital-mineral population (e.g., zircons), representing a minimal part of a minor part of the total sediment flux (e.g., sand bedload), brings on the risk of ignoring the bulk of what lies out there in nature (Weltje and von Eynatten, 2012).…”