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Hydrodynamic fractionation of zircon age populations

Rebecca L. Lawrence, Ronadh Cox, Russell W. Mapes and Drew S. Coleman
Hydrodynamic fractionation of zircon age populations
Geological Society of America Bulletin (January 2011) 123 (1-2): 295-305


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+ or -42 mu m (lower fine sand), whereas grains >2000 Ma have average ESD 67+ or -14 mu m (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 fine sand (p<0.0001 in both cases). In samples with >50% medium sand, average zircon size is 100 mu m, compared with 80 mu m in samples with >50% very fine 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 significantly different detrital zircon age spectra, apparently the result of hydraulic processes. Peak mismatch (age peaks failing to overlap at 2sigma 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.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 123
Serial Issue: 1-2
Title: Hydrodynamic fractionation of zircon age populations
Affiliation: Williams College, Geosciences Department, Williamstown, MA, United States
Pages: 295-305
Published: 201101
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 60
Accession Number: 2011-012650
Categories: HydrogeologySedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2010138
Illustration Description: illus. incl. 3 tables, sketch map
S02°25'00" - S02°23'60", W54°36'00" - W54°36'00"
Secondary Affiliation: University of North Carolina at Chapel Hill, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201108
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