A 4463 Ma apparent zircon age from the Jack Hills (Western Australia) resulting from ancient Pb mobilization

Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half-billion years of Earth's history. Determining accurate and precise crystallization ages of these ancient zircons is a prerequisite for any interpretation of crustal evolution, surface environment, and geodynamics on...

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Bibliographic Details
Published in:Geology (Boulder) 2018-04, Vol.46 (4), p.303-306
Main Authors: Ge, Rongfeng, Wilde, Simon A, Nemchin, Alexander A, Whitehouse, Martin J, Bellucci, Jeremy J, Erickson, Timmons M, Frew, Adam, Thern, Eric R
Format: Article
Language:English
Subjects:
absolute age
Age
Australasia
Australia
cathodoluminescence
Crystallization
Den föränderliga jorden
Earth
Earth crust
Evolution
geochemistry
Geochronology
Geodynamics
Geology
Hadean
Hills
Imaging techniques
Ions
isotope ratios
Isotopes
Jack Hills
Lead
Lead isotopes
metals
mobilization
Modelling
nesosilicates
orthosilicates
Patches (structures)
Pb-207/Pb-206
Precambrian
Ratios
Sedimentary geology
silicates
stable isotopes
Stratigraphy
Tectonophysics
The changing Earth
U/Pb
Western Australia
Zircon
zircon group
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Summary:Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half-billion years of Earth's history. Determining accurate and precise crystallization ages of these ancient zircons is a prerequisite for any interpretation of crustal evolution, surface environment, and geodynamics on the early Earth, but this may be compromised by mobilization of radiogenic Pb due to subsequent thermal overprinting. Here we report a detrital zircon from the Jack Hills (Western Australia) with 4486-4425 Ma concordant ion microprobe ages that yield a concordia age of 4463 ± 17 Ma (2σ), the oldest zircon age recorded from Earth. However, scanning ion imaging reveals that this >4.4 Ga apparent age resulted from incorporation of micrometer-scale patches of unsupported radiogenic Pb with extremely high 207Pb/206Pb ratios and >4.5 Ga 207Pb/206Pb ages. Isotopic modeling demonstrates that these patches likely resulted from redistribution of radiogenic Pb in a ca. 4.3 Ga zircon during a ca. 3.8 Ga or older event. This highlights that even a concordia age can be spurious and should be carefully evaluated before being interpreted as the crystallization age of ancient zircon.
ISSN:0091-7613
1943-2682
1943-2682
DOI:10.1130/G39894.1