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Mobilization of radiogenic Pb in zircon revealed by ion imaging; implications for early Earth geochronology

Monika A. Kusiak, Martin J. Whitehouse, Simon A. Wilde, Alexander A. Nemchin and Chris Clark
Mobilization of radiogenic Pb in zircon revealed by ion imaging; implications for early Earth geochronology
Geology (Boulder) (January 2013) 41 (3): 291-294

Abstract

Zircon is arguably the most commonly used geochronometer, but the reliability of ages obtained requires a full understanding of processes that might compromise the integrity of its U-Pb systematics. Here we present the results of a multifaceted ion microprobe study of zircon grains from the Napier Complex, East Antarctica, a region affected by pervasive high-temperature metamorphism at 2.5 Ga, and from which previous zircon geochronological interpretations have been problematic. Both U-Pb spot analysis ( approximately 15 mu m) and high spatial resolution ( approximately 2 mu m) scanning ion imaging of Pb isotopes have been applied in an attempt to quantify the effects of metamorphism. Spot analyses spread along concordia yielding (super 207) Pb/ (super 206) Pb ages from 2.5 Ga to 3.9 Ga, with the oldest grains reversely discordant. Ion images of uranogenic Pb reveal a surprising micrometer-scale patchy distribution that is unrelated to crystal morphology or damage. The (super 207) Pb/ (super 206) Pb ratios within these subdomains correspond to apparent zircon ages as old as 4.2 Ga. These are interpreted as artifacts of ancient redistribution of radiogenic Pb, a process that can generate meaningless ages, and are not relicts of ancient (including Hadean) zircon. Scanning ion imaging thus facilitates identification of unsupported radiogenic Pb and enables testing of the validity of old ages from zircon known to have a long and complicated history.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 41
Serial Issue: 3
Title: Mobilization of radiogenic Pb in zircon revealed by ion imaging; implications for early Earth geochronology
Affiliation: Swedish Museum of Natural History, Stockholm, Sweden
Pages: 291-294
Published: 20130104
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 18
Accession Number: 2013-014359
Categories: Geochronology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2013099
Illustration Description: illus.
S90°00'00" - S65°00'00", W40°00'00" - E171°00'00"
Secondary Affiliation: Curtin University, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, 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: 201310
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