URANIUM-LEAD GEOCHRONOLOGY BY TITANITE, ADVANTAGES AND LIMITATIONS
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The uranium-lead isotope method is used to date titanites from granitoids of the Ros'-Tikych megablock of the Ukrainian Shield (middle reaches of the Ros' river, below Bila Tserkva). Their isotopic behavior is analyzed along with published results, of other titanites found in crystalline rocks of the Ukrainian Shield. Titanites of granitoids from the Ros'-Tikych megablock contain impurity lead and this leads to an abnormal isotopic composition. The overestimation in age values calculated by methods that do not require correction for ordinary lead (method normalized, agreed differences and the slope of the regression line on the graph in the coordinates 207Pb/204Pb — 206Pb/204Pb) is up to several tens of millions of years, compared to, calculated on the model of Ahrens-Wetherill. The influence of impurity lead of anomalous isotopic composition on the distortion of the calculated values of the isotopic age, in the ratio 207Pb/206Pb is modeled. It was found that the entry into the isotope system, 2.0 billion years old, of ordinary lead with a modern isotopic composition, even with a ratio of impurity lead to radiogenic lead 1:1, will reduce the age by 207Pb/206Pb by only 4 million years. Significantly greater distortions (overestimations) of the isotopic ages is will be caused by impurity lead, in the isotopic composition of which there is an ancient (for example, 500 million older than the age of crystallization of titanite) radiogenic lead. Thus, when the ratio share of impurity lead is 20%, the overestimation of age is more than 50 million years, and it reaches 235 million years forat a ratio of 1:1 (50%). The numerical values of the isotopic age will be even more inflated at the ratio of 207Pb/206Pb in cases when impurity lead contains radiogenic lead of older age, even with a smaller share of it in the composition of impurity lead.Keywords:
Radiogenic nuclide
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Biogeochemical Cycle
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Research Article| March 01, 2013 Mobilization of radiogenic Pb in zircon revealed by ion imaging: Implications for early Earth geochronology Monika A. Kusiak; Monika A. Kusiak * 1Swedish Museum of Natural History, Box 50007, SE104 05 Stockholm, Sweden2Department of Applied Geology, Curtin University, PO Box U1987, Perth, WA 6845, Australia3Institute of Geological Sciences, Polish Academy of Science, ul. Twarda 51/55, 00-818 Warsaw, Poland *E-mail: monika.kusiak@twarda.pan.pl. Search for other works by this author on: GSW Google Scholar Martin J. Whitehouse; Martin J. Whitehouse 1Swedish Museum of Natural History, Box 50007, SE104 05 Stockholm, Sweden Search for other works by this author on: GSW Google Scholar Simon A. Wilde; Simon A. Wilde 2Department of Applied Geology, Curtin University, PO Box U1987, Perth, WA 6845, Australia Search for other works by this author on: GSW Google Scholar Alexander A. Nemchin; Alexander A. Nemchin 2Department of Applied Geology, Curtin University, PO Box U1987, Perth, WA 6845, Australia Search for other works by this author on: GSW Google Scholar Chris Clark Chris Clark 2Department of Applied Geology, Curtin University, PO Box U1987, Perth, WA 6845, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Monika A. Kusiak * 1Swedish Museum of Natural History, Box 50007, SE104 05 Stockholm, Sweden2Department of Applied Geology, Curtin University, PO Box U1987, Perth, WA 6845, Australia3Institute of Geological Sciences, Polish Academy of Science, ul. Twarda 51/55, 00-818 Warsaw, Poland Martin J. Whitehouse 1Swedish Museum of Natural History, Box 50007, SE104 05 Stockholm, Sweden Simon A. Wilde 2Department of Applied Geology, Curtin University, PO Box U1987, Perth, WA 6845, Australia Alexander A. Nemchin 2Department of Applied Geology, Curtin University, PO Box U1987, Perth, WA 6845, Australia Chris Clark 2Department of Applied Geology, Curtin University, PO Box U1987, Perth, WA 6845, Australia *E-mail: monika.kusiak@twarda.pan.pl. Publisher: Geological Society of America Received: 02 Aug 2012 Revision Received: 09 Oct 2012 Accepted: 10 Oct 2012 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2013 Geological Society of America Geology (2013) 41 (3): 291–294. https://doi.org/10.1130/G33920.1 Article history Received: 02 Aug 2012 Revision Received: 09 Oct 2012 Accepted: 10 Oct 2012 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Monika A. Kusiak, Martin J. Whitehouse, Simon A. Wilde, Alexander A. Nemchin, Chris Clark; Mobilization of radiogenic Pb in zircon revealed by ion imaging: Implications for early Earth geochronology. Geology 2013;; 41 (3): 291–294. doi: https://doi.org/10.1130/G33920.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search 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 (∼15 μm) and high spatial resolution (∼2 μm) scanning ion imaging of Pb isotopes have been applied in an attempt to quantify the effects of metamorphism. Spot analyses spread along concordia yielding 207Pb/206Pb 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 207Pb/206Pb 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Geochronology
Geological survey
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Lead is contained as trace element in uranium ores and propagates throughout the production process to intermediate products like yellow cake or uranium oxide. The lead isotopes in such material originate from two sources: natural lead and radiogenic lead. The variability of the isotopic composition of lead in ores and yellow cakes was studied and the applicability of this parameter for nuclear forensic investigations was investigated. Furthermore, the chemical impurities contained in these materials were measured in order to identify characteristic differences between materials from different mines. For the samples investigated, it could be shown, that the lead isotopic composition varies largely from mine to mine and it may be used as one of the parameters to distinguish between materials of different origins. Some of the chemical impurities show a similar pattern and support the conclusions drawn from the lead isotope data.
Radiogenic nuclide
Isotopic signature
Lead (geology)
Natural uranium
Uranium oxide
Uranium ore
Trace element
Isotopes of uranium
Nuclear material
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Geochronology
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