Pb nanospheres in ancient zircon yield model ages for zircon formation and Pb mobilization

• Published in: Scientific Reports
• Main Authors: Lyon, Ian, Kusiak, Monika, Wirth, Richard, Whitehouse, Martin, Dunkley, Daniel, Wilde, Simon, Schaumlöffel, Dirk, Malherbe, Julien, Moore, Katie
• Other Authors: GeoForschungsZentrum - Helmholtz-Zentrum Potsdam (GFZ), Laboratory for Isotope Geology, Swedish Museum of Natural History (NRM), The Institute for Geoscience Research Perth (TIGeR), School of Earth and Planetary Science Perth - Curtin university, Curtin University Perth, Planning and Transport Research Centre (PATREC)-Planning and Transport Research Centre (PATREC)-Curtin University Perth, Planning and Transport Research Centre (PATREC)-Planning and Transport Research Centre (PATREC), Institut des sciences analytiques et de physico-chimie pour l'environnement et les materiaux (IPREM), Université de Pau et des Pays de l'Adour (UPPA)-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), ANR-11-EQPX-0027,MARSS,Centre de Spectrometrie de Masse pour les Sciences de la Réactivité et de Spéciation(2011)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2019
• Subjects: [CHIM.ANAL]Chemical Sciences/Analytical chemistry; [CHIM.THEO]Chemical Sciences/Theoretical and/or physical chemistry; [CHIM.POLY]Chemical Sciences/Polymers; [CHIM.MATE]Chemical Sciences/Material chemistry; East Antarctica; Napier; Antarc*; Antarctica; Enderby Land
• Online Access: https://hal-univ-pau.archives-ouvertes.fr/hal-02299468; https://hal-univ-pau.archives-ouvertes.fr/hal-02299468/document; https://hal-univ-pau.archives-ouvertes.fr/hal-02299468/file/s41598-019-49882-8.pdf; https://doi.org/10.1038/s41598-019-49882-8

International audience Nanospheres of lead (Pb) have recently been identified in zircon (ZrSiO4) with the potential to compromise the veracity of U-Pb age determinations. The key assumption that the determined age is robust against the effects of Pb mobility, as long as Pb is not lost from the zircon during subsequent geological events, is now in question. To determine the effect of nanosphere formation on age determination, and whether analysis of nanospheres can yield additional information about the timing of both zircon growth and nanosphere formation, zircons from the Napier Complex in Enderby Land, East Antarctica, were investigated by high-spatial resolution NanoSIMS (Secondary Ion Mass Spectrometry) mapping. Conventional SIMS analyses with >µm resolution potentially mixes Pb from multiple nanospheres with the zircon host, yielding variable average values and therefore unreliable ages. NanoSIMS analyses were obtained of 207Pb/206Pb in nanospheres a few nanometres in diameter that were resolved from 207Pb/206Pb measurements in the zircon host. We demonstrate that analysis for 207Pb/206Pb in multiple individual Pb nanospheres, along with separate analysis of 207Pb/206Pb in the zircon host, can not only accurately yield the age of zircon crystallization, but also the time of nanosphere formation resulting from Pb mobilization during metamorphism. Model ages for both events can be derived that are correlated due to the limited range of possible solutions that can be satisfied by the measured 207Pb/206Pb ratios of nanospheres and zircon host. For the Napier Complex zircons, this yields a model age of ca 3110 Ma for zircon formation and a late Archean model age of 2610 Ma for the metamorphism that produced the nanospheres. The Nanosphere Model Age (NMA) method constrains both the crystallization age and age of the metamorphism to ~±135 Ma, a significant improvement on errors derived from counting statistics.