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Proc Natl Acad Sci U S A. 2015 Apr 21;112(16):4958-63. doi: 10.1073/pnas.1415264112. Epub 2015 Apr 6.

Metallic lead nanospheres discovered in ancient zircons.

Author information

1
Institute of Geological Sciences, Polish Academy of Science (ING PAN), 00-818 Warsaw, Poland; Department of Geosciences, Swedish Museum of Natural History, SE104 05 Stockholm, Sweden; monika.kusiak@twarda.pan.pl.
2
Department of Applied Geology, Curtin University, Perth, WA 6845, Australia;
3
Experimental Geochemistry, GeoForschungsZentrum Potsdam, 3.3 D-14473 Potsdam, Germany; and.
4
Department of Geosciences, Swedish Museum of Natural History, SE104 05 Stockholm, Sweden;
5
Experimental Geochemistry, GeoForschungsZentrum Potsdam, 3.3 D-14473 Potsdam, Germany; and Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany.

Abstract

Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly distributed with respect to zircon crystallinity, and their association with a Ti- and Al-rich silica melt suggests that they represent melt inclusions generated during ultra-high temperature metamorphism. Metallic Pb is exceedingly rare in nature and previously has not been reported in association with high-grade metamorphism. Formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic Pb from zircon. Both the redistribution and phase separation of radiogenic Pb in this manner can compromise the precision and accuracy of U-Pb ages obtained by high spatial resolution methods.

KEYWORDS:

Antarctica; early Earth; metallic Pb; nanospheres; zircon

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