Metallic lead nanospheres discovered in ancient zircons

Monika A. Kusiak; Daniel J. Dunkley; Richard Wirth; Martin J. Whitehouse; Simon A. Wilde; Katharina Marquardt

doi:10.1073/pnas.1415264112

New Research In

Edited by Richard W. Carlson, Carnegie Institution of Washington, Washington, DC, and approved March 11, 2015 (received for review August 8, 2014)

Significance

Metallic lead nanospheres have been discovered in ancient (>3.4 Ga) zircon grains from an Archean (2.5 Ga) high-grade metamorphic terrain in East Antarctica. Native Pb is present as 5–30 nm nanospheres, commonly in association with an amorphous silica-rich phase, along with titanium and aluminium-bearing phases. Together, these phases form nanoinclusions generated during the recovery of crystallinity in radiation-damaged zircon under high-grade metamorphic conditions. Once formed, the entrapment of nanospheres in annealed zircon effectively arrests Pb loss, explaining why zircon that has experienced such extreme conditions is not completely reset to its metamorphic age. The heterogeneous distribution of Pb can, however, affect isotopic measurement by microbeam techniques, leading to spurious age estimates. Metallic Pb is extremely rare in nature and has never previously been observed in high temperature rocks.

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.

Footnotes

↵¹To whom correspondence should be addressed. Email: monika.kusiak{at}twarda.pan.pl.

Author contributions: M.A.K., D.J.D., and R.W. designed research; M.A.K., D.J.D., R.W., M.J.W., S.A.W., and K.M. performed research; M.A.K., R.W., and K.M. analyzed data; and M.A.K., D.J.D., R.W., M.J.W., S.A.W., and K.M. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.