Metallic lead nanospheres discovered in ancient zircons

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Kusiak, Monika A., Dunkley, Daniel J., Wirth, Richard, Whitehouse, Martin J., Wilde, Simon A., Marquardt, Katharina
• Format: Text
• Language: English
• Published: National Academy of Sciences 2015
• Subjects: Physical Sciences; East Antarctica; Antarc*; Antarctica
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4413306/; http://www.ncbi.nlm.nih.gov/pubmed/25848043; https://doi.org/10.1073/pnas.1415264112

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.