Unsupported radiogenic Pb in zircon: a cause of anomalously high Pb-Pb, U-Pb and Th-Pb ages

Abstract

Ion microprobe U-Th-Pb isotopic analyses of zircons from a granodioritic orthogneiss from the Napier Complex, Mount Sones, Enderby Land, Antarctica, have identified an unambiguous example of unsupported radiogenic Pb in a 3,950 Ma-old crystal. At one 40 μm spot on the crystal an unusually large heterogeneity in Pb content was found, the concentration of radiogenic Pb ranging from 5 to 50 percent higher than could have been generated in 3,950 Ma by radioactive decay of the co-existing U and Th. This relative excess of radiogenic Pb is attributed to Pb gain rather than to U and Th loss because first, the Pb content varied by more than the U or Th contents and secondly, changes in the Pb/U, Pb/Th and Pb isotopic composition correlated directly with changes in the Pb concentration. The individual ²⁰⁷Pb/²⁰⁶Pb apparent ages ranged from 4,000 Ma to 4,145 Ma, all greater than the inferred age of the crystal. A correlation between ²⁰⁷Pb/²⁰⁶Pb and Pb/U shows that the Pb excess has not resulted from recent Pb movement. The spot apparently gained radiogenic Pb about 2,500 Ma ago, at the same time as the majority of the other zircons in the rock suffered substantial Pb loss. The Pb movement occurred in response to a discrete geologic event. Reverse discordance is a phenomenon that must be considered when interpreting zircon U-Pb ages, especially ²⁰⁷Pb/²⁰⁶Pb ages of single crystals or portions of crystals.