Redistribution and oxidation of lead in experimentally heated zircons

Abstract

The disturbance of the U-Pb isotopic system in zircon (ZrSiO4) is an important factor for U-Th-Pb geochronology, therefore understanding zircon nanostructure is crucial for reliable age determination. To understand the process of Pb mobilization in zircon, heating experiments on zircon grains from two samples of the Central Iberian Zone (CIZ), a Variscan tonalite and a Cambro-Ordovician orthogneiss, were performed. Samples were heated at 1400 °C for 30, 90 and 180 days in a horizontal furnace in N2 at 1 atm. Following the experiment, nano- and microstructural analyses were performed using Transmission Electron Microscopy (TEM). Mobilization of Pb was documented only in zircons from orthogneiss – these grains contain Pb nanospheres. Zircon grains from orthogneiss showed a significant influence of heating during the experiment on the microstructure. We observed Pb nanospheres (Pb0) and Pb nanoinclusions containing Pb in different oxidation states, namely Pb2+ and Pb4+. This is the first paper documenting nanospheres of metallic Pb existing together with Pb oxides (PbO, Pb2O3, Pb3O4 and PbO2) in one sample. As a result of heating, Pb was oxidized and after 6 months of heating, the spheres of metallic Pb were no longer found. The formation of metallic Pb nanospheres is explained by an annealing process at elevated temperature, that caused the concentration of Pb in noncrystalline, metamict domains of zircon. This study indicates that more than one mechanism can be responsible for the formation of nanospheres. In this case, chemical elements present in zircon as inclusions react with Pb causing its oxidation.