Mechanisms and consequences of intra-crystalline enrichment of 1 ancient radiogenic Pb in detrital Hadean zircons from the Jack Hills , 2 Western Australia 3 4

23 The recent discovery of Pb* (radiogenic lead) -enriched domains (PEDs) in zircon using 24 high-resolution scanning ion imaging, atom probe tomography and transmission electron 25 microscopy indicates that the U-Pb isotopic system in zircon can be affected not only by Pb* loss 26 but also by intra-crystalline Pb* enrichment. However, the formation mechanism of PEDs and the 27 consequences for in-situ U-Pb dating remain elusive. To further understand these issues, scanning 28 ion imaging and U-Pb dating were carried out on 51 Hadean detrital zircon grains from the Jack 29 Hills, Western Australia. Of these, 8 grains were found to contain micrometer-scale PEDs with 30 elevated 207 Pb and 206 Pb in the ion images. Isotope profiles across these PEDs confirm that they 31 represent unsupported Pb* resulting from intra-grain mobilization and local enrichment of Pb* 32 during ancient overprinting events. The PEDs are rare in most grains, with only 1 – 3 Pb* hotspots 33 present in any ~70 μm × 70 μm imaged area, and are located either in dark-CL, high-U growth 34 zones, or associated with bright-CL, low-Th recrystallized domains, suggesting that they were 35 likely formed by local-scale processes. The PEDs located in dark-CL, high-U growth zones likely 36 resulted from Pb* trapped in local, interconnected pathways produced by radiation damage during 37 thermal overprinting events. Those associated with bright-CL, low-Th domains are interpreted as 38 Pb* concentrated in nanoscale pores formed during fluid-present recrystallization of non-metamict 39 zircon. Our interpretations therefore imply that intra-crystalline enrichment of Pb* in zircon can 40 occur at various scales and through diverse mechanisms. Age calculations based on the ion images 41 demonstrate that incorporation of PEDs in conventional in-situ SIMS spots produces spuriously 42 old ages that are either reversely discordant or concordant, depending on the time of Pb* 43 mobilization and the analytical precision. The implication is that careful investigation of 44 intra-grain distribution of U-Th-Pb isotopes is required in order to better understand the U-Pb 45 isotopic system and to obtain reliable crystallization ages for ancient zircons affected by complex 46 Pb* mobilization and redistribution. 47