A scanning ion imaging investigation into the micron - scale U - Pb systematics in a 1 complex lunar zircon 2 3

12 Abstract 13 14 The full U-Pb isotopic systematics in a complex lunar zircon ‘Pomegranate’ 15 from lunar impact breccia 73235 have been investigated by the development of a 16 novel Secondary Ion Mass Spectrometry (SIMS) scanning ion imaging (SII) 17 technique. This technique offers at least a four-fold increase in analytical spatial 18 resolution over traditional SIMS analyses in zircon. Results from this study confirm 19 the hypothesis that the Pomegranate zircon crystallized at 4.302 ± 0.013 Ga and 20 experienced an impact that formed, U-enriched zircon around primary zircon cores at 21 4.184 ± 0.007 Ga (2 , all uncertainties). The increase in spatial resolution offered by 22 this technique has facilitated targeting of primary zircon that was previously 23 inaccessible to conventional spot analyses. This approach has yielded results 24 indicating that individual grains with a diffusive distance of less than ~4 m have 25 been reset to the young impact age, while individual grains with a diffusive distance 26 larger than ~6 m have retained the old crystallization age. Assuming a broad range 27 in cooling rate of 0.5-50 °C/year, which has been observed in a suite of similar lunar 28 breccias, a maximum localized temperature generated by the impact that reset small 29 primary zircon and created new, high-U zircon is estimated to be between 1100 and 3