Lunar Zircons : What ’ S the Big Picture ?

Introduction: Zircon is a late stage accessory mineral that incorporates radiogenic parents such as uranium and thorium, while preferentially excluding their daughter product lead, allowing for precise U-Pb and Pb-Pb ages to be determined for individual grains [1]. Zircons also incorporate other trace elements such as the rare earth elements (REEs) and titanium, which can reflect the composition and temperature of the parent magma [2]. Microstructural studies of terrestrial and lunar impact zircons found that some types of shock deformation may result in increased Pb mobility, but the extent to which these microstructures affect radiogenic ages and/or trace element compositions in lunar grains is still unclear (e.g. [3-6]). The lunar zircons analyzed to date have Pb-Pb ages that range from 3.9 to 4.4 Ga (e.g. [6-8]). These relatively old ages, predating the hypothesized late heavy bombardment (LHB) [9], and the ability of these grains to retain primary crystallization ages and signatures of secondary impact processes, make them suitable for investigating the early magmatic and impact history of the Moon. We have conducted an extensive U-Pb, REE, and microstructural survey of Apollo zircons that, in combination with previous studies, represents a robust lunar zircon dataset from which we can draw constrains regarding the duration of KREEP magma-tism, zircon formation mechanisms, and possibly the early impact history of the Moon. Methods: Zircons were separated from Apollo 14, 15, and 17 samples by crushing and heavy liquid density separation. All grains were imaged by scanning electron microscopy (SEM) to search for cracks, inclusions, and regions of cathodo-luminescence (CL) variation. After preliminary characterization , U-Pb and Pb-Pb ages for 155 zircons and trace elements of 89 zircons were collected with the UCLA Cameca IMS-1270. A selection of 30 grains were then searched for the presence of crystallographically controlled shock microstructures at the University of Western Ontario, Zircon and Accessory Phase Lab (ZAP Lab) by using a combination of secondary electron (SE), low kV backscatter electron (BSE), CL, and electron backscatter diffraction (EBSD) mapping. Results: Microstructures. The 30 zircons survied for shock microstructures were from breccias 14305, 14321, and