Geocheminar spring-2014

We conducted a survey of Apollo 14 lunar zircon microstructures to search for and compare shock features common in zircons from terrestrial impact structures. The zircons were separated from two breccia rocks (14305 and 14321) and one soil (14259) sample by crushing and heavy liquid density separation. Pb-Pb crystallization ages range from ~3.9 to 4.4 b.y. for both breccia and soil grains, making them ideal candidates for investigating the early impact history of the Moon. Previous microstructural studies of lunar zircons have found evidence of low grade shock deformation. The presence of crystallographically controlled shock microstructures and associated lattice misorientation was first searched for using low kV secondary electron (SE), low kV backscatter electron (BSE), and color cathodoluminescence (CL). Electron backscatter diffraction (EBSD) maps of potential shock features were then collected using a Hitachi SU6600 variable pressure, analytical field emission gun - scanning electron microscope (FEG-SEM) at University of Western Ontario, Zircon and Accessory Phase Laboratory (ZAP Lab). A wide range of microstructural states and textures were observed. The low-strain end of the spectrum included featureless low-strain grains, with primary oscillatory and sector zoning. Shock microstructures in intermediate shock levels include planar and curviplanar fractures hosting micron scale melt inclusions (now partly devitrified glass). Highest shock pressures show domains of granular texture zircon. The highest-grade shock features (microtwins and granular texture) are seen in zircons from both breccias 14305 and 14321, while the soil zircons only exhibit lower grade shock deformation such as curviplanar fractures and small degrees of misorientation. Some grains have multiple sets of microstructures possibly due to different impacts events on the Moon or different stages of shock loading and unloading not previously documented. Further high special resolution investigations into the relationship between the U-Pb system and lunar microstructures will validate and refine the nature of the early lunar impact history, which likely mirrors that of our own planet.