WHEN CRUST MEETS CHONDRULES; ZIRCON RESPONSE WITHIN A SMALL-SCALE IMPACTOR IN THE METEORITE NORTHWEST AFRICA 869

Northwest Africa 869 is a chondritic regolith breccia meteorite with reported signs of impact accretion (Hyde et al. 2015). In one case, a quartz gabbro impactor, likely from the crust of the eucrite parent body, is observed surrounded by a breccia/melt rim. The textures resulting from this impact are captured in a single thin section, providing a glimpse of small-scale impact processes. Moreover, the impactor component is zircon-rich offering a unique opportunity to characterize the preservation/modification of zircon grains found in impactor materials from primordial crusts. The goal is to discriminate between primary and metamorphic zircon formation in preparation for dating small-scale impact processes.

The zircon morphology, chemical zoning and orientation microstructure in the quartz gabbro impactor were analyzed at the Zircon and Accessory Phase Laboratory (ZAPLab) at the University of Western Ontario. This study utilized cathodoluminescence (CL), low kV backscatter electron (BSE) imaging and electron backscatter diffraction (EBSD) mapping.

Zircon grains within, and associated with, ilmenite are fractured and consist of crystalline domains exhibiting crystal plastic deformation as well as areas that appear to have been amorphized by shock. Some of these grains have randomly oriented granules around their edges suggesting local impact heating/recrystallization. A zircon in the breccia/melt rim partially shielded by ilmenite has even retained apparent igneous zonation. As seen in some eucrites (Roszjar et al. in press), zircon in or near SiO₂-rich zones occur as aggregates of rounded concentrically CL-zoned regions suggesting a pre-impact, metamorphic origin. In this case, misorientations of up to 43° are seen between the regions. Zircon grains show the most significant impact modification near the edge of the impactor. In this region whole grains are replaced by granular (submicron) zircon. High pressure zircon polymorphs and the high temperature breakdown of zircon to baddeleyite+SiO₂ have not been observed. The data provided in this study allow us to distinguish pre-impact zircons and shock recrystallized zircons from grains that might have grown in the melt rim. Coupled U-Pb dating and mineral chemistry will enable us to obtain a more accurate petrogenetic reconstruction of this primordial crust.