Shock Processes in Solar System Zircon

English Abstract:

Zircon is an extremely durable and refractory phase, now known to survive intense shock metamorphism, tectonic recycling and the erosion of continents and craters. It occurs throughout the Earth’s lithosphere, and is increasingly being studied in returned or meteoritic samples of crusts of the Moon, asteroids and, most recently, the ancient highlands of Mars. This expansion to extraterrestrial materials has partly been driven by advances in electron microscopy that allow precise mapping and contextualization of grains as small as one micron in polished thick sections, techniques such as EBSD that afford mapping of lattice orientations, misorientations and high pressure polymorphs in micro-grains at angular and spatial resolutions of 0.2 degrees and 50nm, respectively, and the parallel advances in micro- to nano-scale isotopic analysis by SIMS, laser methods and atom probe tomography (APT). In this talk I will present examples of the growing diversity of shock processes recorded in zircon chemistry and/or microstructure from a range of deformation-temperature pathways experienced by inner solar system samples. Case studies will include; the ~250 km wide Vredefort impact structure, a natural laboratory for terrestrial shock processes in accessory minerals across a gradient of ~1000 degrees and 60 GPa where some zircons record the entire shock loading and unloading sequence, lunar zircons from Apollo and meteoritic samples that show features analogous to those at Vredefort, the 4 Ga population of meteorite NWA 7475 now being explored as our first zircons from the martian regolith, as well as shock-generated zircon in melt bodies on Earth and proximal to launch-melt pockets in 180 m.y. shergottite. The full potential of zircon as a recorder of the nature and timing of shock metamorphic processes is yet to be realized, and its future role will be discussed with regard to improving early bombardment chronologies that relate to the evolution and habitability of planetary crusts.