Microstructural response of U-Pb dateable phases across the Vredefort impact structure, South Africa.

English Abstract:

The precise chronology of impact events on planetary surfaces has implications for Earth’s crustal and bio-evolution. U-Pb isotopic dating of accessory phases such as zircon has proved valuable in the pursuit of an enhanced impact chronology, as sufficient shock damage and heating can cause up to 100% Pb-loss while preserving microstructures diagnostic of specific shock environments. Other geochronology phases (e.g. baddeleyite, monazite) also have the potential to contribute to the understanding of shock processes. However, there have been few systematic studies of how zircon and these minerals respond microstructurally and isotopically in a crater environment relative to benchmark shock indicator minerals such as quartz and plagioclase. Here we present initial findings of such a comparison across a known shock metamorphic gradient of ~60 GPa and 900 °C at the 2.020 Ga Vredefort dome, South Africa. Electron nanobeam techniques using a FEG-SEM were applied to map, contextualize and analyze various coexisting accessory phases and measure chemical and orientation microstructures (e.g. CL, EBSD) along with impact-induced melt inclusions. We report preliminary results from low, medium and high shock metamorphic grades of the deeply eroded crater floor. Coexisting phases at the low-grade site include zircon, baddeleyite and titanite, while the high-grade site hosts zircon, monazite and apatite. In all samples, accessory phases entirely enclosed by rock forming minerals (e.g. albite) show less shock damage than those along grain boundaries, revealing the heterogeneity of shock effects within a given sample. Shock microtwins are pervasive, whereas other diagnostic shock effects such as glass inclusions have only been identified in high and medium-grade samples. A more complete understanding of the microstructural and U-Pb response at Vredefort will become a reference for interpreting age and provenance of shocked detrital or ex-situ minerals from Earth, meteorites, Apollo breccias and future sample return missions.