Untangling impact ages and crustal processes in highly shocked planetary materials using accessory minerals

James Darling, University of Portsmouth, Portsmouth, United Kingdom, Desmond Moser, University of Western Ontario, Department of Earth Sciences, London, ON, Canada, Kim Tait, Royal Ontario Museum, Toronto, ON, Canada, Ivan Barker, University of Western Ontario, London, ON, Canada, Kevin Chamberlain, University of Wyoming, Laramie, WY, United States and Axel K Schmitt, University of California Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, CA, United States

Contact First Author: James Darling; james.darling@port.ac.uk

Previously Published Material: Some of the results were presented at IMA2014, but this presentation will add significant new data and insight.

Abstract ID#: 35163

 

English Abstract:
Separating the effects of shock metamorphism from endogenic crustal processes continues to be a major challenge for the study of meteorites and terrestrial impact structures.

Here we show that in highly shocked Martian and Lunar meteorites this evolution can be resolved by combining electron nanobeam techniques (CL, EBSD, TEM) with in-situ U-Pb isotope analyses by SIMS or LA-ICP-MS. We focus upon the effects of shock metamorphism on the U-Pb systematics of baddeleyite (ZrO2) and zircon (ZrSiO4) – micron-scale phases common to achondrites and terrestrial impact structures. Unlike zircon, the relationship of shock heating and deformation with retention of radiogenic Pb in baddeleyite is poorly known.

In both NWA 5298 (shergottite) and NWA 2200 (Lunar anorthositic breccia), baddeleyite grains from individual polished thin-sections show a wide array of deformation microstructures. These include fracturing, varying degrees of amorphization and granulation, plastic deformation, and recrystallization: reflecting local variations in shock pressures and waste heat. SIMS U-Pb isotope analyses reveals variable degrees of age resetting in both samples. For NWA 5298, variable Pb loss (as high as 80 %) can be directly correlated with observed microstructures and with the extent of post-shock zircon rims that are linked to release of Si-rich fluids during quenching of shock melt pockets during transit to space. In NWA2200, the range of baddeleyite 207Pb/206Pb ages (ca. 4050 to 3850 Ma), is significantly younger than zircons with shock microstructures from the same sample (ca. 4400 to 3950 Ma), reflecting either a distinct provenance or differing shock-response of the two phases.

These findings, contrary to the results of shock loading experiments, indicate that baddeleyite U-Pb ages can be reset under certain shock metamorphic pathways. The combined microstructural and U-Pb data therefore provide a powerful tool for determining both the primary age of the meteorite assemblages and bracketing the time of impact events.

Back to: Enriching Concordia; Microstructure Meets the Geochronology of Earth and Solar System Processes I