Combining Nano Scale Microstructural Analysis and U-Pb Geochronology of Terrestrial Baddeleyite to Unravel Crustal and Bombardment Chronology
Combining Nano Scale Microstructural Analysis and U-Pb Geochronology of Terrestrial Baddeleyite to Unravel Crustal and Bombardment Chronology
Abstract ID#: 34803
English Abstract:
Baddeleyite (monoclinic-ZrO2) commonly crystallizes in mafic and ultra-mafic rock types where zircon (ZrSiO4) does not, rendering it a key geochronometer within these silica under-saturated lithologies more prevalent on the Moon and Mars. It is an especially important geochronology mineral when examining ex-situ meteoritic samples derived from other planetary bodies, as many grains within an igneous population survive ejection and give the timing of primary crystallization whereas others experience shock heating and fluid alteration to cause partial to complete age re-setting to the time of an impact event. Discrimination of these scenarios in other samples so as to best interpret U-Pb age will benefit from a fuller knowledge of the shock metamorphic response of baddeleyite to known natural and experimental conditions. Here we present our first ground-truthing efforts toward this goal with an analysis of the evolution of orientation and isotopic microstructure of baddeleyite from the Sudbury impact structure - a globally unique, terrestrial example of these impact processes. Micro-baddeleyite grains (<15μm) within the 2.45 Ga Matachawan diabase dyke swarm, subjected to shock metamorphism as a result of the 1.85 Ga ~200km diameter Sudbury impact structure are being comprehensively analysed along a (near linear) transect across the Superior province radiating out from the boundary with the melt sheet. Electron Backscatter Diffraction (EBSD) mapping reveals typical igneous cooling twins parallel to [100] but also lattice misorientations up to <15°. Atom probe tomography of coherent material (i.e. no apparent fractures or phase discontinuities) reveals nanoclusters of Fe, Al and Ca presently interpreted to have been introduced following shock metamorphism. Distribution analysis of trace elements including U and Pb is underway and will be compared to isotopic analysis of residual grain material using complimentary mass spectrometry methods. Baddeleyite is a common, though tiny, crustal phase that has great potential to improve inner solar system crustal and impact chronologies.