New petrochronology tools and approaches for reconstructing high-temperature crustal process.

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Earth's crust preserves a nearly 4.0 billion year record of our planet's modification and differentiation by plate tectonics. Both processes lead to localized heating of crustal rocks over timescales of tens of millions of years during episodes of plate collision and mountain building (orogenesis). The record of these ancient collisions leave tell-tale textural, chemical, and isotopic features in minerals that form in rocks during their burial, residence, and exhumation back to the surface. Deciphering the imprints of ancient high-temperature crustal processes is the focus of metamorphic `petrochronology'. This field has undergone a recent renaissance because of new analytical tools capable of measuring isotope ratios at spatial resolutions previously unattainable. The first project will be carried out by a MSc graduate student tasked with documenting the absolute age of structural juxtaposition between two well-defined crustal blocks exposed in a Mesoproterozoic metamorphic complex in southeastern BC. This project will test the applicability of new in situ Rb-Sr dating techniques on micas that also characterized in terms of their trace-element zoning patterns. The results are expected to shed new light on Mesoproterozoic active margin processes along the ancestral margin of North America and will provide a starting point for more ambitious projects in other metamorphic settings. The second project will be carried out by a PhD graduate student in ~400 Ma rocks in the western Cape Breton Highlands, NS. Here a complete metamorphic sequence from chloritoid- to kyanite-zone rocks is exposed and displays diachronous U-Pb monazite and Lu-Hf garnet ages moving up-temperature. The goal here will be to use campaign-style in situ Rb-Sr dating and trace-element mapping to identify structural corridors along which the metamorphic sequences were imbricated. The results will contribute to our overall understanding of how the middle-crust responds to plate collisions and how the timing of deformation and metamorphism are interlinked. The final component of this proposal aims to build a novel new laser ablation sampling in liquid (LASIL) to take full advantage of the much higher sensitivity of ICP-MS measurements performed on solutions. The basic concept is to entrain in a liquid the nanoparticles ejected from a sample during laser ablation. This will require the 3D design, computational fluid-flow modeling, multi-jet 3D printing, and prototype testing. First-order calculations based on volumes of liquid, volume of ablated aerosol liberated, and estimated concentrations of analytes suggest that an increase in sensitivity of at least two orders of magnitude, relative to conventional laser ablation, is possible. If successful, this LASIL method will revolutionize in situ isotope measurements and place Canada at the forefront of modern laser ablation interface development.