MICROPROBE MONAZITE GEOCHRONOLOGY: ANALYTICAL CHALLENGES AND POWERFUL APPLICATIONS FOR DATING TECTONIC PROCESSES

The use of monazite for geochronology, thermometry, geochemistry, and petrology has increased dramatically. Reasons include: 1) monazite is observed in a wide variety of igneous, metamorphic, and sedimentary rocks; 2) slow diffusions rates allow monazite to retain a detailed record of previous geologic conditions; 3) the refinement of techniques for trace element analysis and dating of monazite; 4) increased understanding of the conditions under which monazite can crystallize and recrystallize; and 5) identification of geochemical and petrologic links between silicate assemblages and monazite. Monazite crystals typically contain multiple compositional (and age) domains, which can be linked to silicate textures, fabrics, and reactions. However, very small grains or domains can be too small for in-situ isotopic techniques. The electron microprobe can yield precise dates from such micro-volumes, but new analytical techniques are required for accurate trace element analysis. Background characterization is critical because curvature of the spectrum and background interference can lead to systematic errors of 50 m.y. or more. Spectrum modeling and regression allow accurate determination of background and associated uncertainty. Peak interferences are also important and must be calibrated for each instrument. Specimen beam damage and conductivity effects are significant and must be mitigated. Results from the optimized “Ultrachron” probe at UMass indicate that precision can be on the order of several m.y. even in very small domains. On-going research is aimed at integrating monazite into metamorphic or hydrothermal reactions that in turn have been linked to tectonic events. Examples will be presented from the east Lake Athabasca granulite terrane, Saskatchewan. Monazite from high-P-T migmatites records the evolution of host-rock composition through melting, plagioclase depletion and reintroduction, and retrograde metamorphism. In the terrane-bounding shear zones, monazite records Archean high-grade metamorphism and a detailed record of Proterozoic exhumation and rehydration. The integration of monazite composition, texture, and geochronology with silicate petrogenesis is a powerful tool for extracting P-T paths and tectonic histories from complex tectonites.