HR: 14:35h
AN: V33E-04    [Abstracts]
TI: The role of electron microprobe mapping and dating in tectonic geochronology
AU: * Williams, M L
EM: mlw@geo.umass.edu
AF: Department of Geosciences University of Massachusetts, 611 North Pleasant St, Amherst, MA 01003, United States
AU: Jercinovic, M J
EM: mjj@geo.umass.edu
AF: Department of Geosciences University of Massachusetts, 611 North Pleasant St, Amherst, MA 01003, United States
AU: Dumond, G
EM: gdumond@geo.umass.edu
AF: Department of Geosciences University of Massachusetts, 611 North Pleasant St, Amherst, MA 01003, United States
AU: Mahan, K H
EM: mahank@colorado.edu
AF: Dept. of Geological Sciences University of Colorado, Campus Box 399, 2200 Colorado Ave, Boulder, CO 80309, United States
AU: Flowers, R M
EM: Rebecca.Flowers@Colorado.EDU
AF: Dept. of Geological Sciences University of Colorado, Campus Box 399, 2200 Colorado Ave, Boulder, CO 80309, United States
AB: Electron microprobe geochronology occupies a special niche within the spectrum of geochronological techniques and may be particularly relevant to the question, "What are we dating?" The technique was originally envisioned to be a low-cost, reconnaissance dating tool, opening low-resolution geochronology to a large number of researchers. However, more than a decade of research has shown that, when used in a reconnaissance fashion (i.e. using major-element analytical techniques for trace-element analysis) uncertainties are unsuitably large (several 10s of m.y. or more) for solving most tectonic problems. Using trace element analytical techniques (background modeling, interference correction, highly conductive coating, multi-analysis measurement, etc.) precision and accuracy are dramatically increased, but analysis time and cost are also increased, challenging the “quick, cheap, and easy” description. The power of microprobe geochronology comes from the spatial resolution and the natural integration with compositional data. High-resolution compositional mapping is valuable for all in-situ geochronology. Large area maps provide petrologic and textural context for chronometer phases; small scale maps illuminate the history of the chronometers themselves. Compositional maps associated with monazite are particularly informative, but examples from the East Athabasca granulite terrane using zircon, titanite, and rutile will be discussed. Most monazite crystals are 30μ or less and most have several compositional domains. Rim compositions and dates are particularly critical because they can commonly be tied to reactions and to matrix texture and fabric. Commonly, rims and internal sub domains are several microns in width and can only be analyzed by electron probe. Y has been widely used to tie monazite to Grt growth or breakdown, but current studies use a suite of trace and REE (Y, Sm, Nd, Ca, Eu, Gd, etc) to tie monazite into chemical reactions. A rapidly growing application involves detrital and authigenic monazite (and xenotime). Detrital grains that can be linked with source terrains can have very thin authigenic or metamorphic rims dating digenesis or early metamorphism. The rims involve reactions with monazite and surrounding phases that constrain depositional or metamorphic conditions and fluid compositions. Because concordancy cannot be tested, microprobe monazite applications in the Athabasca granulite terrain are strongest in combination with high-resolution U-Pb TIMS data. Microprobe dates constrain the age of specific deformation or metamorphic processes within the context of the overall high-precision geochronologic spectrum.
DE: 1100 GEOCHRONOLOGY
DE: 3600 MINERALOGY AND PETROLOGY
DE: 8103 Continental cratons
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting