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Paper No. 4
Presentation Time: 2:15 PM


AULT, Alexis K., Department of Geological Sciences, University of Colorado at Boulder, UCB 399, 2200 Colorado Ave, Boulder, CO 80309 and METCALF, James R., Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309,

Application of apatite (U-Th)/He thermochronometry (AHe) to continental interiors holds the promise of providing insight into cryptic Phanerozoic epeirogenic motions in these regions. The method has been little applied in these settings, however, owing to poorly understood scatter in cratonic AHe data. Recent study showed that apparent dispersion of some cratonic data is due to date-eU correlations, which impose important restrictions on thermal histories when interpreted using new apatite He diffusivity models. Yet it is clear that not all dispersed cratonic AHe data can be explained in this manner.

We use apatite cathodoluminescence (CL) images from 25 samples across the Slave craton and environs to help isolate high-fidelity AHe data and evaluate the potential for U-Th zonation to explain dispersed AHe dates for a subset of samples. We divide most results into two groups. Group 1 apatites exhibit weak to no CL zoning and are characterized by AHe dates with <15% sample standard deviation. We infer that these apatites lack significant U and Th zonation, such that the results can be used to obtain reliable thermal history constraints. Group 2 apatites display strong CL zonation, yield AHe dates with >15% sample standard deviation, but exhibit no date-eU correlations as observed in prior study elsewhere in the Canadian shield. We infer that variable zoning in U and Th, mimicked by the CL patterns, causes the data scatter. Dates from apatites zoned in U-Th are less reliable due to an inaccurate alpha-ejection correction and the type of thermal history for this area that magnifies the effect of zoning on the dates. A minority of samples do not fall into either group, suggesting that CL may not always be a perfect proxy for U-Th zonation. The dates yielded by Group 1 samples indicate burial and unroofing of the craton by several kilometers of strata in Paleozoic-Mesozoic time, with some spatio-temporal heterogeneity of this history across the region. Elevation change of the craton appears required by this result and is key for evaluating Phanerozoic geodynamic processes in the region. A protocol involving CL characterization of apatites can aid in identification of high-quality apatites for AHe analysis, assist in AHe data interpretation, and facilitate successful application of the method to problems in continental interiors.

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