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Radiation damage control on apatite (U-Th)/He dates from the Grand Canyon region, Colorado Plateau

¹ Division of Geological and Planetary Science, California Institute of Technology, Pasadena, California 91125, USA
² Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA
³ Division of Geological and Planetary Science, California Institute of Technology, Pasadena, California 91125, USA

Individual detrital apatite grains from the Esplanade, Coconino, and Moenkopi Formations in the Grand Canyon region of the Colorado Plateau yield (U-Th)/He dates from 104 to 5 Ma. The range of dates within each unit far exceeds analytical uncertainty, but correlates with both He concentration [He] and effective U concentration [eU]. These dates are all significantly younger than the sandstone units, indicating partial to complete He loss following deposition. Recently published laboratory diffusion data suggest that He retentivity in apatite increases with radiation damage. Forward models predict that the consequences of this effect will be manifested most clearly as a correlation between (U-Th)/He dates and the [He] and [eU] in suites of apatites that (1) are characterized by a large span of [eU], and (2) had thermal histories in which sufficient time elapsed for the apatite He diffusion kinetics to diverge prior to reheating and partial resetting. Apatites in the sedimentary units investigated fit these cri teria. Using geologically reasonable deposition, burial, and unroofing histories, simulations that include the effect of radiation damage on apatite He retentivity can reproduce the observed distributions of apatite dates and correlations with parent and daughter concentrations. These results suggest that a span of (U-Th)/He dates positively correlated with [eU] may provide important information regarding a sample's thermal history.

Key Words: (U-Th)/He • apatite • radiation damage • Colorado Plateau • He diffusion kinetics • thermochronometry • Grand Canyon

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