"Inverted" Zircon and Apatite (U-Th)/He Dates and Interpretation of High-Damage Zircon from the Southern Rocky Mountains, Front Range, Colorado

Johnson, Joshua E.

Graduate Thesis Or Dissertation

"Inverted" Zircon and Apatite (U-Th)/He Dates and Interpretation of High-Damage Zircon from the Southern Rocky Mountains, Front Range, Colorado Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/x633f135m

Abstract

Radiation damage in zircon has a profound influence on helium retentivity. This study is aimed at understanding the He systematics of high-damage zircons since this end of the damage spectrum has received the least attention in prior work. We acquired 121 zircon (U-Th)/He (ZHe) dates from 29 samples from an ~50 km east-west transect across the Colorado Front Range that span the full range of alpha dosages (radiation damage) encompassed by previous diffusion experiments. Date-eU patterns within our ZHe dataset are broadly consistent with the expected influence of radiation damage, showing positive and then negative correlations. ZHe dates from the range core in Rocky Mountain N.P. record cooling to near-surface temperatures during the Laramide Orogeny (65-45 Ma). Closer to the range front, there is a sharp transition to Oligo-Miocene ZHe dates despite the presence of Laramide apatite He (AHe) dates in the immediate vicinity. Titanite He (THe) dates from the area record cooling through ~200 °C in the Neoproterozoic, precluding reheating above that temperature in the last 600 myr. [object Object]High-damage zircons from Big Thompson Canyon have ~20 Ma ZHe dates that are "inverted" with respect to 65-45 Ma AFT and AHe dates from the same area. This inversion implies that these zircons are sensitive to temperatures of <70 °C, significantly lower than their nominal closure temperature of ~180 °C. At these high damage levels, there is a disconnect between the retentivity predicted by the current damage-diffusivity model, model ZHe dates, and relevant geologic and geochronologic constraints. Despite this disconnect, our results show that damaged zircons can serve as low-temperature chronometers. The utility of applying ZHe in this manner is demonstrated by the detection of a previously unrecognized reheating event on the order of ~50 °C in the Oligo-Miocene, implied by the ~20 Ma ZHe dates. Our preferred explanation for this event invokes the reburial of the range front under ~1 km of sediment derived from erosion of the high topography of the range core followed by the subsequent unroofing in the early Miocene, possibly recorded by the basal units of the Ogallala Formation on the High Plains.

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