ZIRCON and APATITE (U-Th)/He DATES and INTERPRETATION of HIGH-DAMAGE ZIRCON from the SOUTHERN ROCKY MOUNTAINS, FRONT RANGE, COLORADO By

“INVERTED” ZIRCON AND APATITE (U-Th)/He DATES AND INTERPRETATION OF HIGH-DAMAGE ZIRCON FROM THE SOUTHERN ROCKY MOUNTAINS, FRONT RANGE, COLORADO by JOSHUA E. JOHNSON B.A., Middlebury College, 2013 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Master of Science Department of Geosciences 2015 This thesis entitled: “Inverted” zircon and apatite (U-Th)/He dates and interpretation of high-damage zircon from the Southern Rocky Mountains, Front Range, Colorado written by Joshua E. Johnson has been approved for the Department of Geosciences (Dr. Rebecca Flowers) (Dr. Kevin Mahan) (Dr. Craig Jones) Date The final copy of this thesis has been examined by the signatories, and we Find that both the content and the form meet acceptable presentation standards Of scholarly work in the above mentioned discipline. ii Johnson, Joshua E. (MSc., Geosciences) “Inverted” zircon and apatite (U-Th)/He dates and interpretation of high-damage zircon from the Southern Rocky Mountains, Front Range, Colorado Thesis directed by Associate Professor Rebecca Flowers 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. High-damage zircons (>1018 α/g) 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. iii ACKNOWLEDGMENTS I have many people to thank for helping me throughout the thesis process. First and foremost, I would like to thank my advisor Becky Flowers for her unwavering support and incredibly helpful feedback. Jim Metcalf provided incalculable analytical assistance – this project could not have happened without his help. David Liefert, a CU undergraduate, provided excellent field assistance and helped tremendously with the lab work that followed. Dr. Graham Baird (University of Northern Colorado) and Dr. Shari Kelley (New Mexico Institute of Mining and Technology) graciously provided mineral separates that were used in this study. Kevin Mahan (CU) provided valuable information on the geology of Big Thompson Canyon. Paul McLaughlin and others at Rocky Mountain National Park assisted with the permitting process required to conduct research in the park. Nigel Kelley (CU) and Eric Ellison (CU) assisted with the collection and interpretation of Raman data. Ken Cochran (University of Northern Colorado) assisted with the collection of CL images. Lastly, thanks to all my fellow grad students and friends that let me bounce ideas off them these past two years. This project would not have been possible without generous funding from the Rocky Mountain Conservancy, the Geological Society of America, the Colorado Scientific Society, and the Dept. of Geosciences at CU. iv CONTENTS CHAPTER I. INTRODUCTION ....................................................................................1 II. GEOLOGIC SETTING ............................................................................4 2.1 Regional Geologic Setting ............................................................4 2.2 Big Thompson Canyon .................................................................9 2.3 Rocky Mountain National Park ..................................................12 III. PREVIOUS THERMOCHRONOLOGY IN THE COLORADO ROCKIES ...............................................................................................13 IV. SAMPLES AND METHODS ................................................................15 4.1 Samples .......................................................................................15 4.2 Cathodoluminescence Imaging and Raman Spectroscopy .............................................................................15 4.3 (U-Th)/He Thermochronology ....................................................18 V. RESULTS ...............................................................................................21 5.1 Zircon Characterization: CL Imaging and Raman Spectroscopy .............................................................................21 5.2 (U-Th)/He Thermochronology ....................................................24 VI. DISCUSSION .........................................................................................32 6.1 (U-Th)/He Systematics of High-Damage Zircons ......................32 6.1.1 Alpha dose of analyzed zircons .........................................32 6.1.2 Reproducibility and temperature sensitivity of He data from high-damage zircons ..............................36 6.1.3 Testing the high-damage end of the damage-diffusivity model .................................................38 6.2 Geologic Implications .................................................................41 VII. CONCLUSIONS ....................................................................................49 REFERENCES….……………………..…………………………………………51 v TABLES Table 1. (U-Th)/He data ..............................................................................................25 vi FIGURES Figure 1. Simplified geologic map of the northern Front Range ...................................5 2. Map of sample locations and plots of ZHe data ...........................................16 3. CL and Raman data .......................................................................................22 4. Age-elevation plot for Longs Peak ...............................................................29 5. Plots of AHe and THe data ...........................................................................30 6. Simplified date-eU plot of all ZHe data ........................................................31 7. Alpha dose comparison plot ..........................................................................34 8. HeFTy simulation results ..............................................................................40 SUPPLEMENTARY FIGURES Figure 1. Results by individual sample and lithology ..................................................50 vii CHAPTER I INTRODUCTION Zircon’s impressive durability, longevity, ubiquity, and ability to incorporate uranium and other trace elements into its structure have enabled a wide array of geochronologic, trace element, and isotopic investigations with extensive insights into the geologic record. Zircon is now increasingly applied as a (U-Th)/He thermochronometer (ZHe), with initial diffusion work suggesting a nominal closure temperature of ~180 °C (Reiners et al., 2002; Reiners, 2005). It is well known that zircon’s typically high U-Th concentrations can cause significant radiation damage accumulation within its crystal structure (e.g., Meldrum et al., 1998). Recent work has shown that zircon He diffusion is strongly dependent on its accrued radiation damage (Guenthner et al., 2013). Recent publication of a zircon radiation damage-He diffusion kinetic model unlocks the potential to understand and exploit the ZHe data dispersion that is a consequence of this effect. The effect of radiation damage on He diffusion kinetics was first recognized in apatite, where damage accumulation decreases the He diffusivity (Shuster et al., 2006; Flowers et al., 2009). For some thermal histories, this phenomenon is evident as a positive correlation between date and effective uranium concentration (eU = U + 0.235 × Th). eU serves a proxy for the rate of alpha production, and hence, relative radiation damage in a suite of crystals that have experienced the same thermal history (Flowers et al., 2007). In zircon, progressive damage accumulation initially causes a decrease in He diffusivity that is interpreted

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