Coupling vertical transect zircon (U-Th)/He and Raman spectroscopy data to constrain Colorado Front Range evolution
by Rachel E. Havranek
B.A., Pomona College, 2014
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 2017
This thesis entitled: Coupling vertical transect zircon (U-Th)/He and Raman spectroscopy data to constrain Colorado Front Range evolution written by Rachel E. Havranek has been approved for the Department of Geosciences
(Dr. Rebecca Flowers)
(Dr. G. Lang Farmer)
(Dr. Nigel Kelly)
Date
The final copy of this thesis has been examine 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 Havranek, Rachel E. (M.S., Geosciences) Coupling vertical transect zircon (U-Th)/He and Raman spectroscopy data to constrain Colorado Front Range evolution Thesis directed by Associate Professor Rebecca Flowers The Front Range is the largest and easternmost Laramide uplift in Colorado and exposes primarily Proterozoic basement. Front Range “fourteeners” with their large vertical relief and abundant zircon provide the opportunity to 1) use the variable He retentivity of a suite of highly damaged zircons to better constrain the Neoproterozoic – Late Cretaceous evolution of the Colorado Front Range and 2) integrate zircon Raman spectroscopy data with zircon (U-Th)/He (ZHe) data to improve the understanding of how radiation damage influences the kinetics of ZHe diffusion and annealing. Pikes Peak samples were collected at elevations from 2084 m to 4297 m. ZHe dates for 6 samples range from 762 ± 87 Ma to 82 ± 6 Ma and display uniform negative date-eU correlations, consistent with reduced He retentivity at high radiation damage doses. There is no correlation between elevation and date. ZHe data are in broad agreement with previously published apatite fission-track (AFT) dates from the same elevation range that vary from 449 ± 57 Ma to 45 ± 4 Ma and were used to interpret the position of the Late Cretaceous 110°C isotherm at a modern elevation 2600 m. In contrast to the Pikes Peak results, existing ZHe and AFT dates from an elevation profile on Longs Peak, a “fourteener” in the northern Front Range are uniformly Laramide (76 ± 21 Ma to 43 ± 5 Ma) in age, implying a higher Late Cretaceous geothermal gradient in the northern Front Range than the southern Front Range. The ZHe data are compatible with the hypothesis that north to south differences in the thickness of the Pierre Shale prior to denudation caused the spatial variability of the geothermal gradient. Raman data indicate that partial annealing of radiation damage did not strongly affect the zircon suite from Pikes Peak. Alpha dose estimates obtained from Raman spectroscopy data are in broad agreement with those suggested the by ZHe results. Further integration of Raman spectroscopy and ZHe data in the future will improve understanding of how damage accumulation, damage annealing, and parent isotope zonation influence complex ZHe data sets.
iii ACKNOWLEDGEMENTS
I would like first to thank my advisor Becky Flowers for her unflagging support and feedback throughout this process. Jim Metcalf has provided an incalculable amount of support and analytical help. Nigel Kelly provided an enormous amount of assistance in interpreting my
Raman Spectroscopy data and had many useful conversations with me throughout the process.
Eric Ellison assisted with the collection and interpretation of Raman data. Julien Allaz assisted with BSE and CL image collection. Finally, I would like to thank my friends and family for their helpful and stimulating conversations and unwavering support. This project was supported by the
Geological Society of America and CU-Boulder Department of Geological sciences.
iv
CONTENTS CHAPTER
I. INTRODUCTION ...... 1
II. BACKGROUND ...... 4
2.1. (U-Th)/He thermochronology ...... 4
2.2. Zircon characterization by BSE, CL, and Raman spectroscopy ...... 6
2.3. Interpreting alpha dose from raman spectroscopy data ...... 7
III. GEOLOGIC SETTING ...... 9
3.1 Regional Geology ...... 9
3.2. Previous Thermochronology in the Colorado Rockies ...... 14
IV. SAMPLES AND METHODS ...... 17
4.1 Samples ...... 17
4.2 Grain-Mount Construction ...... 17
4.3 BSE Imaging, CL Imaging, and Raman Spectroscopy ...... 18
4.4 (U-Th)/He Thermochronology ...... 20
V. RESULTS ...... 22
5.1 BSE and CL results ...... 22
5.2 Raman spectroscopy results ...... 22
5.3 ZHe thermochronology results ...... 32
5.4 Radiation damage parameters versus ZHe dates ...... 36
VI. DISCUSSION ...... 41
v 6.1. Implications of Front Range vertical profile “14’er” datasets ...... 41
6.1.1. Pikes Peak vertical profile ZHe data and comparison with AFT results ...... 41
6.1.2. Comparison of northern and southern Front Range vertical profile “14’er” datasets ...... 45
6.1.3. Thermal history forward modeling ...... 47
6.1.4. Geologic implications ...... 50
6.2 Raman spectroscopy and ZHe data integration and implications ...... 52
6.2.1. Evaluating zircon annealing from raman spectroscopy data ...... 53
6.2.2 Implications of alpha dose estimates and radiation damage accumulation time calculations 55
6.2.3. A suggested workflow to optimize integration of zircon Raman and He datasets ...... 59
CONCLUSIONS ...... 61
REFERENCES ...... 63
vi TABLES Table
1. Raman Data ……………………………………………………………………………24 2. ZHe Data ………………………………………………………………………………35
vii FIGURES
Figure
1. Regional Geologic Map ………………………………………………..……………..…10
2. BSE, CL images and Raman spectra.……………………………………………………23
3. All Raman Data………………………………………………………………………..…34
4. ZHe Data ……………………………………………………………………………...…36
5. Raman data for dated grains only………………………………………………………..38
6. Plots of ZHe data compared to Raman spectroscopy damage proxies…………………..39
7. Plots of eU data compared to Raman spectroscopy damage proxies…………….………40
8. ZHe elevation-date plots for Pikes Peak and Longs Peak with associated AFT data……42
9. Alpha dose estimation using the ZRDAAM……………………………………………..44
10. HeFTy simulation results for Pikes Peak and Longs Peak………………………………48
11. Structural explanation of ZHe data ……………………………………………………..52
12. Alpha dose estimation using Raman data………………………………………………..57
viii CHAPTER I
INTRODUCTION
Zircon (U-Th)/He thermochronology (ZHe) is a useful tool to constrain the unroofing of structural blocks because of zircon’s impressive durability and ubiquity throughout the crust
(Reiners, 2005; Guenthner et al., 2013; Reiners et al., 2015; Guenthner et al., 2016; Orme et al.,
2016). However, self-irradiation from the decay of U and Th in zircon can lead to crystallinity degradation and an amorphous, or, metamict state. Radiation damage is manifested in decreased crystallinity, decreased density, and the production of color (Ewing et al., 2003). Recent work has shown that high levels of radiation damage in zircon leads to reduced helium retentivity and thus a lowered ZHe temperature sensitivity (Guenthner et al., 2013). The zircon radiation damage and annealing model (ZRDAAM) represents a significant step forward in our understanding of ZHe thermochronology, because it raises the possibility of using a suite of zircons from a single sample with variable effective uranium (eU, [U]+0.235*[Th], Shuster et al.,
2006; Flowers et al., 2007) concentrations and thus variable helium retentivities to decipher a temperature history from 200°C to ≤50°C (Guenthner et al., 2013, 2014, 2016; Orme et al., 2016;
Johnson et al., 2017). This approach is used to elucidate protracted thermal histories (e.g.
Guenthner et al., 2014; Orme et al., 2016; Johnson et al., 2017), where date-eU trends in these data sets have been interpreted with the ZRDAAM (Guenthner et al., 2013) using the forward and inverse modeling capabilities of the HeFTy computer program (Ketcham, 2005). Complex
ZHe date-eU datasets highlight the potential to exploit accurate estimations of radiation damage dose as additional information for these thermal history interpretations (e.g. Johnson et al.,
2017). Raman spectroscopy is a reliable semi-quantitative measure of radiation damage dose in
1 zircon (Nasdala et al., 2001; Palenick et al. 2003; Nasdala et al., 2004; Marsellos and Garver,
2010; Pidgeon 2014). There is great potential to couple such data with ZHe dates to gain a better understanding of the effects of radiation damage on helium retentivity (e.g. Danišík et al., 2017).
Although recent work marks a significant step forward in accurate interpretation of ZHe data sets, uncertainty remains in the relationship between ZHe closure temperature and alpha dose at high damage levels (Guenthner et al., 2013) as well as in the kinetics of radiation damage annealing at high dose. For example, the annealing kinetics of zircons at differing levels of metamictization may be variable (Váczi and Nasdala, 2016; Geisler, 2002), such that point defects in grains that have undergone significant self-irradiation may anneal at much lower temperatures than in less damaged grains (e.g. Nasdala et al., 2001; Váczi and Nasdala, 2016).
This is significant as the ZRDAAM (Guenthner et al., 2013) utilizes zircon fission track annealing kinetics of (after Yamada et al., 2007) that places the partial annealing zone of zircon at 310 – 225°C. The uncertainty in radiation damage annealing and accumulation makes it unclear what the effect of long retention times at moderate temperatures (≤250°C) has on ZHe data sets.
Here I present a ZHe and Raman spectroscopy dataset for a suite of Proterozoic basement samples collected from an elevation profile on the Pikes Peak ‘fourteener’ in the Colorado Front
Range. Pikes Peak is an advantageous location because it has a protracted time-temperature history constrained through sedimentological and thermochronological data (i.e. 40Ar/39Ar data and AFT vertical transect data), it exposes a >2000 m transect of zircon-rich Proterozoic basement, and interesting geologic questions remain regarding the region’s uplift history. My goals in this study are twofold. First, I aim to use the variable He retentivity of zircons from a single sample to better constrain the Neoproterozoic – Cenozoic evolution of Pikes Peak. These
2 results are compared with a ZHe vertical transect on Longs Peak in the northern Front Range
(Johnson, 2015) as well as with published vertical transects of apatite fission-track data (Kelley and Chapin, 2004) from both Pikes Peak and Longs Peak. These comparisons will allow me to better describe north-to-south variability in the thermal history of the Front Range. Second, I will consider how zircon Raman spectroscopy data can inform ZHe data interpretations and improve understanding of how radiation damage influences the kinetics of ZHe diffusion and annealing.
3 CHAPTER II
BACKGROUND
2.1. (U-Th)/He thermochronology
Uranium is readily incorporated at a trace element concentration into the zircon crystal lattice, replacing zirconium. 238U, 235U and 232Th decay to lead with the emission of 8, 7, and 6 alpha particles, respectively. At high temperatures, He readily diffuses out of the mineral. At lower temperatures helium is partially lost to the mineral’s surroundings, a temperature range referred to as the partial retention zone (PRZ). At still lower temperatures, He is fully retained in the crystal. The rate of helium diffusion is in part controlled by the crystal lattice structure, damage to the crystal lattice (e.g. radiation damage; Shuster et al., 2006; Flowers et al., 2009;
Guenthner et al., 2013; Baughman et al., 2017), grain size (e.g. Reiners and Farley, 1999; Farley,
2000) and U and Th distribution (e.g. Hourigan et al., 2005; Ault and Flowers, 2012). Initial work on zircon demonstrated a He closure temperature (Tc) of ~180°C (Reiners et al., 2002,
2005).
The emission of an alpha particle during radioactive decay and the recoil of the heavy nucleus create structural damage to the crystal lattice. Heavy nucleus recoil causes the majority of the damage (e.g. Ewing et al., 2003; Guenthner et al., 2013), and alpha particles cause long narrow zones of damage. For samples that have experienced the same thermal history, eU can be used as a proxy to accumulated damage. The accumulated radiation damage can be approximated through alpha dose, which incorporates the concentration of uranium and thorium as well as the time over which damage has been accumulating. Alpha dose is sensitive to both damage accumulation and annealing of radiation damage. Recent work has shown that the accumulation
4 of radiation damage acts as a primary control on the diffusivity of He in zircon and the effective closure temperature (e.g. Guenthner et al., 2013). Helium diffuses more rapidly along the c-axis of the zircon crystal lattice, and at low damage levels, those diffusion pathways become obstructed. A positive correlation between date and eU is observed at these lower damage levels.
However, at high damage levels diffusion fast-pathways are created through the interconnection of damage zones, referred to as the first percolation. The first percolation threshold is interpreted to occur at an alpha dose of ~150x1016 �/g (Geunthner et al., 2013). Above the first percolation threshold, the ZHe Tc is predicted to decrease to as low or lower than the closure temperatures of the AFT (Tc ~ 110 °C depending on chemistry, Carlson et al., 1999) and apatite (U-Th)/He
(AHe) systems (Tc ~ 70-110 °C depending on radiation damage, Farley, 2000; Shuster et al.,
2006; Flowers et al., 2009). Recent ZHe results from the northern Colorado Front Range include a subset of higher-damage zircons with He dates consistently younger than AHe and AFT dates for the same or nearby samples, and thus are consistent with this prediction (Johnson et al.,
2017). Published ZHe data sets commonly display dominantly negative date-eU correlations, suggesting that most grains in these data sets are above the first percolation threshold (e.g.
Guenthner et al., 2014; Orme et al., 2016, Johnson et al., 2017). The kinetics of annealing radiation damage in zircon continues to be actively researched (e.g.Váczi and Nasdala, 2016).
Annealing of damage at high temperatures has long been recognized (e.g. Nasdala et al., 2001;
Geisler, 2002; Reiners, 2005; Guenthner et al., 2013). Subsequent research has suggested that annealing occurs in stages (e.g. Geisler, 2002), and may be induced at low temperatures by an electron beam (Váczi and Nasdala, 2016).
Other considerations for interpreting (U-Th)/He dates include the alpha-ejection correction and U-Th zonation. During radioactive decay, the alpha particle is ejected <20�m, and
5 a correction for this He loss from the crystal must be made based on the grain geometry. Parent isotope zonation may also effect the date of a grain, owing to the subsequent variation of He present in the grain (e.g. Hourigan et al., 2005; Danišík et al., 2017). For example, zonation may also effect the relative levels of radiation damage present throughout the grain.
2.2. Zircon characterization by BSE, CL, and Raman spectroscopy
The elastic interaction of electrons with atoms of a sample surface produce back-scattered electrons (BSE). High atomic numbers have a higher probability of producing an elastic reaction, and therefore, the intensity that a BSE detector measures is therefore a function of atomic number. In BSE images, brighter zones have a higher density, and thus a higher inferred uranium and thorium content (e.g. Ono, 1976; Hanchar and Miller, 1993). Darker zones can therefore be interpreted to represent (U-Th) poor zones. Recent work has demonstrated that the zonation in
BSE images correlates well with He distribution in zircon (Danišík et al., 2017).
Cathodoluminescence (CL) imaging is a complementary tool to BSE and has been used since the
1960’s to constrain zonation in zircon (e.g. Ono, 1976, Hanchar and Miller, 1993, Harley and
Kelly, 2007). Luminescence is the result of a focused beam of electrons exciting electrons in the crystal lattice. Radiation damage caused by the decay of U, Th and Sm can suppress overall luminescence (e.g. Ono, 1976, Hanchar and Miller, 1993, Harley and Kelly, 2007).
Raman spectroscopy uses the inelastic scattering of light cause by molecular vibrations.
The �3[SiO4] “peak” is an internal vibration of the SiO4 units, and because it is the most intense peak, it is used to semi-quantitatively assess the breakdown of these SiO4 units due to radiation damage. An undamaged, laboratory produced zircon will have a characteristic �3[SiO4] peak position of ~1008 cm-1 with a full width half-maximum (FWHM) of <3 cm-1 (Nasdala et al.,
6 2001). However, as the crystal lattice becomes more disordered because of radiation damage, the peak progressively broadens up to 35.64 cm-1, and the peak position will shift to lower wave numbers as low as ~ 996cm-1 (Nasdala et al., 2001). These data allow for qualitative comparison of metamictization levels between samples. These data may also be used to illuminate grain zones where damage has begun to anneal, as denoted by a narrowing of the peak to lower
FWHM without a corresponding increase in raman shift (Nasdala et al., 2001; Geisler et al.,
2002; Váczi and Nasdala, 2016).
2.3. Interpreting alpha dose from raman spectroscopy data
Raman spectroscopy provides the opportunity to both qualitatively assess radiation damage, as was described above, and quantitatively assess radiation damage. Building on the work of Nasadal and others (e.g. Nasdala et al., 1995; Zhang et al., 2000; Nasdala et al., 2001)
An empirically derived equation relating full width half-maximum (FWHM), which describes
-1 � the width of the peak at half the height of the ~1008 cm raman peak and alpha dose ( ��) was presented in Palenik et al., (2003):
FWHM=A 1-e-BD [1] where: A= 35.64, the asymptotically approached FWHM, B= -5.49*10 (g-1), and D=damage
� dose ( ��). This equation presents a means to calculate the alpha dose of a sample based on the measured FWHM. The accumulation time is determined by the alpha dose through the equation:
D= (8*[U]*0.9928)(�( . ∗ �) − 1)+((7*[U]*0.0072)( � . ∗ � −
[2] 1))+((6*[Th]*) (� . ∗ � − 1))
7 where t is the accumulation time, and the concentration of uranium and thorium at the point of analysis are used (Nasdala et al., 2001; Palenik et al., 2003; Váczi and Nasdala, 2016). This approach has subsequently been built upon by Presser and Glotzback (2009). When only bulk uranium and thorium concentrations are available, the range of accumulation time for a given grain provides a qualitative estimate of the range over which the grain may has integrated damage into the crystal lattice, and provides a rough framework from which to interpret the time over which radiation damage has accumulated. Early work suggested that the accumulation time should be nearly equivalent to a zircon fission track (ZFT) date (e.g. Nasdala et al., 2004). In a fashion similar to (U/Th)-He dating, this method of dating integrates the time spent in and above the partial annealing zone.
8 CHAPTER III
GEOLOGIC SETTING
3.1 Regional Geology
The Rocky Mountains are a set of ranges that span >5000 km from New Mexico through
Canada. The Southern Rockies are located farther east than the rest of the mountain chain, and stretch from New Mexico to southern Wyoming. The Southern Rockies separate the Colorado
Plateau in the west from the High Plains in the east and are bisected by the Rio Grande Rift. The rift is a >1000 km zone of extension that extends from New Mexico through central Colorado.
The highest elevations in the Southern Rockies, at > 4000 m, are found in Colorado. The Front
Range is the largest and easternmost uplift in Colorado and exposes primarily Proterozoic basement (Fig. 1). The basement is composed of metamorphic and igneous rocks that were generated during three distinct Proterozoic intervals of deformation and magmatism at 1.78-1.7
Ga, approximately 1.4 Ga and 1.1 Ga (e.g. Whitmeyer and Karlstrom, 2007).
The Yavapai Province is a >1300 km wide province that was accreted onto the southern margin of the Wyoming and Superior cratons during an orogeny that lasted from approximately
1.78 Ga – 1.70 Ga (e.g. Bowring and Karlstrom, 1990; Hill and Bickford, 2001). The margin between the Wyoming craton and the Yavapai province occurs at the northeast-southwest trending Cheyenne belt. The Cheyenne belt is denoted by steeply southward dipping shear zones
(e.g., Hill and Houston; 1979, Reed et al., 1987; Bowring and Karlstrom, 1990), and has classically been described as the convergent margin between magmatic arcs and the North
American craton (e.g., Reed et al., 1987). The Yavapai terrane is composed primarily of amphibolite facies rocks of igneous and sedimentary origin as well as locally common granitoid
9 Figure 1. A. Regional Map (adapted from Kelley and Chapin, 2004). B. Simplified geologic map of the Colorado Front Range. Locations of elevation transects are shown (green stars). C. Elevation transect on Longs Peak along the east Longs Peak trail for samples collected by Kelley and Chapin (2004). D. Elevation transect on Pikes Peak along the Barr Trail. Sample elevations are noted in C. and D.
plutons (e.g., Hill and Bickford, 2001; Sims and Stein, 2003). The area remained relatively quiet for the following 300 million years as additional crust was accreted farther to the south and west
(Whitmeyer and Karlstrom, 2007).
Renewed tectonism occurred 1.44 - 1.36 Ga, resulting in the emplacement of A-type granites and was synchronous with the development of high-strain zones (e.g. Anderson and
Cullers, 1999; Sims and Stein, 2003). Shear zones, such as the Idaho Springs – Ralston Shear zone, cross cut both the 1.4 and the 1.7 Ga crust (e.g. Sims and Stein, 2003). Plutons, including the Silver Plume Granite of Longs Peak (e.g. Peterman et al., 1968; Anderson and Cullers, 1999;
Cole and Braddock, 2009), locally intrude through 1.7 Ga crust (Tweto, 1987; Cole and
10 Braddock, 2009). The Silver Plume Granite is a peraluminous, two mica granite associated with intra-continental plutonism (e.g., Anderson and Cullers, 1999). Some authors contend that the region was likely a transtensional environment during this interval, as inferred from study of the
Idaho Springs-Ralston Shear Zone (e.g. Sims and Stein, 2003). Locally, the magmatism reset the
K-Ar isotopic system in the 1.7 Ga host rock, as demonstrated by 40Ar/39Ar dates on muscovite and biotite that record cooling through 350-300 °C after ~1.4 Ga (Shaw et al., 1999; Shaw et al.,
2005). Mica from the Silver-Plume granite on Longs Peak record cooling through ~300°C ca.
1200 Ma (Shaw et al., 2005).
Protracted tectonism during the Grenville orogeny is interpreted to be associated with the assembly of Rodinia (Whitmeyer and Karlstrom, 2007). The expression of the Grenville orogeny is limited in Colorado to the emplacement of the Pikes Peak composite batholith, which both emplaced juvenile material and reworked existing crust (e.g. Smith et al., 1999; Guitreau et al.,
2016). The Pikes Peak batholith is composed of three granitoids: 1) coarse grained biotite- amphibole syenogranite which is dominantly exposed lithology in the batholith, 2) monzogranite, and 3) quartz syenite. The monzogranite and quartz syenite are fine grained intrusions within the greater batholith (Smith et al., 1999; Guitreau et al., 2016). The primary body of the Pikes Peak Batholith (PPB) has low MgO and CaO abundances and high
Fe/(Fe+Mg) contents, consistent with A-type granites (Smith et al., 1999; Frost et al., 2001). The monzogranite belongs to a potassic series which has a mantle and crust magma mingling geochemical signature, while the quartz syenite belongs to a sodic series (Guitreau et al., 2016).
Because of its greater resistance to weathering, the summit of Pikes Peak is composed of the monzogranite. The syenogranite that dominates the pluton has a U/Pb LA-ICP-MS date of
1097.8 ± 6.9 Ma (Howard et al., 2015). The monzongranite bodies date to 1088.2 ± 7.6 Ma
11 (Howard et al., 2015) and the quartz syenites have a U/Pb LA-ICP-MS date of 1115 ± 12 Ma
(Guitreau et al., 2016).
There is little record of perturbation to the crust for much of the Neoproterozoic and early
Paleozoic. Sandstone injectites that occur in the PPB, adjacent to the Ute Pass Fault, contain detrital zircons that were dated at 1.7 Ga, 1.4 Ga, and 1.3-0.9 Ga that most closely match the signature of (and therefore suggest a provenance from) Neoproterozoic sandstones associated with the “great Grenvillian sedimentation episode” (Siddoway and Geherels, 2014). Kimberlite emplacement occurred in Northern Colorado during the Neoproterozoic and Devonian (Lester et al., 2001). Kimberlites extend in a >200 km long belt from the State Line province proximal to the Cheyenne belt, southward to the Green Mountain kimberlite near Boulder, Colorado (Lester et al., 2001). The kimberlites, which entrained fine grained, mafic two pyroxene garnet–bearing granulites of the lower crust, intruded through Proterozoic crust (e.g. McCallum, 2005). In the
Wet Mountains, the southern extension of the Front Range, the McClure complex was emplaced
523.98 ± 0.12 Ma (Schoene and Bowring, 2006). The complex is an ultramafic to low silica alkalic igneous intrusion.
The Ancestral Rockies were a set of ranges in Colorado and the surrounding region, formed during Pennsylvanian – Early Permian time. The extent and driving mechanisms of these uplifts are still debated. The Ancestral Rockies of Colorado consisted of two primary northwest- southeast trending uplifts, the Uncompaghre and Frontrangia uplifts (e.g. Kluth and Coney,
1981). The coarse arkosic sediments shed off the Ancestral Rockies were deposited in the deep flexural Denver Basin to the east, as well as in other basins developed at that time (e.g. Cullers and Stone, 1991; Weimer, 1996; Sweet and Soreghan 2010). By the early Permian, the Ancestral
Rockies had been beveled and the Western Interior Seaway (WIS) inundated Eastern Colorado.
12 The transgression of the WIS deposited nearly three quarters of the sediment that remain in the
Denver Basin today (Weimer, 1996). The seaway persisted through the Late Cretaceous until the onset of the Laramide Orogeny approximately 70 Ma. The regression of the seaway and uplift of the modern Rockies in Colorado is denoted in the sedimentary record by transition from the shallow marine Pierre Shale to the Fox Hills Sandstone, and into the conglomeratic Arapahoe
Formation (Weimer, 1996). A similar transition from shallow marine to terrestrial facies have been noted in other local basins adjacent to Laramide uplifts (e.g. Dickinson et al., 1988).
Laramide deformation was accommodated by a series of high angle, basement-cored faults such as the Golden Fault, accompanied by deformation and limited volcanism (Kellogg et al., 2004; Weimer, 1996). Igneous and metamorphic clasts first appear in the Arapahoe
Formation in the Denver Basin, suggesting that by 66 – 64 Ma, basement was exposed at the surface (Weimer, 1996). Deformation persisted through approximately 35 Ma (e.g. Yonkee and
Weil, 2016; Bird, 1988; Dickinson and Snyder, 1979). The development of the Colorado Mineral
Belt, a northeast trending series of epithermal polymetallic mineral deposits, occurred concurrently over this time interval (e.g. Jones et al., 2011; Caine et al., 2010).
At the end of the Laramide Orogeny an Eocene erosional surface developed, creating a piedmont on the eastern flank of the Rocky Mountains (e.g., Epis and Chapin, 1975; Eaton,
1986; Weimer, 1996). Sporadic volcanism continued in parts of the Front Range, most notably the ~30-28 Ma Braddock Peak intrusive-volcanic complex exposed in the Never Summer
Mountains of the northern Front Range (Jacob et al., 2015; Epis and Chapin, 1975). Steep canyons with high relief developed during renewed incision of the Rockies during the Miocene-
Pliocene, which resulted in the deposition of the Ogallala Formation on the High Plains (e.g.
Eaton, 1986, Steven et al., 1997, McMillan et al., 2006). What drove this episode of erosion
13 remains controversial; some authors contend that tectonism is required to explain paleoaltimetry and paleobotanical data (e.g. Eaton, 1986, Steven et al., 1997, McMillan et al., 2006, Eaton,
2008). Others, however, argue that there has been little uplift since the Eocene, and that paleobotanical observations and paleoelevevation estimates can be explained by climate change alone (e.g. Molnar and England, 1990). Pleistocene glaciation dramatically altered the highest portions of the Front Range during a series of glacial cycles (Langston et al., 2015; Chadwick et al., 1997).
3.2. Previous Thermochronology in the Colorado Rockies
Previous thermochronology in the Colorado Rockies constrains the thermal history of the region over the last billion years. 40Ar/39Ar dating of hornblende, muscovite, biotite and microcline in ~1.7 Ga units has been utilized to determine the high temperature cooling history of the Mt. Evans Batholith (Shaw et al., 1999; 2005). Hornblende, with an 40Ar/39Ar closure temperature of 500°C - 550°C, yields dates between 1600-1340 Ma. These results have been used to infer partial to no argon loss during 1.4 Ga tectonism, owing to a short lived thermal pulse (<20Ma) that reached peak temperatures of 525°C- 600°C (Shaw et al., 2005). Muscovite and biotite, with a 40Ar/39Ar closure temperatures in the range of 300°C - 350°C, yield dates between 1400-1340 Ma. These data indicate that post-1.4 Ga events were insufficient to cause complete resetting of the mica Ar systematics (Shaw et al., 1999; Shaw et al., 2005). Microcline
40Ar/39Ar thermochronology, which has a closure temperature of approximately 200°C, documented slow cooling between 1300 Ma and 800 Ma following the plutonism and metamorphism associated with the ca. 1.4 Ga tectonism in the region (Shaw et al., 2005). In the
Pikes Peak Batholith, hornblende 40Ar/39Ar dates range between 1.08 and 1.06 Ga. Potassium
14 feldspar 40Ar/39Ar dates indicate the batholith cooled to below 200°C by 1000 Ma (Unruh et al.,
1995). These data demonstrate that the Pikes Peak batholith cooled relatively rapidly after its emplacement, and that it has not been sufficiently reheated to reset 40Ar/39Ar systematics in the last billion years.
An extensive AFT dataset is available for the Front Range, which includes results for elevation transects on Pikes Peak (4297 m) and Longs Peak (4343 m) (Kelley and Chapin, 2004;
Naeser et al., 2002; Bryant and Naeser, 1980). At temperatures >110°C, fission tracks in apatite of a typical chemistry are completely annealed. Conversely, fission tracks are completely retained at temperatures <60°C. Fission tracks are partially shortened and annealed while apatites reside between 60°C – 110°C, in the partial annealing zone (PAZ). The boundary of the late
Cretaceous PAZ present in Colorado is interpreted to represent the 110°C isotherm. Elevation transects presented in Kelley and Chapin (2004) define the location of the fossil apatite PAZ, which deepens from higher elevations in the north to lower elevations in the south (Kelley and
Chapin, 2004). AFT data from Long’s Peak, the northernmost fourteener, have a mean track length of 13.5-14.7 µm and cooling dates of 55 ± 4 Ma to 45 ± 9 Ma along the entire transect.
These data are indicative of rapid Laramide cooling, and demonstrate that the summit of Long’s
Peak was below the PAZ at the onset of the Laramide Orogeny (Kelley and Chapin, 2004). In contrast, the Late Cretaceous 110 °C isototherm is located at approximately 2600 m on Pike’s
Peak. Based on these, and other, observations, Kelley and Chapin (2004) document longwave tilting of the base of the PAZ in the Colorado Front Range. The sample from the summit of Pikes
Peak yields an AFT date of 449 ± 58 Ma, with a mean track length of 12.1 ± 2.1 µm (Naeser et al., 2002). Samples from below the fossil PAZ yield dates of 211.7 ± 21.3 - 44.5 ± 5.3 with track lengths of ~12 µm (Kelley and Chapin, 2004). The authors explain the north-south deepening of
15 the fossil AFT PAZ by plutonic activity that provided long wave structural tilting of the Front
Range such that the Cretaceous Pierre Shale thickened to the north, and provided thermal shielding from the highly insulating shale.
There is relatively little published (U-Th)/He thermochronology for the Colorado Rockies except for investigations in the Gore Range (Landman and Flowers, 2013) and the northern Front
Range (Johnson et al., 2017). The former presents an apatite (U-Th)/He (AHe) dataset that records middle to late Cenozoic exhumation, in contrast to most AFT data from the larger region that demonstrate solely Laramide cooling (Landman and Flowers, 2013). The authors associate the post-Laramide cooling phases with the opening of the Rio Grande Rift. The second study is a
(U-Th)/He investigation in Big Thompson Canyon (BTC) of the northern Front Range (Johnson et al., 2017). ZHe dates from the BTC record a range of dates spanning from the Neoproterozoic at low radiation damage levels to Laramide and more recent ages at high damage levels. This study demonstrated high alpha dose zircons can yield reproducible ZHe dates, with a temperature sensitivity of <70°C. AHe dates from BTC record Late Cretaceous to Eocene cooling, while titanite (U-Th)/He dates are Neoproterozoic (Johnson et al., 2017).
16 CHAPTER IV
SAMPLES AND METHODS
4.1 Samples
13 samples were collected spaced every ~200 vertical m from Pikes Peak along a 2213 m elevation transect from 2084 m to 4297 m (Fig. 1). Samples were collected along the Barr Trail on the eastern face of the peak and were selected to represent the primary Pikes Peak batholith as well as the mozogranite intrusions. The calcic quartz syenite intrusions were excluded because they are only exposed sporadically throughout the batholith. Zircons from six samples were separated and analyzed. Grains were isolated using standard mineral separation techniques. Of the six samples which were processed, a subset of 3 samples were chosen for more detailed characterization of zircon. Individual grains were handpicked using a Leica M165 binocular microscope capable of both reflected and transmitted polarized light and equipped with a calibrated digital camera. Mounted grains were measured both before they were mounted and after they were plucked.
4.2 Grain-Mount Construction
Three samples, from elevations of 2084 m, 2907m and 3971 m, were chosen for more detailed characterization based on their elevation and spread of damage levels, which was assessed visually by the crystal clarity. Initially, 30 grains from each of these three samples were included in the grain mounts. Subsequently, 10 grains from the 2084 m elevation sample were included to better capture the high damage end of the radiation damage spectrum. To choose grains that represent the full spectrum of radiation damage, grains were picked on the basis of
17 color such that there were even contributions from populations that ranged from clear to opaque.
Grains were also selected on the basis of size; as much as possible grains of a similar size were selected so that grains were all polished to a similar depth to better accommodate alpha ejection corrections for zircons later plucked and dated by ZHe. To further control for size and subsequent polishing needs, two mounts were used to separate large and small grains. The selected grains were placed on double-sided packaging tape on glass. Epoxy was poured on top of the grains in 1-inch ring mold. The mount was polished such that grains were exposed at their approximate half-width.
4.3 BSE Imaging, CL Imaging, and Raman Spectroscopy
Back-scatter electron images were collected to characterize zircon zonation and to inform the later collection of Raman spectra. Initial BSE images were collected on a Jeol JXA 8600 electron microprobe in the CU Boulder LEGS lab. Only the 10 zircons from sample 2084 that were included in the second round mount were characterized by both BSE and greyscale cathodoluminescence (CL) using a Jeol 8230 Superprobe in the CU Boulder LEGS lab. CL images display zoning patterns more clearly than BSE images in the grains. All captured images were processed using Photoshop to better illuminate zoning.
Raman spectra were collected using a Horiba LabRAM HR Evolution confocal dispersive Raman spectrometer with a 532nm frequency–doubled Nd:YAG laser (Laser
Quantum, Torus 532 + mpc3000) and coupled to an Olympus BXFM optical microscope with a
100 x 0.90 NA dry objective lens. Spectra were collected and processed with LabSpec version
6.3 software (Horiba Scientific). A1024 x 256 pixel thermoelectrically-cooled CCD detector, with a 50 µm confocal pinhole and 1800 lines/mm diffraction grating were used resulting in ~1
18 µm lateral spatial resolution and a spectral resolution of 1 cm-1 as measured by the full width at half maximum of the 585 nm neon emission line. Raman shift calibration was performed daily prior to analysis using the 520.7 cm-1 Raman peak of silicon. We collected spectra from 200 –
-1 -1 1100 cm in two pieces to capture the 1008 cm �3[SiO4] peak. The laser was set to 25% power and each half of the spectra was scanned three times for three seconds each. Samples which demonstrated high levels of complexity or did not emit a strong enough spectra were analyzed 5 times for 6 seconds each. Each grain was sampled in 3 – 6 spots, depending on the complexity of zoning in the grain. Due to complexity revealed during initial rounds, grains that were characterized in the second round were sampled with >10 spots. Spectra were corrected for instrument bias and baseline corrected using a fourth degree polynomial function. Peaks were fit to the spectra using a Gaussian-Lorentzian fitting function. Any data that exceeded the theoretical FHWM limit of 36 cm-1 were removed (Palenick et al., 2003). Similarly, spots with a
FWHM less than 3 cm-1 were removed because they are below the theoretical limit of the
�3[SiO4] peak (Nasdala et al., 2003). Finally, peak curve fits were rated at poor, good or excellent visually on the basis of curve height and width. Curve fits where there was no visual gap between the spectra and the fitted curve were rated as excellent, if the fitted curve accurately captured the peak height and peak width at approximately half the height of the peak but misestimated the curve shape it was rated as good, and if the fitted curve misestimated the peak height by greater than approximately 5% or the peak width was misestimated by greater than approximately 1 cm-1 on either side of the curve, it was rated as poor. All fitted curves that were rated as poor were removed from the data sets.
19 4.4 (U-Th)/He Thermochronology
After picking, grains were then placed into Nb tubes that were crimped on both ends.
Zircon from the Fish Canyon Tuff standard were also packed at this time to be analyzed with the samples. Packets were then loaded into an ASI Alphachron He extraction and measurement line.
Radiogenic 4He was extracted from the selected grains under ultra-high vacuum (~3 X 10-8 torr), and heated with a diode laser to ~800-1100 °C for 5 to 10 minutes. The degassed 4He was then spiked with approximately 13 ncc of pure 3He, cleaned via interaction with two SAES getters, and analyzed on a Balzers PrismaPlus QME 220 quadrupole mass spectrometer. Zircon grains that did not fully degas during the initial laser heating were subjected to re-extraction steps
(15A/10 min) until the resultant 4He/3He ratios were at blank levels. Once degassed, grains were then taken to a class 10 clean lab and were dissolved in Parr Large Capacity dissolution vessels.
The packets were placed in Ludwig-style Savillex vials, spiked with a 235U/230Th mixture, and mixed with 200 ml of Optima grade HF. The vials were then capped, stacked in a 125 mL Teflon liner, placed in the dissolution vessel, and baked at 220 °C for 72 hours. After cooling, the vials were uncapped and placed on a 90 °C hot plate until dry. The vials then underwent a second round of acid-vapor dissolution, with 200 ml of Optima grade HCl in each vial, and baked at 200
°C for 24 hours. Vials were completely dried down a second time. Once dry, 200 ml of a 7:1
HNO3:HF mixture was added to each vial. They were then capped and dried on the hot plate at
90 °C for 4 hours. Before taking samples to the ICP-MS at the Institute of Arctic and Alpine
Research, samples were spiked with 1 – 3 ml of doubly - deionized water; these solutions were analyzed for U, Th, Sm, using a Thermo Element 2 magnetic sector mass spectrometer. Alpha ejection corrections were made using measured grain dimensions according to Farley (2002).
Grains that were polished were corrected assuming that they were polished half way through.
20 The FT correction uses measurements made after the grain was picked from the mount. Volume and surface area equations assume that the 'trunk' of the zircon is 75% of the height, and each tip is 12.5% of the height. Measured grain dimensions were also used to calculate dimensionless masses for U and Th concentrations.
21 CHAPTER V
V. RESULTS
5.1 BSE and CL results
Zircon BSE images reveal moderate to low levels of zoning in all 102 zircons imaged from the three samples (Fig. 2). Zoning in BSE images was most commonly visible in zircon with grains that were associated Raman spectra that indicated more radiation damage (Fig. 2).
Irregular zoning patterns were more common than oscillatory patterns, though both were present.
Subsequent CL imaging of 5 grains from sample 2084 revealed either oscillatory or irregular zoning in all grains, including those that did not show zoning in BSE images.
5.2 Raman spectroscopy results
326 Raman analyses were acquired from 102 zircons from 3 samples. All Raman data are presented in Table 1. Spectra rated as “poor” (N=82) according to the criteria described previously were removed from the data set. For the 137 Raman spot analyses for zircons from sample 2084 m, 13 were rated “poor”, 59 as “good”, and 65 as “excellent.” Of the 126 analyses for sample 2907 zircons, a “poor” rating was given to 43 spots, “good” to 69 spots, and
“excellent” to 13 spots. Out of 64 data points for sample 3971 m, 22 were rated as “poor”, 12 as
“good” and 28 as “excellent.” Thus, sample 2084 had the lowest percentage of points rejected
(~9%), while samples 2907 and 3971 each had higher proportions of rejected analyses (~34% each).
22 Figure 2. A. Theoretical prediction of how Raman spectra change with increasing damage from Nasdala et al. (2001). The ~1008 cm-1 peak broadens and shifts to the left with increasing damage. B. Variation in radiation damage within a single sample as seen by cross polarized light, BSE, CL and Raman spectroscopy. C. Variation in radiation damage within a single grain.
23 Table 1. Raman Data
Radiation Calculated Full width Peak damage α-fluence Grain Number Spot at half max Rating Position date (when (when (FWHM) available) available)
PP_2084 /cm /cm (Ma) [10 16 /mg]
zr10 1 4.3 1006.0 excellent zr10 3 6.7 1005.0 excellent
zr11 1 9.5 1003.5 good zr11 2 5.8 1005.3 excellent zr11 3 6.1 1005.8 excellent
zr12 1 6.2 1004.6 excellent zr12 2 8.1 1003.9 excellent zr12 3 5.4 1005.5 excellent
zr13 1 17.7 998.6 good zr13 2 4.9 1006.6 excellent zr13 3 12.5 1001.5 good
zr14 1 4.6 1007.0 good zr14 2 8.3 1003.3 good zr14 3 6.4 1005.6 good
zr15 1 9.2 1002.9 good
zr16 1 9.6 1003.8 good 3 12.0 1002.2 good
zr17 1 4.3 1006.5 excellent 2 7.1 1005.5 good 3 4.8 1006.1 good
zr19 1 11.6 1002.89 good 3 9.0 1003.46 good
zr01 1 6.6 1005.6 excellent 2 6.6 1005.8 excellent 3 15.5 1000.8 excellent
zr20 1 12.7 1001.3 excellent PP_2907 2 24.3 998.9 excellent 3 8.8 1003.5 excellent
zr22 1 4.3 1006.9 excellent 2 4.5 1006.7 excellent *Grains have accompanying ZHe data available
24 *Grains have accompanying ZHe data available
Table 1 (cont). Raman Data Full width at Radiation Calculated α- Peak Grain Number spot half max rating damage fluence Position (FWHM) date (when (when PP 2084 /cm /cm (Ma) [10 16 /mg] zr30 1 5.7 1005.6 excellent 2 4.9 1006.4 excellent 3 4.9 1006.0 excellent
zr31* 1 8.3 1004.1 good 1045 48.1 2 5.5 1006.1 excellent 695 30.9 3 5.6 1005.7 excellent 698 31.0 1 8.1 1003.1 good 1030 47.3 2 6.9 1005.2 good 865 39.1 3 5.7 1006.2 good 715 31.8 4 5.1 1006.3 good 634 28.0 5 5.7 1006.3 good 716 31.9 6 6.6 1004.9 good 830 37.4 7 13.4 1002.5 good 1738 86.5 8 7.1 1004.6 good 890 40.3 9 6.6 1003.8 good 830 37.4
zr4 3 17.1 1002.3 good
zr6 1 4.2 1006.8 excellent 2 15.4 1001.0 excellent 3 4.2 1006.6 excellent
zr7 1 13.3 1001.1 good 2 6.1 1005.9 excellent 3 7.8 1003.8 excellent
zr8 1 4.7 1006.6 excellent 2 4.7 1006.5 excellent 3 8.6 1003.9 excellent
zr32* 1 4.2 1006.5 excellent 443 22.7 2 4.0 1006.9 excellent 397 20.3 3 4.1 1006.3 excellent 440 22.5 4 4.3 1006.5 excellent 458 23.5 5 5.6 1006.5 excellent 596 31.0 6 4.1 1007.1 excellent 440 22.5 7 4.1 1007.2 excellent 435 22.3 8 4.9 1006.5 excellent 516 26.6 9 9.9 1004.9 excellent 1087 59.5 10 8.8 1005.4 excellent 961 51.9
zr33* 1 23.5 998.3 good 845 195.5 2 23.2 999.0 good 833 192.5 3 4.9 1004.4 excellent 124 26.8 *Grains have accompanying ZHe data available
25 *Grains have accompanying ZHe data available Table 1 (cont). Raman Data Radiation Calculated Full width Peak damage α-fluence Grain Number spot at half max rating Position date (when (when (FWHM) available) available) PP 2084 /cm /cm (Ma) [10 16 /mg] zr33* 4 7.7 1003.1 good 203 44.2 5 12.6 1001.2 good 360 79.6 6 20.6 1000.7 good 690 157.3 7 16.8 997.6 good 520 116.6
zr34 1 5.3 1005.6 excellent 2 4.6 1006.2 excellent 3 4.1 1006.4 excellent 4 4.2 1006.5 excellent 5 4.7 1006.2 excellent 6 6.3 1005.5 excellent 7 5.9 1005.7 excellent 8 5.6 1005.9 excellent 9 9.3 1003.3 excellent 10 15.2 1000.2 excellent 11 17.1 999.2 good
zr35 1 9.2 1004.2 good 2 7.9 1003.7 good 3 7.1 1004.3 good 5 8.5 1003.7 good 6 11.8 1002.0 good 7 13.1 1000.5 good 8 18.5 998.1 good 10 16.4 999.3 good
zr36* 1 17.8 999.9 excellent 294 126.4 2 11.8 1001.2 excellent 172 73.1 3 5.8 1005.1 excellent 77 32.5 4 5.7 1005.5 excellent 75.5 31.8 5 5.0 1006.7 excellent 65.2 27.5 6 5.8 1005.9 excellent 76.6 32.3 7 13.6 1001.3 excellent 206 87.9 8 5.1 1006.0 excellent 67 28.2 9 6.8 1006.6 excellent 91 38.4 10 10.8 1003.0 excellent 155 65.8 11 11.9 1003.0 excellent 175 74.4
zr09 1 10.4 1002.9 excellent 2 5.8 1006.0 excellent 3 5.6 1005.6 excellent
zr18 1 12.6 1001.5 excellent 2 6.0 1005.7 excellent 3 7.5 1005.1 excellent *Grains have accompanying ZHe data available
26 Table 1. Raman Data Table 1 (cont). Raman Data
Calculated α- Full width Radiation Calculated α- fluence Grain at half Peak damage date Spot Rating fluence (when (when Number max Position (when available) available) (FWHM) available)
[10 16 /mg] PP 2084 /cm /cm (Ma) [10 16 /mg]
zr23* 2 9.7 1004.5 good 1380 57.6 3 6.5 1005.7 good 915 36.4 1 11.8 1002.2 good 1690 73.1 2 10.3 1003.3 good 1480 62.5 3 5.7 1005.8 good 800 31.5 4 6.9 1005.6 good 980 39.3 5 5.8 1005.6 good 820 32.3 6 7.2 1005.3 good 1019 41.0 7 4.6 1006.0 good 650 25.2 8 5.7 1005.6 good 805 31.7 9 6.5 1005.5 good 925 36.9 10 6.0 1006.0 good 850 33.6
zr24 1 5.9 1006.0 excellent 2 4.2 1006.8 excellent 3 5.1 1006.2 excellent PP_3971 zr 25 1 4.3 1006.41 excellent 2 4.0 1006.92 excellent
zr26 1 4.6 1006.5 excellent 2 4.2 1006.7 excellent 3 6.0 1005.3 excellent
zr28 1 5.5 1006.3 good 2 8.1 1006.5 good 3 8.9 1006.3 good
zr29 1 4.0 1006.5 good 2 5.6 1005.7 good 3 12.6 1000.9 good
zr02 1 9.5 1005.51 good 2 9.2 1005.18 good
PP_2907 zr 11 2 8.2 1004.5 good 3 16.4 1003.1 good 4 22.0 1002.7 good
zr 13 1 14.9 1000.4 good
*Grains have accompanying ZHe data available *Grains have accompanying ZHe data available
27 *Grains have accompanying ZHe data available *Grains have accompanying ZHe data available
Table 1 (cont). Raman Data Table 1 (cont). Raman Data Calculated α- Full width Radiation Calculated α- Grain Peak fluence spot at half rating damage date fluence (when Number Position (when max (when available) [10 16 /mg] PP 2907 /cm /cm (Ma) [10 16 /mg] zr 14 2 21.5 999.3 good 4 22.7 1000.2 good 5 14.7 1000.4 good 6 33.0 1000.0 good 48.1 2 16.6 998.7 good 30.9 3 26.0 1000.2 good 31.0 4 14.6 1000.8 good 47.3 5 18.9 1000.5 good 39.1 6 20.7 998.4 good 31.8 7 27.5 998.1 good 28.0 31.9 zr 15 3 14.0 1001.8 good 37.4 86.5 zr 1 2 7.7 1004.5 excellent 40.3 37.4 zr 21* 2 8.7 1005.0 excellent 298 50.8 3 6.5 1005.4 excellent 215 36.4
zr23 1 19.9 1000.7 excellent 2 9.3 1002.1 3 13.0 1001.7
zr24 1 13.4 1004.8 good
3 20.9 1000.0 good
zr25 2 16.4 1002.5 good 3 12.4 1002.7 good 4 10.3 1003.3 good 1 20.5 999.6 good 2 12.9 1001.3 good 22.7 3 18.2 1000.2 good 20.3 4 10.9 1002.1 good 22.5 5 11.0 1001.9 good 23.5 6 10.1 1002.5 good 31.0 7 15.4 1000.8 good 22.5 8 9.4 1003.0 good 22.3 9 21.5 999.8 good 26.6 59.5 zr02 1 21.9 1000.0 good 51.9 2 19.2 999.9 good 3 14.7 1002.4 good 195.5 *Grains have accompanying ZHe data available
28 Table 1 (cont). Raman Data Table 1 (cont). Raman Data Calculated α- Full width Radiation calculated α- fluence Grain at half Peak damage date spot rating fluence (when (when Number maximum Position (when available) available) (FWHM) available) [10 16 /mg] PP 2907 /cm /cm (Ma) [10 16 /mg] 44.2 zr03 1 12.2 1001.9 excellent 79.6 2 5.6 1006.0 excellent 157.3 3 10.1 1003.2 excellent 116.6 zr04 1 9.9 1003.3 excellent 3 8.9 1004.1 excellent
PP_2907_5 1 21.1 1000.8 excellent 3 21.9 1000.4 excellent
zr17* 1 8.6 1005.2 good 325 49.9 2 6.3 1005.3 good 232 35.4 2 10.0 1004.2 good 390 60.3 3 6.1 1004.2 good 220 33.5 4 6.4 1004.9 good 232 35.4 5 6.5 1004.8 good 240 36.6 6 7.7 1003.9 good 290 44.4 7 7.9 1003.4 good 295 45.2 8 6.6 1004.7 good 241 36.8 9 8.5 1002.3 good 322 49.4 10 11.8 1000.5 good 475 73.9
zr18 1 5.7 1005.5 good
2 13.6 1005.4 good 3 14.5 1002.0 good 126.4 73.1 zr19 3 8.3 1005.9 good 32.5 31.8 zr20 1 5.7 1006.2 good 27.5 2 10.1 1003.6 good 32.3 3 7.9 1004.4 good 87.9 28.2 zr26 2 7.9 1005.9 good 38.4 3 15.8 979.9 65.8 74.4 zr27* 1 13.0 1002.6 good 645 82.1 2 9.5 1002.1 good 450 56.2 3 11.8 1003.4 good 576 72.8 2 6.9 1004.5 good 320 39.5 3 10.2 1002.7 good 489 61.3 4 11.7 1001.5 good 575 72.7 4 10.9 1002.0 good 530 66.7 6 16.8 1000.8 good 887 115.5 *Grains have accompanying ZHe data available 29 Table 1 (cont). Raman Data Table 1 (cont). Raman Data
Radiation Calculated α- Full width at Calculated α- Grain Peak damage date fluence (when Spot half max Rating fluence (when Number Position (when available) (FWHM) available) available)
[10 16 /mg] PP 2907 /cm /cm (Ma) [10 16 /mg] zr27* 7 8.3 1003.4 good 384 47.7 57.6 9 10.1 1001.8 good 485 60.8 36.4 10 11.0 1003.0 good 535 67.4 73.1 11 9.2 1001.8 good 438 54.7 62.5 31.5 39.3 zr28 1 13.4 999.7 good 32.3 2 10.3 1000.9 good 41.0 4 11.5 1002.5 good 25.2 31.7 zr29 1 11.6 1001.5 good 36.9 2 12.9 1001.6 good 33.6 3 30.9 999.0 good
zr08 1 9.4 1001.3 good 4 9.0 1002.8 good
PP_3971 zr 1 3 18.7 997.7 good
zr2* 1 9.2 1004.5 excellent 162 0.00 2 7.5 1005.1 excellent 128 0.00 3 7.3 1005.1 excellent 125 0.00
zr3 2 7.2 1004.1 excellent
zr5 1 16.8 998.7 excellent
zr6* 1 5.5 1005.5 excellent 103 29.9 2 5.9 1005.7 excellent 113 32.8
zr8* 1 3.8 1006.7 excellent 188 20.1 2 3.5 1006.9 excellent 174 18.5 3 5.6 1006.0 excellent 284 30.6
zr 9 3 10.8 1000.3 excellent
zr 10 2 7.2 1005.7 excellent 3 7.0 1005.7 excellent 4 5.7 1005.9 excellent
*Grains have accompanying ZHe data available
30 *Grains have accompanying ZHe data available
Table 1 (cont). Raman Data Table 1 (cont). Raman Data Calculated α- Full width at Radiation calculated α- Grain Peak fluence (when spot half maximum rating damage date fluence (when Number Position available) (FWHM) (when available) [10 16 /mg] /cm /cm (Ma) [10 16 /mg] zr 13 1 4.9 1003.8 excellent 2 4.7 1006.1 excellent 3 4.4 1006.1 excellent
zr14 1 5.9 1005.3 excellent 2 4.5 1006.9 excellent 3 7.4 1004.4 excellent
zr15* 1 4.3 1006.4 excellent 380 22.7 2 5.2 1005.2 excellent 470 28.3
zr16 2 7.2 1000.3 good
zr17 3 18.1 1001.7 good 1 9.3 1004.5 good 50.8 2 7.5 1005.1 excellent 36.4 3 7.3 1005.1 excellent
zr19 1 5.8 1005.5 good
zr20* 1 18.2 999.1 good 278 130.8 2 14.2 999.5 good 198 92.5 zr24 3 16.3 998.7 good
zr26 2 5.2 1006.6 excellent 3 7.1 1005.3 excellent
zr29 1 11.3 1001.3 good 2 8.1 1005.4 excellent 3 6.0 1003.6 excellent 4 11.3 1001.0 good
zr 30 1 6.4 1005.5 good
zr32 1 5.4 1005.6 good 2 5.4 1006.3 good 3 6.7 1005.0 good 4 7.8 1005.2 good *Grains have accompanying ZHe data available
31 Figure 3 is a plot of FWHM versus Raman Shift for the retained analyses for all 3 samples.
Previous work has demonstrated that Raman Shift is linearly and inversely related to FWHM when the zircon has not undergone partial annealing (Fig. 3; e.g. Nasdala et al., 2001). The majority of the Raman spectroscopy data (195/250 data points) fall within error (estimated at ±
1.5 cm-1 for FWHM) of the “Nasdala” line (Fig. 3A). Approximately 22/250 points (8.8%)
(samples 2084 m, N=3, Fig. 3B; 2907 m, N= 13, Fig. 3C; and 3971 m, N=6, Fig. 3D) fall below and to the left of the “Nasdala line” (Fig. 3a). Similarly, a suite of Raman data points fall to the right of the line, thus showing a higher degree of band broadening than is expected for a given frequency.
For zircons with greater than 10 spot analyses, the average and standard deviation of
FWHM and frequency were calculated. The standard deviation for FWHM ranged from 1.8 cm-1 to 6.5 cm-1. The standard deviation of Raman shift ranged from 0.9 cm-1 to 1.4 cm-1. This demonstrates that the variability of zoning is similar in samples 2907 m and 2084 m. There were no grains with 10 or more spot analyses from sample 3971 m. Each sample encompasses nearly the full range of raman shift and FWHM observed in the entire data set (Figs. 3 B – D). Single grains show considerable variation in raman shift and FWHM (Fig. 2 B, Figs. 3 B – D).
5.3 ZHe thermochronology results
We acquired single grain (U-Th)/He data for 32 zircons from 6 samples. Results are reported in Table 2. Uncertainties for (U-Th)/He dates are reported in the tables, text, and figures at 1σ. Reported uncertainty includes propagated analytical uncertainties for He, U, and Th measurements as well as for alpha ejection corrections. Dates range from 81.6 ± 5.9 Ma to 762.8
± 64.3 Ma across an eU span of 51 ppm to 1955 ppm (Fig. 4a). Results for all samples display similar strong negative date – eU correlations that asymptotically approach ~100 Ma. There are
32 no distinguishable differences in the date-eU patterns among the samples despite the 2213 m elevation range over which the samples were collected. This result is well-displayed in figure 4B by the similar date-eU patterns for samples 3971 and 2084 that are separated in elevation by
1887 m. The highest elevation sample (4297 m) also has results similar to the other samples, but there are no high eU (less than approximately 800 ppm) zircon dates to allow for comparison across the full eU range. Figure 4C similarly shows the absence of a correlation between date and elevation.
33
Figure 3. A. Full width at half-maximum (FWHM) versus Raman shift for all Raman data from the three analyzed samples. Inset: theoretical prediction that raman shift decreases and FWHM increases as radiation damage increases. In response to partial annealing FWHM is thought to decreases while raman shift does not. B-D. Plots of FWHM versus Raman Shift for samples 2084, 2907, and 3971 respectively. In each plot, each color represents a different grain.
34 Table 2. Zircon (U/Th)-He Data Dim Raw Analytic Samplea lb rb Ft U Th Sm eUd He Corr Date 1σe Mass Date Unc µg µm µm ppm ppm ppm ppm nmol/g (Ma) (Ma) (Ma) (Ma) PP_2084 z01 36.1 469.2 77.9 0.87 127.8 74.5 4.1 145.3 444.3 540.4 616.4 43.4 4.8 z02 36.2 537.1 67.1 0.87 253.0 124.0 0.8 282.1 872.2 546.1 624.7 45.3 8.7 z05 57.9 528.5 86.9 0.90 380.4 57.3 5.7 393.8 433.5 200.5 223.0 15.8 1.8 zr23* 17.9 306.0 62.5 0.89 102.2 43.5 1.3 112.41 353.8 555.3 622.1 46.3 5.0 zr31* 1.19 178.9 20.8 0.72 118.5 40.9 3.9 128.15 367.2 507.4 695.4 58.4 4.8 zr32* 32.2 430.5 70.0 0.90 144.0 31.8 4.1 151.47 237.6 283.6 312.9 23.3 4.1 zr33* 40.9 378.0 98.5 0.91 595.7 268.8 9.8 658.88 266.3 74.5 81.6 5.9 0.8 zr36* 24.9 569.6 55.3 0.87 1235.4 229.8 10.9 1289.4 1208.5 171.3 197.3 14.3 1.8
PP_2497 z01 7.6 296.2 41.3 0.79 106.0 87.7 2.3 126.6 401.1 559.6 699.3 50.2 4.3 z02 4.6 254.6 32.5 0.74 178.2 135.1 0.0 210.0 609.6 515.0 685.9 49.3 4.1 z03 1.3 167.5 23.0 0.63 410.2 318.3 0.0 485.0 824.8 307.2 477.7 34.1 2.9 z04 3.5 207.5 34.0 0.74 374.8 508.9 7.7 494.4 882.8 322.4 432.5 30.9 2.8 z05 3.6 250.5 30.75 0.72 388.9 404.4 21.8 483.9 1193.4 440.6 600.4 43.0 3.2
PP_2907 z01 14.4 376.7 50.1 0.83 284.2 170.8 12.4 324.4 875.6 479.9 574.9 41.2 4.0 z02 32.2 412.4 74.3 0.88 707.6 540.1 8.6 834.5 910.7 198.9 226.5 16.1 1.8 z03 12.3 314.8 52.0 0.83 1665.4 1231.4 15.8 1954.8 907.0 85.4 103.0 7.3 0.7 zr21* 2.3 192.7 28.9 0.67 376.7 577.9 12.5 512.5 555.6 197.7 253.8 60.5 1.9 zr17* 7.1 215.8 48.0 0.85 399.9 255.0 9.2 459.84 523.2 207.3 243.2 18.3 1.8 zr27* 4.0 225.6 34.8 0.81 325.4 190.0 16.6 370 593.8 290.2 354.3 27.6 3.6
PP_3597 z01 7.6 358.0 37.4 0.77 52.3 38.7 0.5 61.4 206.9 593.1 761.6 86.9 9.1 z02 16.6 369.2 54.8 0.84 480.2 245.4 5.0 537.9 856.6 288.0 341.1 24.4 2.9 z03 16.4 365.8 55.2 0.84 827.3 489.9 3.7 942.4 748.1 145.4 172.9 12.3 1.4
PP_3971 zr02* 21.4 342.4 63.5 0.89 911.9 437.1 6.1 1014.7 568.5 103.0 115.8 8.5 0.8 zr06* 28.7 263.8 83.9 0.90 804.6 343.0 26.1 885.22 590.9 122.5 135.7 10.0 1.7 zr08* 27.6 442.5 63.7 0.90 305.9 72.8 1.4 322.97 738.0 408.4 453.6 33.7 4.2 zr15* 5.2 202.2 41.7 0.81 161.3 69.5 0.0 177.62 598.4 592.3 719.8 64.3 6.7 zr20* 27.4 354.5 70.7 0.90 1206.0 886.4 52.3 1414.3 729.7 94.9 105.8 8.1 2.1
PP_4297 z01 45.3 491.6 80.1 0.89 165.8 85.9 3.1 186.0 601.9 570.4 639.3 45.7 6.2 z02 12.3 270.0 53.1 0.83 78.0 62.7 2.9 92.7 267.1 511.1 613.0 45.1 9.8 z03 3.6 221.2 33.2 0.73 630.8 731.0 3.7 802.5 1363.8 307.2 414.7 29.6 3.2 z04 4.0 221.0 39.0 0.73 439.9 270.3 0.0 503.4 744.6 268.1 361.7 24.6 2.7 z05 10.6 321.0 47.3 0.81 38.1 53.6 0.0 50.7 160.0 558.6 682.4 48.7 5.3 a z - zircon; a - apatite; t - titanite b l - length, r - radius cFt is alpha-ejection correction of Farley et al. (2002). d eU - effective uranium concentration, weights U and Th for their alpha productivity, computed as [U] + 0.235 * [Th] e1σ - includes propagated analytical uncertainty for He, U, and Th measurements, as well as an estimated uncertainty on the alpha-ejection correction *Indicates that grains were mounted, polished, and have accompanying Raman data. Grains that were polished were corrected assuming that they were polished 1/2 through. The FT correction uses measurements made after the grain was picked from the mount. Volume and surface area equations assume that the 'trunk' of the zircon is 75% of the height, and each tip is 12.5% of the height 35
Figure 4. ZHe date versus eU for A. all grains, and B. for samples with Raman data available. C. Sample elevation versus ZHe date for all samples. In all plots, data marked with an O indicate that there is Raman data to accompany ZHe data.
36 5.4 Radiation damage parameters versus ZHe dates
Raman spot analyses from single grains with ZHe dates and thus bulk U-Th concentration data (Figs. 5 A-C) span the full range of Raman shift and FWHM observed in the entire Raman spot data set (Fig. 3). Sample 2907 displays greater scatter about the Nasdala line
(Fig. 5B) than zircons from samples 2084 or 3971 (Fig 5A, 5C). The fewer number of analyses per zircon from sample 3971 may not adequately represent the full range of Raman shift and
FWHM in each grain.
Plots of ZHe date versus Raman shift (Figs. 6 A-C) and ZHe date vs. FWHM (Figs. 6 D-
F) also show the broad span of Raman shift and FWHM within single grains, especially for zircons from samples 2907 m and 2084 m. The zircons that yielded the youngest ZHe dates in samples 2084 m and 3971 m have lower mean Raman shift values (Fig. 6A, 6C) and higher mean FWHM (Fig. 6D, 6F) than zircons that yielded the older He dates in the same samples.
There is no correlation between mean Raman shift or mean FWHM and ZHe date for sample
2907, likely because this sample has a limited range of ZHe dates.
Figures 7 shows plots of eU versus Raman shift (Figs. 7 A-C) and eU versus FWHM
(Figs.6 D-F). Samples 2084 and 2907 show no clear correlation between eU and Raman shift or
FWHM. Sample 3971 shows a generally negative correlation between eU and Raman shift, and a positive correlation between eU and FWHM, although additional analyses per grain would make these relationships more robust.
37
Figure 5. FWHM versus Raman Shift for grains that were analyzed by Raman spectroscopy and have accompanying ZHe data for samples A) 2084, B) 2907, and C) 3971.
38 Figure 6. ZHe date versus Raman Shift for grains that were analyzed by Raman spectroscopy and have accompanying ZHe data samples A) 2084, B) 2907, and C) 3971. D-F) ZHe Date versus FWHM for same grains as in A-C.
39
Figure 7. eU versus Raman Shift for grains that were analyzed by Raman spectroscopy and have accompanying ZHe data samples A) 2084, B) 2907, and C) 3971. D-F) eU versus FWHM for same grains as in A-C.
40 CHAPTER VI
VI. DISCUSSION
6.1. Implications of Front Range vertical profile “14’er” datasets
6.1.1. Pikes Peak vertical profile ZHe data and comparison with AFT results
ZHe dates from the Pikes Peak sample suite show no correlation between ZHe data and elevation. Instead the negative date-eU correlations for each sample are consistent across over the nearly 2000 m elevation range that the samples were collected (Fig. 4). These data patterns have two general implications. First, the negative correlations between date and eU are consistent with the ZRDAAM, indicating that the higher eU and more radiation damage grains are less He retentive and therefore yield younger than the lower eU zircons. Second, the consistency of the date-eU patterns suggests that the entire elevation transect underwent a similar thermal history.
Comparison of these new ZHe results with the published vertical profile AFT data from
Pikes Peak (Kelley and Chapin, 2004) can yield additional insights into the ZHe data temperature sensitivity and the geologic significance of the date-eU patterns (Fig. 8a). Below
2680 m, AFT dates range from 44 .5 ± 4.3 Ma to 66.6 ± 3.9 Ma, with one anomalously old date at 171 Ma ± 9.8 Ma. Between 2680 and 2771 m, AFT dates range from 123.6 ± 11.2 Ma to 211.7
± 21.3 Ma (Kelley and Chapin, 2004). There are no AFT data between 2963 m and 4297 m because the Pikes Peak granite, which lacks apatite, is exposed here. At 4297 m, the summit, an
AFT date of 449 ± 58 Ma on an apatite-bearing late potassic Pikes Peak Batholith intrusion was reported by Naeser (2002). For apatites of a typical chemistry, the AFT partial annealing zone occurs in a temperature range of ~ 60°C to 110°C, and temperatures >110 °C are required to
41 completely anneal fission tracks and reset the apatites to zero age (e.g., Ketcham et al., 1999).
The apparent roll-over of AFT dates from Laramide dates below ~2600 m to older dates above
this A 4500 Pikes Peak Elevation vs. Date for ZHe and AFT data 2000
1800 4000
1600
3500 1400
1200
3000 1000 eU (ppm)
Elevation (m) 800 2500 600 AFT data
ZHe data 400 2000 ZHe data with accompanying 200 Raman data 1500 0 0 100 200 300 400 500 600 700 800 900 Corrected Date(Ma) B 4500 Longs Peak Elevation vs. Date for ZHe and AFT data 2000
1800 4000 1600
1400 3500
1200
3000 1000 eU (ppm)
800 Elevation (m) 2500 600
400 2000 AFT data 200 ZHe data 1500 0 0 100 200 300 400 500 600 700 800 900 Corrected Date(Ma) Figure 8. A. Date vs. elevation for Pikes Peak for ZHe data from this thesis and AFT data from (Kelley and Chapin, 2004). B. Date vs. elevation for Longs Peak for ZHe data from (Johnson thesis) and AFT data from (Kelley and Chapin, 2004).
42 elevation (Fig. 8a) was interpreted by Kelley and Chapin (2004) to represent the base of the Late
Cretaceous partial annealing zone, equivalent to the 110°C isotherm. Thus, the AFT data suggest that there was sufficient reburial during late Mesozoic and early Cenozoic time to raise the temperatures in rocks that now outcrop below 2600 m to temperatures > 110°C, which is substantial enough to completely anneal the apatites such that only Laramide dates are recorded.
In contrast, the preservation of dates older than those corresponding to Laramide thermal events in samples that outcrop at elevations >2600 m indicate peak temperatures during Laramide or earlier (post intrusion) events must have not exceeded <110°C, conditions that were insufficient to cause complete apatite annealing.
Below elevations of 2500 m, all AFT dates (N=7 samples) are younger than the youngest
ZHe dates. The highest eU grain from 2084 m has a date of 82 ± 6 Ma that is only slightly older than the AFT date of 57 ± 9 Ma at a nearly identical elevation of 2054 m. The youngest ZHe dates at an elevation of 2971 m are younger than AFT dates from elevations between 2482 m and
2771 m, where AFT dates ‘roll over’ (Fig 8a). Sample 2479, taken from a late-stage potassic intrusion that does not yield high eU (> 500 ppm) zircon, yields dates that are only substantially older than AFT data. The AFT date from the summit, at 4297 m, is older than the two highest eU zircon dates.
Using the AFT constraint on Late Cretaceous paleotemperatures of ~110 °C at a modern elevation of ~2600 m, I can estimate 1) the temperature profile of Pikes Peak during the Late
Cretaceous and 2) the maximum predicted alpha doses for any zircon in a sample given those temperatures and incomplete resetting of ZHe data during the Late Cretaceous using zircon Tc versus alpha dose plot of Guenthner et al., (2013) (Fig. 9). Assuming a 25°C /km geothermal gradient, my lowest and highest samples at elevations of 2084 m and 4297 m would have resided
43
at temperatures of ~ 122°C and ~73 °C
respectively during the Late Cretaceous.
Late Cretaceous isothermal holding for
10 myr at ~122°C is predicted to fully
reset grains with an alpha dose of
~325 × 10 �/� , therefore suggesting
all individual grains from sample 2084 m
have an alpha dose <325 × �/�(Fig. 9
A). Using the same approach I estimate
that any given zircon from the Pikes
Peak summit must have an alpha dose <
500 × 10 �/� assuming a Late
Cretaceous temperature of ~73 °C,
otherwise we would expect to observe
fully reset grains (Fig. 9B). These alpha
dose estimates only provide a framework
with which to approach alpha dose
estimates, for any given grain with a
variable eU, the actual alpha dose of the Figure 9. Closure temperature (Tc) versus alpha dose from Guenthner et al., 2013.The red line grain will differ. shows the approximate temperature in the Late Cretaceous for A. Pikes Peak Summit, where The date-eU pattern from the grains are only partially reset B. Pikes Peak Base, where grains are only partially reset and C. Pikes Peak batholith, together with the Longs Peak summit, where all grains are reset regardless of eU. AFT results, suggest that the ZHe data
44 record a protracted thermal history in which zircons of variable eU (and therefore variable radiation damage and variable He retentivity) were reset to different degrees by burial after beveling of the Ancestral Rockies. ZHe data from late stage potassic plutons, like that of the summit, have low uranium and thorium concentrations, such that eU < 500 ppm, and therefore do not have lowered retentivity and only display dates > 350 Ma. All samples taken from the syenite, which dominates the batholith, show a similar data trend in which there are zircons with dates < 350 Ma and are as old as ~750 Ma. These oldest ZHe dates suggest initial cooling of the batholith through the closure temperature of zircon at ca. 750-800. The plateau of the youngest dates ca. 100 Ma, which are older than Laramide, suggests that even the lowest retentivity zircons were only partially reset prior to Laramide exhumation. I will test these ideas further in section 6.1.3 below with forward modeling of thermal histories.
6.1.2. Comparison of northern and southern Front Range vertical profile “14’er” datasets
Previous AFT work has documented north to south variation in the modern elevation of the 110°C isotherm, and thus the temperature profiles of Colorado “14’ers”. To understand how this variation is expressed in ZHe data, I can compare my Pikes Peak ZHe data from the southern
Front Range with previously acquired vertical profile ZHe data from Longs Peak in the northern
Front Range (Johnson thesis, 2015). The Longs Peak elevation transect extends from 2835 m to
4343 m. All single grain ZHe dates fall between 43 ± 5 Ma and 76 ± 21 Ma, except for one analysis at the summit with a date of 137 ± 25 Ma (Fig. 8b). These dates are uniform despite an eU range that spans over 1800 ppm. Thus, the dates show no correlation with either elevation or eU. The absence of a date-eU correlation contrasts with the Pikes Peak ZHe data that show a strong negative correlation with eU and a date range of over 650 Ma. The AFT dates from Longs
45 Peak fall between 45 ± 9 Ma and 55 ± 4 Ma, and like the ZHe dates show no relationship with elevation (Fig. 8b). These data indicate that the summit of Longs Peak had a temperature >110°C in the Late Cretaceous. Assuming a 25°C/km geothermal gradient and using the 1508 m elevation difference of the transect, this result suggests that the lowest elevation sample was at a temperature of >145°C during the Late Cretaceous. The Pierre Shale is interpreted to have significantly elevated the geothermal gradient in the Northern Front Range (Kelley and Chapin,
2004), such that a 25 °C/km geothermal gradient and the associated temperature estimate of 145
°C for the lowest elevation sample likely represent the minimum allowable values.
Using the same method as described in section 6.1.1, the ZHe closure temperature versus alpha dose plot of (Guenthner et al., 2013) was used to predict the alpha dose at which the zircon would be fully reset given the estimated Late Cretaceous temperature based on the AFT data
(Fig. 9). Unlike Pikes Peak where the ZHe dates were incompletely reset by Late Cretaceous burial such that the estimated alpha dose was a maximum value, the Longs Peak zircons were completely reset and so instead constrain the minimum alpha dose for any grain, given a unique eU value (Fig. 9C). Due to uncertainty in the geothermal gradient imposed by insulation of heat by the Pierre Shale the following doses only provide a qualitative framework under which to understand the Longs Peak data set, and may vary significantly given the steep negative correlation between alpha dose and closure temperature past the first percolation point
(Guenthner et al., 2013). The minimum zircon alpha doses predicted for complete He loss in the low and high elevation samples with Late Cretaceous temperatures of >110 °C and >145°C are
375×10 �/�, and 250×10 �/�., respectively (Fig. 9C). It is possible that the zircon alpha doses for Longs Peak are higher than those of Pikes Peak because of the increased time for radiation damage accumulation owing to the older age of the Longs Peak lithologies (~1400 Ma)
46 relative to those from the Pikes Peak batholith (~1100 Ma). Biotite 40Ar/39Ar data indicate the
Longs Peak batholith cooled through ~ 300 – 400°C by 1200 Ma, this is approximately 400 million years than the Pikes Peak batholith cooled through the same temperatures, which indicates that radiation damage accumulation may be more prolonged on Longs Peak.
ZRDAAM uses the damage annealing kinetics of Yamada et al., (2007), which gives a partial annealing zone of 310 to 223°C for zircon fission tracks. Based on this annealing zone, the
Longs Peak – St. Vrain Batholith potentially had up to 200 Ma of additional radiation damage accumulation time over the Pikes Peak batholith.
6.1.3. Thermal history forward modeling
I can test my ideas above for the thermal history significance of the ZHe dataset through some preliminary forward modeling using the HeFTy computer modeling software (Ketcham et al., 2005), ZRDAAM diffusion kinetics (Guenthner et al., 2013), and the AFT annealing kinetics of (Ketcham et al., 1999). To understand the first-order thermal history of Pikes Peak, I modelled grains from sample 3971 m that show a negative date-eU pattern consistent with the general trends of the other samples (Fig. 10). The forward modeling allows me to evaluate the degree of reheating experienced in the early Paleozoic and again during the late Paleozoic – early
Mesozoic. I impose the following constraints on the models: 1) Pikes Peak emplacement at 1078
± Ma, 2) cooling to temperatures <325°C between 1079 ± 2 Ma and 1073 ±2 Ma based on biotite
40Ar/39Ar dates from the Pikes Peak batholith, 3) exposure at the surface by the Neoproterozoic, based on work demonstrating that the Pikes Peak batholith provided detritus to the local
47 Neoproterozoic ‘Tava Sandstone’ (Siddoway
and Gehrels, 2014), 4) probable continued
exposure at the surface until reburial beginning
Cambrian time, because the Cambrian Sawatch
formation lies unconformably on the Pikes
Peak batholith (Siddoway and Gehrels, 2014)
5) sedimentation continued through the Middle
Pennsylvanian based on the preserved nearby
sedimentary sequences, 6) unroofing to or near
the surface during the Ancestral Rockies
Orogeny, when the Pikes Peak batholith
provided detrital zircon material to the
Fountain Formation (Kluth and Coney, 1981,
Siddoway and Gehrels, 2014), 7) reburial and
reheating occurred between the Early Permian
Figure 10. A. Forward modeling workflow B. Plots ZHe dates versus eU for both Pikes and Longs Peak. The blue and purple lines show the predicted date- eU correlation for the time-temperature histories for Pikes and Longs Peak respectively. The predicted Pikes Peak date-eU correlation fits the highest and lowest eU grains well, but does not match date for an eU ~400 ppm. The predicted Longs Peak date-eU correlation does not fit dates for eU < 400 ppm well, but captures the high eU dates well. C. Model time-temperature histories are plotted in blue and purple for Pikes Peak and Longs Peak, respectively. 48 and Late Cretaceous based on the preserved nearby sedimentary sequences of ≤4000 m (Weimer,
1996), 8) erosion and cooling during the Laramide Orogeny (approximately 80 – 40 Ma), and 9) return to the surface by 66 – 64 Ma based on igneous and metamorphic basement clasts present in the Arapahoe formation (Weimer, 1996).
Models were designed to test the sensitivity of ZHe data to the magnitude of reheating and cooling events. For example, in a series of models, Mesozoic reheating was scaled by 25°C increments between 125°C – 225°C. In this initial modeling effort, I found it challenging to generate a time-temperature history that honored geologic constraints while predicting a date of
762 Ma for a zircon with ~175 ppm eU as well as a date of 476 Ma for a zircon with ~325 ppm eU (Fig. 10). The large change in date over such a small eU range is challenging to reproduce.
The best fit is presented in figure 10. The results that the maximum temperature reached during the late Paleozoic – early Mesozoic primarily controls the dates of ‘intermediate’ eU grains, and the temperature reached during rapid burial after beveling the Ancestral Rockies serves as the primary control on dates for ‘high’ eU grains. Additional forward and inverse modeling of this sample as well as others in the vertical transect can further refine the interpretations of the data patterns in the future.
The Longs Peak model was intended to replicate the Pikes Peak model as closely as possible with the aim of testing the hypothesis that higher Late Cretaceous temperatures in the northern relative to the southern Front Range are responsible for the observed flat date – eU pattern. The constraints on the Longs Peak model were: 1) pluton emplacement ca. 1400 Ma 2) cooling through approximately 300°C based on mica 40Ar/39Ar (Shaw et al., 2005) 3) exposure at the surface by the Neoproterozoic that continued through the beginning of Cambrian time, 4) sedimentation continued through the Middle Pennsylvanian based on the preserved nearby
49 sedimentary sequences, 5) unroofing to or near the surface during the Ancestral Rockies
Orogeny, 6) reburial and reheating between the Early Permian and Late Cretaceous based on the preserved nearby sedimentary sequences of ≤4000 m (Weimer, 1996), 9) erosion and cooling during the Laramide Orogeny (approximately 80 – 40 Ma). Grains from 3 samples from elevations 2835 m, 3383 m, and 4121 m were modeled together to capture a large eU range in the Pikes Peak data set. Initial models suggest that the Longs Peak data were sensitive to the timing of rapid unroofing during the Cenozoic, and are best reproduced with unroofing beginning at 50 Ma (Fig. 10). The higher Late Cretaceous temperatures of Longs Peak relative to
Pikes Peak predict the uniform ca. 55 Ma ZHe dates at >400 ppm eU owing to complete resetting of the zircons followed by Laramide cooling. However, I have difficulty inducing complete resetting of ZHe dates at eU<400 ppm and thus cannot reproduce the Laramide dates at lower eU values (Fig. 10). Additional forward and inverse modeling are needed to further refine the interpretations of the data patterns in the future.
6.1.4. Geologic implications
My initial HeFTy modeling results shown in Figure 10B suggest a cooling and unroofing event during the Neoproterozoic, consistent with regional constraints. Initial breakup of the super continent Rodinia is interpreted to have begun in Western Laurentia ca. 800 – 750 Ma (e.g. Li et al., 2007; Whitmeyer and Karlstrom 2007). North of this study, the Gunbarrel mafic dikes, which span from Northern Canadian provinces into northwest Wyoming ca. 780 Ma are interpreted to be the result of crustal extension and asthenosphere upwelling associated with the break-up of
Rodinia in western Laurentia (Harlan et al., 2003). Southwest of the study area, the
Neoproterozoic – early Paleozoic Chuar group and Pahrump group are interpreted as recording
50 the Rodinian Rifting from ca. 780 Ma – 520 Ma (e.g. Timmons et al., 2001; Dehler et al., 2001).
ZHe data from the Big Horn Mountains in the Northern Rockies suggest an unroofing event ca.
700 – 900 Ma (Orme et al., 2016). More proximal to the study site, Titanite (U-Th)/He data from
Big Thompson canyon, < 25 km ENE from Longs Peak, imply cooling between 976 ± 71.4 Ma and 614 ± 44 Ma (Johnson, 2015). Microcline 40Ar/39Ar data from the nearby Mt. Evans batholith records cooling through 200°C by 800 Ma (Shaw et al., 2005). Together, these data are compatible with a cooling event in the Rocky Mountain region ca. 750 – 800 Ma that may have been caused by Rodinia rifting (Orme et al., 2016).
The ZHe data from Pikes Peak indicate incomplete resetting of zircon in Late Cretaceous time, in contrast to the results from Longs Peak that record complete Late Cretaceous He loss from the zircon. These results indicate higher peak Late Cretaceous paleotemperatures in the northern than southern Front Range, consistent with the AFT results and compatible with the conclusion of Kelley and Chapin (2004) that the thick Pierre Shale in the northern Front Range thermally shielded the basement. On Longs Peak both ZHe and AFT data record rapid exhumation through the temperature sensitivity range of both systems ca. 70 – 45 Ma.
The uniform date-eU trend observed in the ZHe data from Pikes Peak could be explained by sampling along a single geotherm. Two high angle reverse faults, the Oil Creek Fault and the
Ute Pass Fault, which bound the fault block in the west and east, respectively, may have provided sufficient tilting of the fault block in the Early Mesozoic during the Ancestral Rockies
Orogeny, such that the modern topographic profile may dip at, or below, the dip of the overall fault block (Fig. 11). Given present day topography, tilting the block 5 -10° would eliminate the geothermal gradient we expect to observe. This could have resulted in the sampling of a single
51 Figure 11. Tilting of the fault block by two high angle reverse faults on the western and eastern edges of the batholith may have created a paleogeotherm which mimics the modern day topography. The sampling of a uniform paleogeotherm could explain the observed uniformdate- eU trends.
paleo geotherm. Further structural analysis of the region is needed to understand how the ZHe
data may have been effected.
6.2 Raman spectroscopy and ZHe data integration and implications
My thesis included the additional goal of exploiting grain imaging and Raman
spectroscopy data to inform interpretation of my ZHe results. Specifically, I aim to constrain 1)
the influence of partial annealing of radiation damage within and between samples, and 2) how
empirical estimates of alpha dose through FWHM compare with those made by ZHe and
geologic data.
52 6.2.1. Evaluating zircon annealing from raman spectroscopy data
I compared my Raman data with the “Nasdala line”, which plots FWHM vs. raman shift in unannealed zircon (Nasdala et al., 2005), to evaluate if my Pikes Peak zircons were annealed during their history. The majority of the Raman spectroscopy data (195/250 data points) fall within error (estimated at ± 1.5 cm-1 for FWHM) of the “Nasdala” line (Fig. 3A). The narrowing of the �3(SiO4) FWHM without accompanying increase in Raman Shift is interpreted to represent the partial annealing of radiation damage such that points falling to the lower left of the Nasdala line imply partial annealing of radiation damage (Marsellos and Garver, 2002; Nasdala et al.,
2005). Approximately 22/250 points (8.8%) fall below and to the left of the “Nasdala line” (Fig.
3a). Given that no single grains have spot analyses that all fall into the partial annealing regime, the data do not support whole grains being partially annealed. Raman spot data that fall above and to the right of the “Nasdala line” have been reported in the literature, discussion of such results significance has been limited (e.g. Nasdala et al., 2004; Váczi and Nasdala, 2016). Such data occur in all three samples from Pikes Peak, and may have analytical causes. First, the error may have been introduced by inaccuracies in the baseline correction, because at high levels of metamictization, the overall spectra bow upwards (Fig. 2B). The baseline correction then makes it possible to subtract the bowing effect, and measure the Raman shift and FWHM accurately.
However, if the baseline correction anomalously heightens or shortens the peak, then the half maximum height can be over or underestimated, yielding an anomalously wide or narrow
FWHM. Second, the spectra bowing may be a consequence of my use of a 532 nm Nd:YAG laser, which may be inappropriate for metamict grains. A 632 nm laser may be better equipped to accurately measure Raman shift and FWHM in metamict grains. This hypothesis is not readily testable at CU-Boulder, but may be tested at facilities elsewhere.
53 To understand if all samples are responding uniformly to a t-T history, as suggested by the ZHe data, intrasample variability was assessed (Figs. 3 b –d). It may be reasonable to expect that the lowest elevation, and thus hottest sample during the Late Cretaceous, would be more partially annealed than a sample nearly 2 km structurally higher. However, there is no strong correlation between sample elevation and the proportion of data which fit to the line, which suggests no annealing in any samples despite the 1900 m range of elevation change. This result is consistent with the 73 - 122°C Late Cretaceous paleotemperatures that we inferred earlier for the samples, because they are insufficient to anneal zircon fission tracks according to the model of Yamada et al., 2007, which suggests that temperatures of 262 to 330 °C are required for annealing.
To understand the relationship between annealing of radiation damage and partial retention of helium, I compared the results from Raman spectroscopy to ZHe data (Figs. 5,6 and
7). Grains that have paired data yield representative dates and spread in FWHM that are representative of the overall data sets. (Fig. 4b, Fig. 5). A nonlinear relationship between date and raman radiation damage proxies (Fig. 6) is consistent with the nonlinear, logarithmic relationship between radiation damage and helium retentivity at alpha doses greater than
~150x1016 �/g in the ZRDAAM (Guenthner et al., 2013). The correlation between date and radiation damage proxies is not robust in sample 2907, and further data is needed to elucidate patterns in this sample (Figs. 6 B, E). The negative correlation between date and FWHM is expected because a higher FWHM implies a more damaged grain that will be less He retentive and therefore yield a younger date, consistent with the negative ZHe date-eU correlation for these grains (Fig. 4B). Similarly, a positive correlation between raman shift and ZHe date is expected because a less damaged grain will have a higher raman shift and an older date.
54 The lack of correlation between radiation damage proxies and eU can be accommodated given that we expect eU to be a weighted average of all the zones within a given grain, and therefore would be an average of damage levels. From primary relationships between Raman spectroscopy data and ZHe data, the two methods to first order support relationships observed in each system separately. These data indicate that partial annealing of radiation damage is not a dominant control on ZHe dates, but accumulated radiation damage in particular zones may be exerting an influence on ZHe dates. However, without spot uranium and thorium concentration data, and significant diffusion studies, the extent to which zonation is influencing this data set cannot be conclusively evaluated.
6.2.2 Implications of alpha dose estimates and radiation damage accumulation time calculations
The goal of this section is to independently estimate alpha dose so that I may 1) evaluate the variation of damage levels within a grain, 2) assess the accumulation time of radiation damage, and 3) compare alpha dose estimates based on Raman data with those from ZHe data.
Unfortunately, the strong U-Th zonation of my dated grains, manifested by the broad variation in the Raman spot analysis results (Fig. 2c, Fig. 3), unexpectedly limited the extent to which I can achieve this goal because I lack spot U-Th concentration data to match the Raman spot data and instead must use the bulk U-Th concentration data. I also have fewer Raman spot analyses for some zircons than ideal because in my initial Raman data acquisition phase, the extent of zonation was unknown. Nonetheless, I still can use the available results to draw some preliminary conclusions.
Nasdala and others empirically created a relationship between FWHM and alpha dose using zircons with a well know thermal history (Fig. 12 A) (Nasdala et al., 2001; Palenick et al.,
55 2003; Presser and Glotzback, 2009; Pidgeon, 2014; Váczi and Nasdala, 2016). The equations for this calculation were presented in section 2.3. Using fixed bulk uranium and thorium concentration data, for each spot the accumulation time can be varied so that a given FWHM
“falls onto” the empirically derived relationship between FWHM and alpha dose (Fig. 12 A).
From this work, researchers predict that the ‘time’ term represents the time over which alpha and fission track damage has accumulated (Nasdala et al., 2001). This process makes it possible for me to qualitatively evaluate the range of alpha doses present. Owing to zonation, all individual zircons have a range of FWHM and because I use bulk U and Th concentration data, there is a range of inferred alpha doses (Fig. 12B). For samples 2084, 2907, and 3971, the full range of alpha dose estimates vary between 20 x1016– 200 x1016 �/g, 33x1016 – 115x1016 �/g, and 18 x1016 – 130 x 1016 �/g, respectively (Fig. 12 B). These alpha dose estimates do not exceed the maximum allowable alpha doses presented above for Pikes Peak (Fig. 9), and so to first order appear to be consistent with ZHe data. However, given that the alpha dose estimates for samples
2907 and 3971 are below the ~150x1016 �/g percolation threshold of the ZRDAAM, we would expect a positive date-eU trend, which is in opposition with what I observe. Further refinement of the above estimates using spot [U/Th] data would allow for further integration of ZHe data with alpha dose estimates.
56 Figure 12. A. FWHM versus alpha dose for all grains with both ZHe and Raman data available. The accumulation time for each data point so each point fell onto the empirical relationship between FWHM and alpha dose proposed in Nasdala et al., (2001). B. Plots FWHM versus alpha dose for the same data from sample 3971 plotted in A. However, in this plot, accumulation time was varied based on different geologic constraints. Each ‘row’ of data represents a single raman spot. Note, bulk eU was used in this calculation in conjunction with spot Raman analyses. C. Plots the alpha dose predicted by the ZHe date and the alpha dose predicted by the empirical relationship created by Nasdala and others (2001). In all three plots, each ‘row’ of data is a single grain. Empirically derived alpha doses are typically, but not strictly, greater than those that would be estimated by ZHe date.
57 It is possible to compare the alpha dose estimates predicted by the model presented by
Nasdala and others with the alpha dose predicted by the accumulation of radiation damage since
1) the ZHe date of each grain (Guenthner et al., 2013) 2) passage through ~325°C at approximately 1073 Ma based on mica 40Ar/39Ar data and 3) since the time of crystallization at
1078 Ma (Fig. 12 B). Alpha dose estimation was accomplished using the same equation as was used for the Nasdala model, but, for example, instead of altering the accumulation time to fit onto the Nasdala curve, the ZHe date was used for the accumulation time. This results in a uniform alpha dose estimate for each grain predicted by the ZHe date, passage through 325°C, and the crystallization age respectively (Fig. 12 B). In figure 12 B, each ‘horizontal line’ of data points corresponds to a single spot analysis. The accumulation of damage since 1.1 Ga overestimates alpha dose (Fig 12 B). This is expected given fission-track annealing kinetics (e.g.
Yamada et al., 2007). The accumulation of damage since the samples cooled through ~325°C via
40Ar/39Ar dating also overestimates the alpha dose (Fig. 12 B). This indicates that damage has not been accumulating since the batholith passed through ~325°C, which is only slightly hotter than the zircon fission track closure temperature suggested by Yamada and others (2007), and instead would suggest that radiation damage is annealing at lower temperatures. If this trend is confirmed through the use of spot [U/Th] data, this would support the hypothesis that radiation damage anneals at temperatures <300°C at high levels of metamictization (e.g. Geisler, 2002;
Váczi and Nasdala, 2016).
In figure 12 C, I compare the alpha dose predicted by radiation damage accumulation time model with the alpha dose predicted by the ZHe date for samples 2084, 2907 and 3971.
Each ‘horizontal line’ of data represents a single grain. In samples 2084 and 2907 the estimated alpha dose predicted by the radiation damage accumulation time model is, for most grains,
58 higher than the alpha dose predicted by the helium date alone (Fig. 12 C). This makes sense given the negative date – eU correlation observed among all grains (Fig. 4b). This trend is expected because the time over which radiation damage is accumulating prior to reaching the percolation point of the ZRDAAM may be ‘lost’ if the zircon becomes sensitive to lower closure temperatures than it was previously. More analyses for sample 3971 are needed to understand if the sample is behaving differently.
6.2.3. A suggested workflow to optimize integration of zircon Raman and He datasets
Further exploration of both annealing kinetics and the effect of zonation in zircon are needed to improve current ZHe models. Here I suggest a future workflow that may aid in accurate estimation of alpha dose through the zones of a grain, improved understanding of accumulated radiation damage, and better constraints on the thermal history recorded by complex ZHe data sets. From each sample, a wide range of zircon metamictization levels, approximated by color and morphology, should be analyzed. To ensure that zonation is properly characterized, thorough BSE and CL imaging should be carried out. Raman spectroscopy can be done either by spot analyses, which take 9 – 30 seconds each and therefore can be done in large quantities, or by maps of the polished grain surface. The second approach may be helpful in estimating the dominance of damage zones within a single grain and could be done on fewer grains of interest. [U/Th] data should be obtained either through spot analyses, which should correspond with specific raman point analyses, or through quantitative EMPA analyses. Recent work, however, has raised the possibility that long electron beam exposures may anneal zircons
(Váczi and Nasdala, 2016), and so that method may not be fully appropriate. Finally, the
59 collection of ZHe dates would allow for the careful integration of radiation damage information into an understanding of the thermal history of that sample.
60 CHAPTER VII
CONCLUSIONS
New zircon (U-Th)/He (ZHe) thermochronologic data constrain the protracted thermal history of Pikes Peak in the Colorado Front Range. Dates range from 762 ± 87 Ma to 82 ± 6 Ma, define negative date-eU trends, and display similar patterns despite the 2213 m elevation range that the samples were collected. To first order, these data trends are compatible with the
ZRDAAM. The results are broadly consistent with existing AFT, which record the base of the
Late Cretaceous PAZ (at 110°C) at an elevation of 2600 m. AFT data record north to south variation in the elevation of the 110°C isotherm between Pikes Peak and Longs Peak in the southern and northern Front Range, respectively, attributed to north to south variation in the thickness of the Pierre Shale prior to denudation. ZHe data from Longs Peak and Pikes Peak support this hypothesis. Initial HeFTy modeling demonstrates that similar date – eU patterns to those observed in the acquired data sets can be recreated simply by elevating the maximum Late
Cretaceous temperature in the northern relative to the southern Front Range. Further forward and inverse modeling of these data would strengthen geologic interpretations.
Raman data for a subset of grains from the ZHe dataset demonstrate the variability of damage levels within a single grain and between samples in the Pikes Peak sample suite. These data suggest little to no partial annealing of radiation damage in Pikes Peak zircon. Relationships between ZHe date and radiation damage proxies (FWHM and raman shift) are consistent with decreased helium retentivity at high damage levels. Estimates of alpha dose through an empirical relationship between FWHM and alpha dose are consistent with maximum allowable alpha doses constrained by ZHe data. Empirically derived alpha doses are typically, but not strictly, greater
61 than those that would be estimated by ZHe date. An improved workflow is proposed to further investigate the effect of variable intragrain damage on ZHe data interpretation, which would incorporate imaging, spot analyses of U and Th concentrations in conjunction with Raman work, and acquisition of single crystal ZHe data.
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