(U-Th)/He Thermochronology of the Santa Impact Crater (New

Zircon U-Pb and (U-Th)/He thermochronology of the Santa Impact Crater (New

Mexico, USA): evidence for a Paleozoic asteroid breakup?

Rayssa Martins Pimentel

Department of Geological Sciences, University of Colorado Boulder

Defended April 4, 2018

Thesis Advisor Stephen J. Mojzsis

Committee Members Stephen J. Mojzsis, Department of Geological Sciences Nigel M. Kelly, Department of Geological Sciences Nick Schneider, Astrophysical and Planetary Sciences Brian Hynek, Department of Geological Sciences

Abstract

The age of the Santa Fe Impact Structure (SFIS) is poorly constrained. Here, I examine the employability of the U-Th/He system in zircon and apatite extracted from shatter cones of the SFIS – some of which were previously analyzed for their U-Pb ages

– to better constrain the age of this impact structure, which lacks a preserved melt sheet, and has a complex, post-impact tectonic and thermal history. To achieve this, zircon and apatite grains were separated from shatter cone samples. Grains were selected to span a range of apparent radiation damage to access a range of He retentivities. Where possible, grains preserving planar fractures (PFs) were also included. U-Th/He dates from apatite predominantly record cooling from Laramide burial (c. 60 Ma), correlating with published apatite fission track dates. In contrast, U-

Th/He dates from zircon have a wider range (334.87 ± 13.56 Ma to 7.88 ± 0.22 Ma) and show a pronounced negative correlation with effective uranium content. The preservation of dates (n=3) that are older than the age of Laramide resetting in apatite demonstrates that low eU zircons are promising candidates for both recording and preserving the age of the impact. Therefore, further dating of low-eU zircons may provide evidence to support stratigraphic constraints that the Santa Fe impact age is roughly compatible with age estimates of ten other craters across North America,

Europe and Africa, of variable (and lower) geochronological reliability. It is noteworthy that the age coincides with the estimated times of three asteroid breakup events documented to have occurred in the asteroid belt. I propose that both the age overlaps and geographic distributions of these impacts represent terrestrial evidence an asteroid breakup event. Finally, I also report the first planar fractures (PFs) in zircons sampled directly from the central uplift zone of the crater where shatter cone morphologies are

2 preserved. The discovery of PFs in zircons increase the minimum pressure generated by the impact to at least 20 GPa, indicating a much larger crater (>>13 km diameter) than previously estimated.

1. Introduction

Impacts have played an important but commonly overlooked role in the formation and evolution of the solar system beyond the early accretion phase associated with planet formation. A colossal impact has been implicated in the formation of the Moon (e.g. Canup and Asphaug, 2001), and a rain of bombardment may have resurfaced terrestrial planets relatively late in solar system history (Tera et al.,

1974). There is little doubt that impact bombardments contributed to planetary growth after core formation (Bottke et al., 2010), shaped planetary habitability (Abramov and

Mojzsis, 2009) and caused at least one major mass-extinction (e.g. Alvarez and Alvarez,

1980). Nevertheless, fewer than 10% of all known craters have been precisely dated

(Jourdan et al. 2010). Some of the inherent difficulties in estimating the ages and magnitudes of impact events include poor preservation of the crater morphology due to crustal recycling, absence of neoformed minerals owing to the lack of impact melt, and overprint of geo- and thermochronometers by subsequent thermal events associated with normal crustal processes such as orogeny.

Previous attempts to perform radiometric geochronological techniques directly on craters have commonly relied on the U-Pb system in the mineral zircon, which is a common accessory phase. In addition to their highly refractory nature, and resistance to physical and chemical weathering, zircons preferentially incorporate U and Th, while excluding their Pb decay products, making them suitable for geochronology and

3 thermochronology (e.g. Reiners et al., 2005). The U-Pb system has been successfully employed to determine the ages of craters where identifiable melt is preserved (Kamo et al., 1996; Gibson et al., 1997; Petrus et al., 2016) or in cases where the impact provided enough heat to reset zircons in the target rocks (Abramov et al. 2013).

However, in lower energy impacts where no melt is present (or is not preserved due to erosion) or where heating durations are too short to allow resetting of the U-Pb system, the timing of the event may not be recorded. Alternatively, the U-Th/He geochronological system is sensitive to resetting at lower temperature conditions and thus has the capability to document a wider range of thermal events. This method has been tested in craters where melt is present and proved to be in good agreement with the U-Pb ages (van Soest et al., 2011; Wiliecki et al., 2014). In this case study, I examine the suitability of the U-Th/He method to date the Santa Fe impact structure (SFIS), an eroded structure for which impact melt is not preserved and where the target rocks have experienced a complex thermal history both prior to and following the impact.

Deriving an impact age via this method would set precedents for analogous studies to be attempted in other craters with similarly challenging circumstances.

The age of the Santa Fe impact structure is poorly constrained between 1.4 Ga and c. 320 Ma. The minimum age is inferred from the presence of Mississippian sediments deposited conformably on the structure (Fackelman et al., 2008), while the absolute maximum age is limited by the crystallization age of zircons in the target igneous rock (c. 1.4 Ga; Williams et al., 1999; Montalvo et al., 2017). Importantly, the minimum age constraint is consistent with the estimated ages of ten other craters (Table

1), of which three in North America currently align with Santa Fe (Figure 1), while others may roughly align along a paleotrack. These overlaps in age and distribution suggest a

4 genetic relationship between the cratering events, which may be explained by a possible crater chain. Additionally, three asteroid breakups with corresponding ages have been identified (Nesvorny et al., 2015), thus providing a possible candidate mechanism for either a crater chain or a related series of impacts on Earth in the Paleozoic.

Confirmation of this age relationship would have meaningful implications for our understanding of the more recent impact history of the Earth.

Table 1. Potential craters related to crater chains in the Paleozoic Name Country Age (Ma) Reference Serpent Mound USA <330 Watts , 2004 Crooked Creek USA 320±80 Hendricks, 1954 Decaturville USA <320 Offield, 1979 Clearwater West Canada 290±30 Fleischer, 1969 Charlevoix Canada 372 to 335 Rondot, 1968 Dobele Latvia 290±35 Masaitis, 1999 Kursk Russia 250±80 Masaitis, 1999 Mishna Gora Russia 300±50 Masaitis, 1999 Aorounga Chad Upper Devonian Koeberl et al., 2005 Gweni-Fada Chad Upper Devonian Koeberl et al., 2005

5 2. Background

2.1. U-Th/He Thermochronology

Zircon and apatite are common accessory phases in igneous and metamorphic rocks. As they crystallize, they incorporate trace amounts of U, Th and Sm into their structures, which produce He through a series of decays at known rates. At relatively low temperatures, radiogenic 4He will be retained within the crystal structure of a mineral, and by measuring the total He and the concentrations of the parent isotopes in a grain it is possible to estimate how long it has been accumulating He. However, helium diffusivity increases exponentially with increasing temperature, such that (U-Th)/He dates can be wholly or partially reset when a rock is heated to sufficiently high temperatures. Diffusion data show that the temperatures over which resetting occurs, and where the system closes to He diffusion during cooling (the closure temperature,

Tc), vary with crystal structure and grain size (e.g., Farley, 2000; Reiners et al., 2004); different minerals and grains within a single sample may record different dates despite experiencing the same thermal history.

Retentivity of 4He may also be dependent on radiation damage accumulated in a crystal (e.g., Flowers et al., 2009; Guenthner et al., 2013). Moderate amounts of radiation damage to the host crystal suffered by decay of U and Th initially increase 4He retentivity in both zircon and apatite (Figure 1b). However, the higher U+Th concentrations typically found in zircon mean that at higher radiation dosages, zones of damage become interconnected and retentivity declines. Not all minerals necessarily have the same concentrations of trace elements, which translates into a natural range of 4He retentivity that can vary widely within a zircon population even from the same sample. The relationship between crystal structures, variable compositions between

6 crystals, radiation damage and grain size greatly benefits thermochronometry studies because individual grains within a larger population will be susceptible to different degrees of resetting during the same thermal event.

It is also important to point out that the (U-Th)/He date recorded by a single grain does not necessarily reflect its crystallization age, but rather, it is a result of the total time-Temperature (t-T) history. As such, the date-eU patterns recorded by a population of grains (where eU = U + 0.235*Th, a proxy for radiation damage; Flowers et al., 2009), are also an integrated function the entire thermal history experienced by a sample and the evolving diffusivity of He during radiation damage accumulation.

2.2. Geologic Background

2.2.1 Santa Fe Region

The Santa Fe impact structure is hosted in Paleo- and Mesoproterozoic igneous and metamorphic rocks, which are part of the southwest extension of the north-south trending Sangre de

Cristo Mountains and bounded to the

West by the Rio Grande Rift. The host rocks to the structure were emplaced at c. 1.4 Ga (zircon U-Pb ages; Cavosie et al., 2015) during a major magmatic and deformational event (Picuris

Orogeny; Daniel & Pyle, 2005). The

7 region has since experienced a complex thermotectonic history, including at least six major burial and heating episodes. These are possible effects of the c. 1.2-0.9 Ga

Grenville Orogeny and the c. 0.7 Ga rifting of Western Rodinia, c. 550 Ma extension of the Southern Oklahoma Aulacogen, the Ancestral Rocky Mountain Orogeny (c. 325-290

Ma), the Laramide Orogeny (c. 75-35 Ma) and the main phase of Rio Grande rifting (c.

26-10 Ma; Cather et al., 2006).

These events had varying degrees of influence in the thermal history of the region. Cooling from high temperature conditions (500-550°C at 0.35-0.4 GPa, Williams et al., 1999) to temperatures below 300oC-250oC following the Picuris Orogeny was achieved by the onset of the Grenville Orogeny (Cather et al., 2006). Evidence for

Paleozoic and Mesozoic burial is not well preserved in the vicinity of the Santa Fe impact structure due to the sub-Mississippian Great Unconformity. However, in the impact area

Mississippian and Pennsylvanian marine sedimentary rocks locally overlie the crystalline basement and drape breccias interpreted to be associated with impact (Fackelman et al., 2008). These latter sedimentary rocks establish the minimum age for the impact.

Absolute maximum depths of burial may be inferred from the adjacent Albuquerque

Basin, where the maximum sedimentary thickness was estimated to be 3444m (Kelley,

1977). Apatite fission track cooling dates of c. 60 Ma imply the region experienced heating above temperatures required for track annealing during the Laramide Orogeny

(Kelley and Duncan, 1987). However, titanites still preserve Cambrian fission track dates, restricting the maximum temperature conditions reached during Laramide burial to lower than 290 ± 40oC (Kelley and Duncan, 1987; Harrison and McDougall, 1980).

8 2.2.2 Santa Fe Impact Structure

The Santa Fe impact structure is heavily eroded with no preserved geomorphic circular, crater-like structure. Its original discovery was based on the identification of shatter cones over an area of ~5.5 km2 between Santa Fe and Hyde Memorial State Park

(Fackelman et al., 2008). These shatter cones are exposed at the surface at a road cut along the New Mexico State Highway 475 around 8 km from Santa Fe (Figure 2). These nested conical structures with horsetail-like striated surfaces can reach up to 2m in length (Figure 3). They may form under relatively low pressure conditions (<2GPa) and have thus been recognized in a large number of craters (French and Koeberl, 2010).

Shatter cones are generally constrained to the central uplift, where these pressures are pervasive, and are currently the only megascopic feature accepted as diagnostic of an impact. Initial assessments of crater size yielded a 6-13 km wide crater, which were based on scaling models using a 3 km diameter central uplift derived from shatter cone distribution (Montalvo and Cavosie, 2015). However, in addition to the discovery of planar fractures (PFs) in quartz (Fackelman et al., 2008), xenotime (Cavosie et al., 2016) and apatite (Montalvo and Cavosie, 2015), subsequent reports of PFs in detrital zircon

(Lugo-Centeno et al., 2014; Montalvo and Cavosie, 2016; Montalvo et al., 2017; Cavosie,

2017) have raised the minimum pressure during impact to 20 GPa at the central uplift

(Leroux et al., 1999; Wittmann et al., 2006), implying a much larger structure than what was initially estimated.

11 3. Methods

3.1 Sample Selection and Preparation

The samples were processed at the Department of Geological Sciences of the

University of Colorado, Boulder. Initially, five thin sections were produced targeting the shatter cone surface, and possible impact related veins located a few centimeters below the surface (Figure 5. c and d). In addition, small portions of the sample in the vicinity of shatter cones and from veinlets possibly related to impact were crushed and milled following standard procedures. Next, they were subjected to heavy mineral separation

(Tetrabromoethane and Methylene Iodide), and the magnetic phases were removed using a hand-magnet. Finally, a portion of the total heavy mineral separate was mounted in epoxy and polished to half-grain thickness in an attempt to identify zircon and apatite.

However, backscatter electron images of these mounts revealed an insufficient number of grains for age analysis.

Subsequently, a 2 cm thick slab was cut from the bottom of the remaining sample, and was crushed and milled following standard procedures. The resulting material was then separated into five density fractions on the Wilfley table, of which only the highest density fraction was utilized for further separation. This fraction was also subjected to heavy liquid mineral separation and removal of the magnetic phases.

A total of 198 grains were handpicked under a stereoscope, with grains chosen based on morphology (shape, color, size) and placed within two one-inch round areas on double-sided adhesive tape. Optical photomicrograph overview maps were produced to facilitate identification and navigation on the mounts. Scanning Electron Microscope

(SEM) secondary electron images of the grain exteriors were collected using the Field-

Emission Scanning Electron Microscope (FE-SEM) located in Geology and Geological

12 Engineering at the Colorado School of Mines to detect the presence of impact-related microstructures.

Each of the dated apatite and zircon grains were individually transferred to an ethanol filled petri dish using tweezers for further characterization utilizing a Leica M165 binocular microscope equipped with a calibrated digital camera. After they were photographed and their dimensions were determined, each individual grain was packed into Nb tubes, which were crimped at both ends.

3.2. U-Th/He Methods

U-Th/He analyses were carried out at the University of Colorado Boulder TRaIL

Facility (Thermochronology Research and Instrumentation Lab) with the assistance of

Dr. James Metcalf and according to the techniques described in Kelly et al. (2018).

13 Following selection, Nb packets were loaded into the ASI Alphachron He extraction and measurement line. Each packet was placed in the UHV extraction line (~3 X 10 -8 torr) and heated with a 25W diode laser to ~800-1100°C for 5 to 10 minutes to extract the radiogenic 4He. 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. Degassed grains were then removed from the line, and taken to Class 10 clean lab for dissolution. The apatite grains, still enclosed in the Nb tubes, were placed in 1.5 mL Cetac vials, spiked with a 235U - 230Th – 145Nd tracer in HNO3, capped, and baked in a lab oven at 80°C for 2 hours. Zircons were dissolved using Parr large-capacity dissolution vessels in a multi-step acid-vapor dissolution process. Grains (including the Nb tube) are placed in Ludwig-style Savillex vials, spiked with a 235U - 230Th – 145Nd tracer, and mixed with 200 µl of Optima grade HF. The vials were then capped, stacked in a 125 mL Teflon liner, placed in a Parr dissolution vessel, and baked at 220°C for 72 hours. After cooling, the vials were uncapped and dried down on a 90°C hot plate until dry. The vials then underwent a second round of acid-vapor dissolution, this time with 200 µl of 6N Optima grade HCl in each vial that is baked at

200°C for 24 hours. Vials were then dried down a second time on a hot plate. Once dry,

200 µl of a 7:1 HNO 3:HF mixture was added to each vial, the vials were capped, and cooked on the hot plate at 90°C for 4 hours. Once the minerals were dissolved, regardless of the dissolution process, they were diluted with 1 to 3 mL of doubly- deionized water, and taken to the ICP-MS lab for analysis. Sample solutions, along with normal solutions and blanks, were analyzed for U, Th, and Sm content using an Agilent

7900 quadrupole ICP-MS located in the CU TRaIL laboratory. After the U, Th, and Sm

14 contents are measured, He dates and all associated data are calculated on a custom in- house spreadsheet.

5. Results

5.1 Sample Description

Sample SF1014B preserves a shatter cone surface (Figure 5), which suggests that it has experienced peak pressure conditions of ≥20 GPa during the impact. The rock is a fine to medium grained, equigranular granite that ranges between pink and orange in color. It contains quartz, potassium and plagioclase feldspars, biotite, muscovite, and iron oxide minerals, in addition to apatite and zircon as accessory phases. Thin sections of the sample reveal the presence of pervasive planar deformation features in muscovite, and locally preserved PFs in quartz.

For apatite (U-Th)/He (AHe) thermochronometry, all grains from the tape mounts had abundant inclusions and not suitable for dating. Therefore, a further six grains that were selected from the remaining mineral separate. Only optical images of these grains are available. Selected grains were euhedral to subhedral, had visible crystal facies, and included at least one termination. They ranged in length between 122 and

247µm and their aspect ratios were between 1.2 and 2.0 (see Figure 2 of the

Supplementary Files). For zircon (U-Th)/He (ZHe) thermochronometry, grains were selected based on size and color (Figure 1 of Supplementary Files), with the latter being qualitatively indicative of eU (or radiation damage) levels. They were also screened for quality, including crystal shape and the presence of inclusions, in order to achieve the best results. The eleven targeted zircon grains were euhedral to subhedral, had visible crystal facies, and at least one termination. Some grains preserved planar fractures (see

15 below). They ranged in length between 150 and 287µm, while their aspect ratios were between 1.7 and 3.9.

5.2 SEM images

Figure 6 shows SEM secondary electron images of the 11 dated zircons and

Figure 7 shows SEM secondary electron images of 6 apatites that are representative of those also dated in this study. Common to many zircon and apatite grains separated from sample SF1014B, are repeating, planar and parallel fractures, which are interpreted to be planar fractures (PFs; Figure 8). These features are shock-produced parallel micro-deformation structures that have been documented in rocks from a number of craters and are commonly used as diagnostic evidence for an impact (French and Koeberl, 2010). Within this sample, there are at least two intersecting sets of externally visible PFs in the zircons. The are commonly visible for up to 20 µm in length and have 0.5-10 µm spacing between each fracture. The images allow for the qualitative determination of two orientations for the PFs: (110) and (011). However, the number of possible orientations for PFs in zircons and how they relate to the intensity of shock metamorphism is not known (Cavosie et al., 2010). There have been previous reports of

PFs for detrital zircons from near the Santa Fe Impact Structure. However, owing to the fact that these were detrital grains the host rock was not identified (Lugo-Centeno et al.,

2014; Montalvo and Cavosie, 2016; Montalvo et al., 2017; Cavosie, 2017).

5.2 U-Th/He Thermochronometry

The U-Th/He results for zircon and apatite are reported in Table 2. Apatite grains have a compositional range of 5.4 – 16.4 ppm eU and have dates that range between

94.85 ± 14.83 Ma and 53.75 ± 5.29 Ma. Data define a slightly negative date-eU

19 correlation (Figure 9.1). Zircon compositions range between 375 and 3723 ppm eU and have dates that range from 334.87 ± 13.56 Ma to 7.88 ± 0.22 Ma. Data define a robust negative date-eU correlation (Figure 9.2), as well as a less well-defined positive date- size correlation (Figure 9.3).

Table 2. U-Th/He results for 11 zircons and 6 apatite grains from sample SF1014B.

Name rs (µm) Dim 4He U Th Sm eU Raw Ft Corrected Error Mass (nmol/g) (ppm) (ppm) (ppm) (ppm) Date Date (Ma) (Ma) (mg) (Ma) 1-5-9 75,28 14.35 588.877 318.19 240.89 1.51 374.8 284.36 0.845 334.87 13.56 2-2-9 29,95 1.17 105.09 1590.16 1419.54 5.82 1923.8 10.12 0.631 16.02 1.81 2-3-12 52,76 7.2 448.483 3599.38 527.14 5.95 3723.3 22.32 0.787 28.34 0.77 2-3-6 59,46 6.69 454.241 937.23 709.11 3.95 1103.9 75.81 0.806 93.88 3.53 2-4-6 47,57 3.53 89.831 1392.41 1177.82 8.08 1669.2 9.97 0.76 13.11 1.22 2-5-11 49,86 3.92 392.832 1081.32 834.3 3.86 1277.4 56.75 0.771 73.5 4.77 1-8-12 64,56 8.88 455.796 795.93 607.54 3.56 938.7 89.35 0.821 108.67 5.94 1-6-10 38,11 2.04 76.003 2414.98 391.3 3.57 2506.9 5.63 0.714 7.88 0.22 1-7-4 65,65 9.53 209.838 940.59 861.19 5.67 1143 33.93 0.823 41.21 1.59 1-8-7 53,98 4.88 525.783 757 606.48 3.14 899.5 107.4 0.787 136.07 15.91 1-7-2 78,49 15,05 491,286 466,42 200,62 1,49 513,6 174,76 0,853 204,32 12,40

a06 49,51 1.34 1.866 5 1.81 17.58 5.4 61.89 0.644 94.85 14.83 a03 37,20 6.64 3.044 7.32 5.71 27.03 8.7 63.18 0.8 78.56 4.57 a05 72,38 1.81 3.64 11.38 9.54 39.8 13.6 48.18 0.677 70.43 2.84 a04 41,33 1.81 3.563 8.87 32.24 27.42 16.4 39.4 0.646 60.58 1.77 a02 43,07 1.47 2.431 9.83 6.07 55.39 11.3 38.36 0.635 59.37 2.07 a01 38,20 2.34 1.383 5.7 2.44 32.01 6.3 39.08 0.718 53.75 5.29 Fish Canyon Tuff zircons run in conjunction with these grains yield a date of 28.9 ± 2.0 Ma (n=4) rs: grain radius

21 6. Discussion

6.1 Implications of inferred crater size on ZHe: Impact Crater Modeling

In order to investigate the probable conditions affecting the basement during development of the

Santa Fe impact structure, and therefore potential thermal conditions associated with resetting of (U-Th)/He dates, thermal-mechanical models were produced by Dr. Oleg Abramov

(Planetary Science Institute, Tuscon,

AZ) following the methods outlined in

Abramov et al. (2013). Models were constructed based on a maximum crater size inferred from shatter cone distribution (~13 kms; Fackelman et al.,

2008), and simulated impact of a 756m diameter rocky impactor, with an impact velocity of 17 km/s and an impact angle of 45o. Model outputs (e.g., Figure 10) allowed constraints to be placed on peak pressure and temperature conditions immediately after impact, and decay of elevated pressure and temperature back to steady-state.

As shown in Figure 10, the maximum temperatures achieved in the crater center

(and therefore maximum temperatures likely within the central uplift) are ~95°C. With

22 rapid cooling, it is unclear if this would lead to resetting of apatite or zircon He dates, which may require higher temperatures for longer durations. It is interesting to note, however, that the maximum pressure that can be achieved within a crater of these dimensions is ~3.4 GPa, which is in stark contrast to the minimum pressures required to form PFs in zircon (20 GPa; Leroux et al., 1999; Wittmann et al., 2006). Therefore, this crater size estimate (<13 km) is inconsistent with the observed evidence and we propose the diameter of the Santa Fe Impact Structure is actually greater. This would also affect the maximum temperature reached at the center of the impact, and therefore the thermal history recorded by zircons and apatites in samples of the central uplift. Further modeling using peak pressures of 20 GPa or greater will allow us to better constrain the temperature conditions generated by the impact, and therefore interpret zircon and apatite He dates from the crater basement.

6.2 U-Th/He Thermochronology

The first observation that can be derived from the ZHe dates is that, while all grains have experienced the same t-T history, they preserve different dates that define a negative correlation with eU. Therefore, in order to interpret the U-Th/He results, it is imperative to consider how each grain was affected by thermal events. In zircons, low levels of radiation create isolated damage zones, decreasing overall He diffusivity and leading to a corresponding increase in closure temperature (Tc) (Reiners et al., 2005;

Guenthner et al., 2013). However, with increasing radiation damage levels, these damage zones become interconnected creating fast pathways for He diffusion, resulting in diffusivity to increase dramatically and leading to a corresponding decrease in (Tc)

(Guenthner et al., 2013; Baughman et al., 2017). Additionally, because net accumulation

23 of radiation damage is a function of eU and time, grains with higher eU will accumulate radiation damage faster (Figure 11). For most geologic timescale this leads to lower Tc in high eU zircons than low eU ones. As previously stated, the date-eU for the zircons in the Santa Fe data set show a negative date-eU trend (Figure 9.2), that flattens out at eU

>1500 ppm. Similar negative date-eU trends were observed by Guenthner et al. (2013) for samples with older ages and higher eU values. I interpret this as a result of different degrees of radiation damage across this zircon population. However, the preservation of older dates by the lowest eU grains is a good indication that they had not yet reached high levels of radiation damage accumulation by the time of Laramide burial, and thus are most promising as candidates for preserving pre-Laramide dates, and so if heated sufficiently during impact, the age of the Santa Fe crater. Additionally, the oldest grains are also the largest in this population, which collectively shows a weak positive correlation between date and size (Figure 9.3). Therefore, it is possible that other grains with similar low-eU levels and large grain sizes may have preserve older dates and should be targeted in future work.

Next, I evaluate the data in the context of the thermo-tectonic history of the

Santa Fe region. Although the pre-Laramide history is difficult to constrain, we know that no temperatures affecting the rocks were greater than that required to reset U-Pb crystallization ages in zircon (c. 1.4 Ga; Cavosie et al., 2015). Events subsequent to that

(see introduction) are difficult to establish, but did envolve some burial during the

Paleozoic as evidenced by the presence of Miss/Penn sediments draping impact breccias. Full details on depths of burial of the rocks in the immediate vicinity of the

Santa Fe structure are no longer preserved. However, maximum depths of burial during

24 the Paleozoic and Mesozoic are likely much less than those inferred for the adjacent

Albuquerque basin (<3444 meters; Kelley, 1997).

The data from apatite and zircon do allow conclusions to be made about heating during Laramide burial. Apatite dates are consistent with partial to complete He loss during the Laramide Orogeny, and are also in good agreement with apatite fission-track

(AFT) dates from the Santa Fe Range (74.0 - 43.8 Ma; Kelley and Duncan, 1986).

Resetting of these systems typically occurs at ~50oC – 115oC (AHe) and 100oC±25oC (AFT)

(Schuster et al., 2006; Kelley and Duncan, 1986), which implies these samples experienced minimum temperatures of ~100oC at that time. However, in the zircon dataset, three of the ZHe dates, which are from low eU grains, are older than estimates

25 for the Laramide Orogeny, suggesting incomplete resetting. This implies that the maximum temperature during the Laramide Orogeny was below the upper closure temperature limit for low eU zircons, which is estimated to be ~210oC at most

(Guenthner et al., 2013).

The only event that postdates the Laramide Orogeny is the development of the

Rio Grande Rift (26-10 Ma). Dates corresponding to this event are not recorded in our

AHe or AFT dates (Kelley and Duncan, 1986). Nevertheless, four of the ZHe dates, which are from high eU zircons, are compatible with complete or partial resetting during this event (41-13 Ma). This suggests that, in contrast with the low eU grains, these four grains had much lower closure temperatures to He diffusion and were in fact susceptible to resetting by more recent, lower temperature events.

The four oldest ZHe dates recorded in my data set do not correspond to the ages of any known thermal events, but rather, are consistent with varying degrees of partial resetting. Importantly, while a single ZHe date cannot be used to constrain any geologic event, the oldest ZHe date does fall within the expected range of values for the Santa Fe impact age. Based on the 20 GPa minimum, it is possible that the temperatures at the center of the impact would have exceeded zircon He Tc, resetting even low eU grains.

Therefore, it is possible that these four dates reflect resetting by the Santa Fe impact coupled with partial resetting by one or more subsequent events: The Laramide Orogeny or the Rio Grande Rift. Since the Rio Grande Rift heating in the Santa Fe area did not cause resetting of AHe or AFT, it is unlikely that it would have caused resetting of the low eU zircons, suggesting that the Laramide Orogeny is a more probable candidate.

Taking all of these events into account, I offer three possible interpretations for the 334.87 ± 13.56Ma ZHe date: (1) the U-Th/He system in this zircon was reset by the

26 impact and then partially reset by the Laramide Orogeny, in which case the date is younger than the impact; (2) it was reset by the impact and remained undisturbed through the Laramide Orogeny, preserving the age of the impact; or (3) it was not fully reset by the impact, and, therefore, it reflects multiple partial resetting events.

However, the current data is insufficient to confidently discriminate between the three.

Therefore, these preliminary results suggest the date oldest date, defines a lower limit for the age of the impact, which corroborates with stratigraphic evidence. Additional analysis focusing on large, low eU zircons could help provide data that support one of the hypotheses above, by either: (1) yielding older and/or younger dates showing a gradual decrease with eU, in which case there would be no clear evidence of an impact age; or (2) revealing a plateau or a cluster of dates around ~350 Ma, making resetting and preservation of the impact age the most plausible justification.

6.3 Implications for crater chains in the Mesozoic

Asteroid breakups are caused by high speed collisions resulting from perturbations in their orbits, which can deliver fragments into Earth crossing orbits. Such events have previously been connected to an amplified influx of meteorites for millions of years following the breakup (Schmitz et al., 2001). These include an event in the

Middle Ordovician, which has been connected to the Lockne and Malingen craters is

Sweden (Schmitz et al., 2015) and another in the Late Eocene, which has been implicated in the formation of the Popigai and Chesapeake Bay impact structures in the United

States (Ormö et al., 2014). Nesvorny et al. (2015) have estimated ages for three asteroid families whose potential breakups may overlap with Santa Fe and the other ten craters:

Merxia (250 ± 100 Ma), Astrid (250 ± 100 Ma) and Erigone (300 ± 100 Ma).

27 The estimated ages of the ten craters mentioned in Table 1 are based on stratigraphy and paleomagnetism, but no geochronology has yet been attempted.

Accurately determining their ages may present further evidence in support of their relationship and offer another proxy for this kind of event. Additionally, establishing a link between asteroids and impact craters is possible through geochemical analysis of terrestrial samples, which usually focuses on the isotopic composition of siderophile elements such as Cr, Ni, Co and the platinum group elements (Simonson et al., 2009;

Foriel et al., 2013). These elements are relatively rare on the Earth’s surface and high concentrations of them are usually indicative of an extraterrestrial contaminant. Based on their isotopic signatures it is possible to match the crater to a specific type of impactor, provided that they are sufficiently well constrained. Such a study could help clarify the connection between these impacts and possible impactor bodies. Our understanding of the impacts in the solar system between the early and more recent phases has often been reserved to the Lunar record, since it is more well preserved than the Earth’s (Kelly et al., 2018). However, studying the Earth’s corresponding impact record could help elucidate our understanding of the interactions between solar system bodies.

7. Conclusion

U-Th/He dates for eleven zircons from the Santa Fe Impact Structure show a negative correlation with eU, consistent with higher radiation damage and subsequent increased He diffusivity in high eU zircons. The ZHe dates are consistent with multiple thermal events, some of which postdate the impact, causing a partial to complete overprint in the higher eU zircons. Apatite dates are all consistent with the Laramide

28 Orogeny, but three of the zircon dates are older than Laramide, which allows us to constrain the temperature conditions of heating during Laramide burial to between

100oC and 210oC. Future impact modeling coupled with thermal history modeling of ZHe and AHe dates, may further constrain temperature conditions: that experienced during

Laramide burial, and if temperatures reached during impact would have been sufficient to cause complete resetting of all zircon. The preservation of ZHe dates older than those corresponding to Laramide burial is a good indication that, if the impact age was recorded, it should be observed in zircons with low eU values. Further dating of low eU zircons is required to determine whether more grains have recorded an age near 334Ma and to help clarify the age of the impact.

Acknowledgements

I would like thank my advisor Stephen Mojzsis for sharing his amazing ideas and for giving me the opportunity to undertake this project. I would also like to thank Nigel

Kelly for all his generous and patient advice and for showing me how to navigate the laboratory. Additional thanks to James Metcalf for his help in the sample selection and preparation process, to Rebecca Flowers for the use of her laboratory, Oleg Abramov for producing the Impact Models and to my other committee members, Nick Schneider and Brian Hynek for their participation in this Honors Committee.

29 Supplementary Files

31 References

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