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