Detrital Shocked Zircon Provides First Radiometric Age Constraint (<1472 Ma) for the Santa Fe Impact Structure, New Mexico, U

Manuscript I 1 Detrital shocked zircon provides first radiometric age constraint 2 (<1472 Ma) for the Santa Fe impact structure, New Mexico, 3 USA 4 Pedro E. Montalvo1, Aaron J. Cavosie1,2, Christopher L. Kirkland2, Noreen J. Evans2, 5 Bradley J. McDonald2, Cristina Talavera3,Timmons M. Erickson4, Cristina Lugo-Centeno5 6 1Department of Geology, University of Puerto Rico, P.O. Box 9000, Mayagüez, Puerto Rico 7 00681-9000, USA 8 2The Institute for Geoscience Research (TIGeR), John de Laeter Centre, School of Earth and 9 Planetary Sciences, Curtin University, Perth, WA 6102, Australia 10 3School of Geosciences, University of Edinburgh, The King’s Building, James Hutton Road, EH9 11 3FE, Edinburgh, UK 12 4 Center for Lunar Science and Exploration, Lunar and Planetary Institute - USRA, Houston, TX, 13 77058, USA 14 5Department of Earth Sciences, Syracuse University, Syracuse, NY, 13244, USA 15 Date: April 23, 2018 16 ABSTRACT 17 The Santa Fe structure in northern New Mexico is one of the few confirmed impact craters in the 18 western USA. The history of the impact structure is obscure as it is tectonized and eroded to the 19 extent that an intact crater is not preserved, and what remains is located in a complex geological 1 20 setting. Shatter cones and shocked quartz were previously cited to confirm an impact origin, 21 however estimates for both impact age (350 – 1200 Ma) and crater diameter (6 – 13 km) remain 22 poorly constrained. To further evaluate the extent of shock deformation, we investigated ~6600 23 detrital zircon grains for shock features, using material collected from fifteen drainages and other 24 sites within a ~5 km radius of known shatter cone outcrops. Six detrital shocked zircon grains 25 were found at three locations, including two near shatter cones and one near brecciated granitoid. 26 Follow-up studies of bedrock at two sites proximal to detrital shocked zircon occurrences led to 27 the discovery of shocked zircon in situ in a shatter cone-bearing sample of biotite schist; shocked 28 grains were not found in brecciated granitoid at the second site. Electron backscatter diffraction 29 confirms the presence of {112} shock-twin lamellae in five shocked zircon grains, and secondary 30 ion mass spectrometry U-Pb data for three detrital shocked grains yielded 207Pb/206Pb 31 crystallization ages from 1715±22 to 1472±35 Ma. Laser ablation-inductively coupled plasma 32 mass spectrometry U-Pb ages for detrital zircon grains at five of the investigated sites provide 33 the first broad constraints on the local distribution of Paleo- to Mesoproterozoic bedrock in the 34 area. The presence of shock-twinned zircon indicates that some exposed rocks at the Santa Fe 35 structure record impact pressures up to ~20 GPa, which is higher than previous reports of ~10 36 GPa based on planar deformation features in shocked quartz. The 1472±35 Ma date from a 37 shock-twinned zircon yields the first direct radiometric maximum age constraint on the Santa Fe 38 impact event, and expands the possible time period for impact to the Mesoproterozoic. 39 Identification of shocked zircon in modern sediment led to the first discovery of shocked zircon 40 in bedrock at this site, which is notable, as shocked zircon is otherwise not abundant in the 41 studied rock samples. This study thus illustrates that detrital zircon surveys are an efficient way 2 42 to search for diagnostic evidence of shock deformation at putative impact structures where 43 shocked minerals may be present, but are not abundant in the exposed bedrock. 44 INTRODUCTION 45 The Meteorite Impact Record of Earth 46 Meteorite impacts are a major geologic process in the solar system, however only 190 47 terrestrial impact structures have been confirmed (Earth Impact Database, 2018). On Earth, 48 impact structures are susceptible to erosion, burial, and tectonic deformation, which provides 49 motivation to develop new methods for reconstructing the terrestrial impact record, particularly 50 for events that occurred during the Precambrian. The oldest evidence of terrestrial impact 51 processes are spherule deposits up to 3470 Ma in South Africa and Australia (Simonson et al. 52 2000; Byerly et al., 2002; Lowe et al., 2003; Koeberl, 2006; Glikson et al., 2016), some of which 53 contain shocked quartz (Rasmussen and Koeberl, 2004). The largest (~250 – 300 km) and oldest 54 impact structures on Earth are the 2020 Ma Vredefort Dome (e.g., Kamo et al., 1996) and the 55 1850 Ma Sudbury Basin (e.g., Krogh et al., 1984). Of the 190 confirmed impact structures, only 56 20 – 25 potentially formed in the Precambrian, although many have poorly constrained ages 57 (Jourdan et al., 2009; Earth Impact Database, 2018). In this study we present the results of a 58 detrital zircon survey designed to search for new evidence of shock deformation at the Santa Fe 59 impact structure in New Mexico, one of two potential Precambrian impact structures in the USA. 60 Evidence of Impact – Shocked Minerals 61 Meteorite impacts produce shock waves that cause instantaneously high pressures (10s – 62 100s GPa) and temperatures in target rocks and cause shock metamorphism (Melosh, 1989). The 63 brief yet extreme conditions form microstructures diagnostic of shock in some minerals (French, 3 64 1998). Impact-generated microstructures can be manifest in minerals such as zircon, quartz, 65 feldspars, and others (e.g., Ferrière and Osinski, 2013). Shocked minerals thus provide diagnostic 66 evidence of hypervelocity-driven deformation and can be used to confirm an impact event 67 (French and Koeberl, 2010). 68 Shock Deformation of Zircon 69 Planar microstructures, including planar fractures and planar deformation features, are the 70 most common impact-related microstructures described in zircon (e.g., Krogh et al., 1984; Bohor 71 et al., 1993; Erickson et al., 2013a), and form by 20 GPa in shock experiments (Leroux et al., 72 1999). We use the term planar fracture (PF) to describe planar microstructures when imaged on 73 external grain surfaces using backscattered electron (BSE) imaging (e.g., Bohor et al., 1993), and 74 employ additional genetic terminology to describe features quantified by electron backscatter 75 diffraction (EBSD) on polished surfaces (e.g., Erickson et al., 2013a). 76 Zircon {112} twin lamellae are considered diagnostic evidence of shock deformation, as 77 they have only been reported in grains from impact environments, and have been produced in 78 static experiments at 20 GPa (Morozova et al., 2018). Control of twin formation along {112} has 79 been attributed to elastic anisotropy of zircon, as the lowest values of shear modulus (G) and 80 Poisons ratio (ν) occur in the shear direction of twinning (η1, <111>) (Timms et al., 2018). 81 Shock-twinned zircon has been identified at several impact structures using EBSD, including 82 Vredefort (Moser et al., 2011; Erickson et al., 2013a,b; Cavosie et al., 2015a; Erickson et al., 83 2016; Erickson et al., 2017a; Montalvo et al., 2017; Cavosie et al., 2018a; Timms et al., 2018), 84 Sudbury (Thomson et al., 2014), Ries (Erickson et al., 2017b), Rock Elm (Cavosie et al., 2015b), 85 and in impact breccia from the Moon (Timms et al., 2012). The {112} twins occur as closely- 86 spaced (~5 µm) sub-micrometer-wide lamellae misoriented 65° about <110> relative to the host 4 87 zircon (Moser et al., 2011), and occur in up to four orientations in individual grains (Erickson et 88 al., 2013a; Cavosie et al., 2015a; Cavosie et al., 2018a; Timms et al., 2018). Studies have shown 89 that shock-twinned zircon generally does not record impact-age resetting (Erickson et al., 2013b; 90 Cavosie et al., 2015a; Montalvo et al., 2017; Cavosie et al., 2018a), although grains affected 91 post-impact by a thermal or fluid pulse can experience partial loss of radiogenic Pb (Moser et al., 92 2011). 93 The high-pressure ZrSiO4 polymorph reidite is also diagnostic of shock deformation, as it 94 forms at pressures above 30 GPa in shock experiments (Kusaba et al., 1985; Leroux et al., 1999) 95 and has been documented in several impact environments (e.g., Wittmann et al., 2006; Reddy et 96 al., 2015; Cavosie et al., 2015b; Erickson et al., 2017b). Shocked zircon that subsequently 97 experiences high temperature conditions can form a polycrystalline, or granular, texture. 98 Granular zircon consists of aggregates of recrystallized neoblasts ranging from ~1 to ~100 µm in 99 diameter (e.g., Bohor et al., 1993), and has been used to date impact events (e.g., Kamo et al., 100 1996; Moser et al., 2011; Cavosie et al., 2015a). The formation conditions of a granular texture 101 in zircon have not been constrained experimentally, however, orientation analysis indicates 102 formation at high temperature after shock decompression, with many granular zircon grains from 103 impact melt rocks preserving evidence for the former presence of reidite (Cavosie et al., 2016a; 104 Timms et al., 2017; Cavosie et al., 2018b). At temperatures >1673 °C zircon dissociates into 105 constituent oxides (El Goresy, 1965; Timms et al., 2017). In the absence of diagnostic shock 106 deformation or misorientation relations that record characteristic impact-related phase 107 transformations (e.g., Timms et al., 2017), granular zircon alone is not considered diagnostic of 108 impact. Metamorphic and/or tectonically deformed zircon may also show a granular texture (e.g., 5 109 Piazolo et al., 2012; Cavosie et al., 2015a) similar in appearance to that generated in impact 110 environments. 111 THE SANTA FE IMPACT STRUCTURE 112 Regional Geologic Setting 113 The Santa Fe structure is one of only five confirmed impact structures in the western 114 USA (Earth Impact Database, 2018). It is located in the Sangre de Cristo Mountains of northern 115 New Mexico, which host a geological archive of the major tectonic events that influenced crustal 116 growth and modification in the region over the last two billion years (Fig.

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