GEOLOGY | Volume 43 | Number 11 | www.gsapubs.org 999

A terrestrial perspective on using ex situ shocked zircons to date lunar impactsAaron J. Cavosie1,2,3, Timmons M. Erickson1, Nicholas E. Timms1, Steven M. Reddy1, Cristina Talavera4, Stephanie D. Montalvo3, Maya R. Pincus3, Ryan J. Gibbon5, and Desmond Moser6

1TIGeR (The Institute for Geoscience Research), Department of Applied Geology, Curtin University, Perth, WA 6102, Australia2NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA3Department of Geology, University of Puerto Rico–Mayagüez, Mayagüez, Puerto Rico 00681, USA4Department of Physics, Astronomy and Medical Radiation Sciences, Curtin University, Perth, WA 6102, Australia5Department of Anthropology, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada6Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada

ABSTRACTDeformed lunar zircons yielding U-Pb ages from 4333 Ma to 1407 Ma have been interpreted

as dating discrete impacts on the Moon. However, the cause of age resetting in lunar zircons is equivocal; as ex situ grains in breccias, they lack lithologic context and most do not contain mi-crostructures diagnostic of shock that are found in terrestrial zircons. Detrital shocked zircons provide a terrestrial analog to ex situ lunar grains, for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons. Electron backscatter dif-fraction and sensitive high-resolution ion microprobe U-Pb analysis of zircons eroded from the ca. 2020 Ma Vredefort impact structure (South Africa) show that complete impact-age resetting did not occur in microstructural domains characterized by microtwins, planar fractures, and low-angle boundaries, which record ages from 2890 Ma to 2645 Ma. An impact age of 1975 ± 39 Ma was detected in neoblasts within a granular zircon that also contains shock microtwins, which link neoblast formation to the impact. However, we show that granular texture can form during regional metamorphism, and thus is not unique to impact environments. These results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence to establish impact provenance, and then targeting specific age-reset microstruc-tures. With the recognition that zircon can deform plastically in both impact and magmatic environments, age-resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes rather than discrete impacts. Identifying shock microstructures that record complete age resetting for geochronological analysis is thus crucial for constructing accurate zircon-based impact chronologies for the Moon, Earth, or other planetary bodies.

INTRODUCTIONReconstructing the early terrestrial bom-

bardment history in part involves reconciling ages of zircons in lunar breccias and Hadean detrital zircons (Marchi et al., 2014). Both lu-nar and Hadean zircon populations consist of ex situ grains that have been separated from their original host rocks, and if shown to be shocked (Cavosie et al., 2010) offer the potential to date early impacts. However, terrestrial impacts are difficult to date with zircon U-Pb geochronol-ogy, even using samples from known crater environments (Jourdan et al., 2009). To deter-mine if ex situ zircons can date impacts, here we evaluate detrital shocked zircons sourced from an impact of known age to identify microstruc-tures age-reset by impact. We then present evi-dence showing that granular zircon is not unique to impact environments. Lastly, we discuss the implications of these results for dating impacts on the Moon with ex situ lunar zircons.

DATING IMPACTS WITH ZIRCONThree zircon morphotypes are used to date

impact processes: (1) shocked zircons contain-ing planar microstructures, including planar fractures, planar deformation features, planar

deformation bands, microtwins, and reidite la-mellae; (2) granular zircons that have recrystal-lized during impact to form neoblasts of variable size; and (3) unshocked zircons that crystallized in impact-generated melts (Wittmann et al., 2006). Microtwins (Moser et al., 2011; Timms et al., 2012; Erickson et al., 2013a) and the Zr-SiO4 polymorph reidite (Cavosie et al., 2015; Reddy et al., 2015) appear to be the most diag-nostic of shock deformation in zircon, as clear differences between shock-induced planar fea-tures and other planar microstructures that form in tectonically deformed zircon (e.g., Kovaleva et al., 2015) await to be resolved.

The largest terrestrial impacts, including Vredefort (South Africa), Sudbury (Canada), and Chicxulub (Mexico), have been dated using U-Pb analysis of multiple zircon morphotypes (Krogh et al., 1984, 1993; Kamo et al., 1996). Lead loss typically correlates with morphotype; zircons with planar microstructures are com-monly partially age-reset, whereas granular zircons are commonly completely age-reset. Unshocked zircons from impact melts gener-ally yield the most reliable ages (e.g., Moser, 1997). Unambiguous shock-wave ages are not consistently extractable from zircon because of

either incomplete U-Pb resetting during impact or subsequent Pb loss (e.g., Tohver et al., 2012; Schmieder et al., 2015).

SAMPLES AND METHODSData for three detrital zircons from South

Africa are presented (Fig. 1). Grain 07VD07-3 (zircon 3) is from alluvium in the core of the 2020 Ma Vredefort Dome impact structure. Grain 13DG08-24 (zircon 24) is from Paleo-zoic glacial tillite in the collar of the Vredefort structure. Grain 14VD80-205 (zircon 205) is from beach sand on the Atlantic coast at the mouth of the Orange River. Exterior and in-terior features were characterized using back-scattered electron and cathodoluminescence (CL) imaging and electron backscatter dif-fraction (EBSD) mapping. Age determinations were made by sensitive high-resolution ion microprobe (SHRIMP). Details of analytical conditions, U-Pb results, and additional sam-ple information are presented in the GSA Data Repository1.

1 GSA Data Repository item 2015335, analytical methods, electron backscatter diffraction conditions, and sample information, is available online at www .geosociety.org/pubs/ft2015.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

GEOLOGY, November 2015; v. 43; no. 11; p. 999–1002 | Data Repository item 2015335 | doi:10.1130/G37059.1 | Published online 1 October 2015

© 2015 Geological Society of America. For permission to copy, contact editing@geosociety.org.

N

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Figure 1. Simplified map of South Africa showing sample locations in the Orange River basin and regional impact structures. N—Namibia; B—Botswana; L—Lesotho; S—Swaziland; Z—Zimbabwe; M—Mozambique.

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RESULTS

Detrital Shocked Zircon with MicrotwinsZircon 24 has planar fractures on the exterior

and a disturbed CL pattern (see the Data Reposi-tory). EBSD data show that the grain preserves ~35° of cumulative misorientation accommo-dated through crystal-plastic deformation and fractures, resulting in non-systematic dispersion of crystallographic poles about {110} (Fig. 2A; see the Data Repository). The grain contains four orientations of microtwins that relate to the host by 65° misorientation about <110>. U-Pb analyses in the microtwin domain are highly discordant, however three of four are co-linear and define a discordia with an upper intercept 207Pb/206Pb age of 2645 ± 76 Ma (2s, mean square of weighted deviates [MSWD] = 0.83, n = 3) and a lower intercept age of -11 ± 31 Ma (Fig. 2B). While recent Pb loss may obscure evidence of older Pb loss, no indication of Vre-defort age resetting was detected.

Detrital Shocked Zircon with Microtwins and Granular Texture

Zircon 3 has granular texture on the exterior and two distinct CL domains (Figs. 3A and 3B).

The outer domain preserves igneous zoning, low-angle boundaries, and one orientation of microtwins. The boundaries or bands are typi-cally 1–10 µm across, have a dominant [001] rotation axis (see the Data Repository), and are misoriented <2°, producing cumulative misori-entation of 8° across the grain (Fig. 3C). They are similar to planar deformation bands (Kova-leva et al., 2015), however they correlate to mi-

crostructures visible in CL (Fig. 3B, arrow). The inner domain consists of neoblasts (12–97 µm across, average = 42 µm) with straight to curved high-angle boundaries that form ~120° triple junctions and protrude into the igneous domain. The neoblasts are bright in CL, cross-cut all other features, and are nearly strain free (<2° mean internal misorientation; Fig. 4A; see the Data Repository). Their orientations form broad

25 µm

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MSWD = 0.83 (n=3)

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Figure 2. Detrital zircon with planar micro-structures (grain 13DG08-24). A: Map show-ing crystallographic misorientation, relative to reference (red cross). Four microtwin (t) orientations are labeled. Circles are sensitive high-resolution ion microprobe (SHRIMP) spots. B: U-Pb results. Dashed ellipse was not included in regression. MSWD—mean square of weighted deviates.

Figure 3. Detrital zircons with granular texture (A–D: grain 07VD07-3; E–H: grain 14VD80-205). A: Exterior backscattered electron (BSE) image; arrows show triple junctions. B: Cath-odoluminescence (CL) image showing two domains and labeled U-Pb analysis spots. Left arrow indicates a planar microstructure visible in both CL and EBSD images. C: Map show-ing crystallographic misorientation relative to reference (red cross); neoblasts are gray. t—microtwin. D: U-Pb results for grain 07VD07-3. Red star is 2020 Ma impact age (Kamo et al., 1996; Moser, 1997; Gibson et al., 1997). E: Exterior BSE image. F: CL image showing granule core-and-rim structure and labeled U-Pb analysis spots. G: Map showing crystallographic misorientation relative to a reference (red cross). H: U-Pb results for grain 14VD80-205.

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clusters about principal axes of the host grain, with misorientation values ranging from 13° to 90° (average = 44°) (Fig. 4A). Three analyses in the microtwin–low-angle boundary domain are discordant and not co-linear; a weighted mean 207Pb/206Pb age of ca. 2890 Ma is poorly con-strained, but similar to a 2867 ± 15 Ma popula-tion of Vredefort shocked zircons (Erickson et al., 2013b) (Fig. 3D). In contrast, three analyses of neoblasts overlap and are concordant, yield-ing a weighted mean 207Pb/206Pb age of 1975 ± 39 Ma (2s, MSWD = 0.46, n = 3). This age is within uncertainty of the Vredefort impact age derived from impact melt zircons (Kamo et al., 1996; Gibson et al., 1997; Moser, 1997) (Fig. 3D). Average Th/U ratios from both domains show little variation (0.97 versus 0.85).

Detrital Zircon with Granular TextureZircon 205 is composed of granules rang-

ing from 14 µm to 150 µm (average = 54 µm) (Fig. 3E). Most granules contain a core-and-rim microstructure; cores preserve oscillatory zon-ing that can be traced across adjacent granules, whereas rims cross-cut igneous zoning (Fig. 3F). Individual granules (core and rim) are nearly strain free (<2° mean internal misorien-tation) (Fig. 3G) and their margins are defined by low-angle boundaries (average = 5.8°) (Fig. 4B) with various rotation axes, including [001], [112], [211], and [111] (see the Data Reposi-tory). A core yielded a slightly reversely discor-dant 207Pb/206Pb age of 1812 ± 28 Ma (2s) with Th/U = 0.68, whereas a dark CL rim yielded a concordant 207Pb/206Pb age of 1025 ± 24 Ma (2s) with Th/U = 0.02 (Fig. 3H). Nearly identi-cal ages have been reported for cores (1822 ±

36 Ma) and metamorphic rims (1032 ± 18 Ma) of zircons in a granulite facies orthogneiss from the Okiep copper district in the Namaqua Meta-morphic Complex (Fig. 1), a potential proximal source for zircon 205 (Robb et al., 1999).

DISCUSSION

Challenges of Dating Impacts with Ex Situ Shocked Zircons

Dating terrestrial impacts using multiple zircon morphotypes from intact rocks allows an evaluation of microstructure, Pb behavior, and age consistency in grains from known cra-ter environments. In contrast, the use of ex situ zircon to date terrestrial impacts is largely un-tested (cf. Moser et al., 2009). Results for the two Vredefort zircons presented here, together with previous studies, demonstrate that zircon domains containing microtwins and other pla-nar microstructures are generally unaffected or only partially age-reset by shock. Pb loss in pla-nar microstructure domains can be so irregular that discordia regressions need to be “anchored” to a lower concordia intercept based on inde-pendent knowledge of impact age (Kamo et al., 1996; Moser et al., 2011; Wielicki et al., 2014). Granular-textured zircons can yield an impact age (Moser, 1997); however, granular zircons are commonly discordant, show post-impact Pb loss, and yield ages younger than impact melt zircons (Krogh et al., 1993; Kamo et al., 1996; Tohver et al., 2012; Schmieder et al., 2015). Par-tial age resetting, discordance, and non-system-atic Pb loss clearly present challenges for dating impacts with ex situ zircons. Regardless, results for zircon 3 (Fig. 3) demonstrate that it is pos-sible to date an impact with an ex situ shocked zircon if both diagnostic shock features (e.g., microtwins) and age-reset domains (e.g., neo-blasts) are present.

Distinguishing Characteristics of Granular Zircon

The results for zircon 205 demonstrate that granular texture is not uniquely produced by impact. The granular components of zircons 3 and 205 have similar exterior appearances (Fig. 3), and both are low-strain domains that record younger events than their host grains. The most significant difference is that granules in zircon 205 are not neoblastic; they preserve inherited zoned cores surrounded by younger rims, whereas granules in zircon 3 are neo-blasts. Differences in mean misorientation of granules (44° for zircon 3, and 6° for zircon 205) (Fig. 4) suggest different formation mech-anisms. The orientations of neoblasts in zircon 3 form broad clusters similar to crystallo-graphic orientations in the host grain (Fig. 4A), and are consistent with energetically favorable nucleation at sites of former crystal defects, such as radiation damage, fluid inclusions, or

microtwin interfaces. Grain boundary migra-tion during neoblast growth clearly truncated preexisting igneous zoning. In zircon 205, granular texture only occurs in age-reset rims, high-angle boundaries are absent, and the mag-nitude of misorientation is limited (Fig. 4B). The preservation of igneous zoning and older cores is evidence that the host grain did not re-crystallize, which distinguishes these granules from similar looking “Z-grains” (Piazolo et al., 2012). The age, low Th/U ratio, and absence of shock deformation are consistent with forma-tion of granular rims during ca. 1030 Ma gran-ulite facies metamorphism (Robb et al., 1999).

Implications for Ages of Deformed Lunar Zircons

Zircons in lunar breccias are similar to de-trital zircons in that they are ex situ grains that have been separated from their source rocks. Dates for multiple lunar impacts, ranging from 4335 to 1407 Ma, have been proposed based on ages from individual zircons (Pidgeon et al., 2007; Nemchin et al., 2009; Zhang et al., 2012; Grange et al., 2011, 2013a, 2013b). However, most lunar zircons do not contain diagnostic shock features found in terrestrial shocked zir-cons (Pidgeon et al., 2011). Microtwins have been documented in only three lunar zircons (Timms et al., 2012; Crow et al., 2015), and reidite has not been reported from the Moon. Planar deformation bands in lunar zircons (Nemchin et al., 2009) are similar to micro-structures in tectonically deformed zircon (Kovaleva et al., 2015), but have been inter-preted as impact related owing to the assumed lack of tectonites on the Moon. Granular lunar zircons (e.g., Grange et al., 2013a, 2013b) may have formed by melt reactions or metamor-phism (Heaman and LeCheminant, 1993), and as shown here, are not unique to impact envi-ronments. The results presented here represent a perspective based on the idea that for ex situ zircons that lack context, confidently dating an impact requires the presence of an age-reset microstructure in a grain with diagnostic fea-tures known to occur in terrestrial shocked zircons. The recognition that crystal-plastic deformation of zircon can occur in magmatic environments (Reddy et al., 2009) further high-lights the need to apply consistent criteria for identifying and dating ex situ shocked zircons from any source. Until sufficient contextual and/or microstructural evidence is found that links age resetting in individual lunar zircons to shock deformation, ages in lunar zircons re-main equivocal. The role of magmatism (e.g., Valley et al., 2014), metamorphism (Gibson et al., 2002), and other processes, in addition to impact deformation, should be considered as potential causes of age resetting in lunar zir-cons, which may provide further insights into the crustal evolution of the Moon.

{001} {110}B. 14VD80-205 (n=36,558)

{001} {110}A. 07VD07-3 (n=411,235)

host grain neoblasts microtwins

Figure 4. Pole figures of detrital granular zir-cons. A: Data for grain 07VD07-3, color-coded by domain. Small mean internal misorienta-tion for individual neoblasts is evident from observation that each neoblast appears as nearly a single symbol, yet contains hundreds of poles. B: Data for grain 14VD80-205, show-ing systematic misorientation of granules. Projections are equal area, lower hemisphere.

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ACKNOWLEDGMENTSB. Hess, C. Johnson, W. Reimold, J. Valley, and J. Wooden provided assistance and access to facili-ties. R. Gibson, E. Kovaleva, and M. Wielicki pro-vided thoughtful reviews. Support was provided by the National Science Foundation (grant EAR-1145118), the NASA Astrobiology program, and the SHRIMP and Microscopy and Microanalysis facili-ties at Curtin University, Australia.

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Manuscript received 11 June 2015 Revised manuscript received 26 August 2015 Manuscript accepted 26 August 2015

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 Stephanie D. Montalvo, Maya R. Pincus, Ryan J. Gibbon and Desmond MoserAaron J. Cavosie, Timmons M. Erickson, Nicholas E. Timms, Steven M. Reddy, Cristina Talavera, impacts

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