Abstract

Deformed 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 microstructures 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 diffraction 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 microstructures. 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.

INTRODUCTION

Reconstructing the early terrestrial bombardment history in part involves reconciling ages of zircons in lunar breccias and Hadean detrital zircons (Marchi et al., 2014). Both lunar 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 geochronology, even using samples from known crater environments (Jourdan et al., 2009). To determine if ex situ zircons can date impacts, here we evaluate detrital shocked zircons sourced from an impact of known age to identify microstructures age-reset by impact. We then present evidence 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 ZIRCON

Three zircon morphotypes are used to date impact processes: (1) shocked zircons containing planar microstructures, including planar fractures, planar deformation features, planar deformation bands, microtwins, and reidite lamellae; (2) granular zircons that have recrystallized 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 ZrSiO4 polymorph reidite (Cavosie et al., 2015; Reddy et al., 2015) appear to be the most diagnostic of shock deformation in zircon, as clear differences between shock-induced planar features 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 commonly partially age-reset, whereas granular zircons are commonly completely age-reset. Unshocked zircons from impact melts generally 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 METHODS

Data 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 Paleozoic 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 interior features were characterized using backscattered electron and cathodoluminescence (CL) imaging and electron backscatter diffraction (EBSD) mapping. Age determinations were made by sensitive high-resolution ion microprobe (SHRIMP). Details of analytical conditions, U-Pb results, and additional sample information are presented in the GSA Data Repository1.

RESULTS

Detrital Shocked Zircon with Microtwins

Zircon 24 has planar fractures on the exterior and a disturbed CL pattern (see the Data Repository). EBSD data show that the grain preserves ∼35° of cumulative misorientation accommodated 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 (2σ, 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 Vredefort 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 typically 1–10 µm across, have a dominant [001] rotation axis (see the Data Repository), and are misoriented <2°, producing cumulative misorientation of 8° across the grain (Fig. 3C). They are similar to planar deformation bands (Kovaleva et al., 2015), however they correlate to microstructures 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 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 constrained, but similar to a 2867 ± 15 Ma population of Vredefort shocked zircons (Erickson et al., 2013b) (Fig. 3D). In contrast, three analyses of neoblasts overlap and are concordant, yielding a weighted mean 207Pb/206Pb age of 1975 ± 39 Ma (2σ, 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 Texture

Zircon 205 is composed of granules ranging from 14 µm to 150 µm (average = 54 µm) (Fig. 3E). Most granules contain a core-and-rim microstructure; cores preserve oscillatory zoning 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 misorientation) (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 Repository). A core yielded a slightly reversely discordant 207Pb/206Pb age of 1812 ± 28 Ma (2σ) with Th/U = 0.68, whereas a dark CL rim yielded a concordant 207Pb/206Pb age of 1025 ± 24 Ma (2σ) with Th/U = 0.02 (Fig. 3H). Nearly identical 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 Metamorphic 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 crater environments. In contrast, the use of ex situ zircon to date terrestrial impacts is largely untested (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 planar microstructures are generally unaffected or only partially age-reset by shock. Pb loss in planar microstructure domains can be so irregular that discordia regressions need to be “anchored” to a lower concordia intercept based on independent 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). Partial age resetting, discordance, and non-systematic Pb loss clearly present challenges for dating impacts with ex situ zircons. Regardless, results for zircon 3 (Fig. 3) demonstrate that it is possible to date an impact with an ex situ shocked zircon if both diagnostic shock features (e.g., microtwins) and age-reset domains (e.g., neoblasts) 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 neoblasts. Differences in mean misorientation of granules (44° for zircon 3, and 6° for zircon 205) (Fig. 4) suggest different formation mechanisms. The orientations of neoblasts in zircon 3 form broad clusters similar to crystallographic 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 migration 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 magnitude of misorientation is limited (Fig. 4B). The preservation of igneous zoning and older cores is evidence that the host grain did not recrystallize, 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 formation of granular rims during ca. 1030 Ma granulite facies metamorphism (Robb et al., 1999).

Implications for Ages of Deformed Lunar Zircons

Zircons in lunar breccias are similar to detrital 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 zircons (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 microstructures in tectonically deformed zircon (Kovaleva et al., 2015), but have been interpreted 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 metamorphism (Heaman and LeCheminant, 1993), and as shown here, are not unique to impact environments. 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 features 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 highlights 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 remain 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 zircons, which may provide further insights into the crustal evolution of the Moon.

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

1GSA 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.