(U-Th)/He dating of terrestrial impact structures: The Manicouagan example

Authors


Abstract

The accurate dating of meteorite impact structures on Earth has proven to be challenging. Melt sheets are amenable to high-precision dating by the U-Pb and 40Ar/39Ar methods, but many impact events do not produce them, or they are not preserved. In cases where high-temperature shock metamorphism of the target materials has occurred without widespread melting, these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact. However, the (U-Th)/He chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites. We report (U-Th)/He dates for accessory minerals from the Manicouagan impact structure in Quebec, Canada. Nine zircons from a melt sheet sample yield a weighted mean age of 213.2 ± 5.4 Ma (2SE), indistinguishable from the published 214 ± 1 Ma (2σ) U-Pb zircon age for the impact. In contrast, five apatites from this sample yield dates between 205.9 ± 6.5 and 162.0 ± 5.3 Ma (2σ), indicating variable postimpact helium loss due to low-temperature thermal disturbance. Preimpact titanite crystals from a shocked meta-anorthosite sample yield two dates consistent with the impact age, at 212 ± 27 and 214 ± 13 Ma (2σ), and two younger dates of 189.6 ± 6.9 and 192.2 ± 9.8 Ma (2σ), suggestive of postimpact helium loss. These results indicate that (U-Th)/He chronometry is a suitable method for dating impact events, although interpretation of the results requires recognition of possible 4He loss related to reheating subsequent to impact.

1. Introduction

Accurate determinations of the ages of meteorite impact craters on Earth are valuable for determining the projectile flux to our planet, for constraining statistical models of impact risks in the future, and for exploring the causes of major biological extinction and adaptive radiation episodes in Earth history. Isotopic ages have been interpreted for about half of the 178 confirmed impact structures on Earth (Earth Impact Database, http://www.unb.ca./passc/ImpactDatabase/, Planetary and Space Science Centre, University of New Brunswick, accessed January 2011). However, the statistical precision of most of these dates is relatively poor; fewer than 10% of all known terrestrial impact structures can be regarded as both accurately and precisely dated [Jourdan et al., 2009]. All robust ages in the literature were acquired on fresh impact glass, pseudotachylites, or neoblastic minerals from impact melts. Samples experiencing shock metamorphism with little or no associated melt production typically yield U-Pb, 40Ar/39Ar, and Rb-Sr dates significantly older than the assumed age of the impact event, indicative of partial or no resetting of these geochronometers [e.g., Deutsch and Schärer, 1994; Kelley, 2007; Jourdan et al., 2009]. Unfortunately, pristine impact melt rocks are scarce at many impact structures, making precise dating difficult. Melt glass, where formed, is easily altered during impact-related hydrothermal activity and postimpact weathering [Kelley, 2007], while crystalline melt sheets are best developed in the larger (>25 km diameter) impact structures [Kelley, 2007].

One promising isotopic technique for dating the majority of poorly dated or undated impact structures is based on the production of 4He by radioactive decay of U, Th, and (to a much lesser extent) Sm isotopes. The (U-Th)/He method has been applied most commonly to the accessory phases zircon and apatite, and to a lesser extent titanite, all of which occur in many target rocks at terrestrial impact structures as igneous, metamorphic, or detrital grains. These chronometers are especially attractive for impactite dating because the diffusivity of radiogenic helium in zircon, titanite, and apatite is significantly faster than the diffusivity of radiogenic Ar, Sr, or Pb in most datable minerals at the temperatures likely encountered by target materials during impact events [Farley, 2000; Reiners and Farley, 1999; Reiners et al., 2004; Reiners, 2009]. Thermal effects related to shock metamorphism have been proposed as the principal cause of helium loss in apatite in Martian meteorites. Numerical models indicate that the thermal effects (up to 300°C) of shock pressures in excess of 30–40 GPa should result in near total resetting of the (U-Th)/He apatite geochronometer [Min et al., 2004; Schwenzer et al., 2008]. Such pressures may be realized in the centers of target regions during relatively small impact events that do not show evidence of widespread melting [Melosh, 1989].

2. Resetting Kinetics of (U-Th)/He Chronometers During Impact Events

Most discussions in the geologic literature of (U-Th)/He thermochronometer resetting have focused on timeframes and temperature regimes pertinent to orogenic activity (i.e., cooling rates of ≤10°C/Myr), so it is useful to consider resetting in the context of hypervelocity impact (i.e., seconds to minutes duration at temperatures up to several thousand degrees). Table 1 shows the results of modeling impact-induced 100% volume diffusive loss of 4He from three minerals (zircon, titanite, and apatite) that were dated in this study. These calculations assume that the effective diffusive loss dimensions were equivalent to the grain size, which was taken as the average size of the grains analyzed, but we hasten to add that one effect of shock metamorphism on relict crystals from target rocks may be physical damage of the crystal structure and the development of intracrystalline fast diffusion pathways (e.g., microfractures). In such instances, the resetting times shown in Table 1 could be overestimates. Table 1 shows how long it would take to reset (U-Th)/He thermochronometers, with commonly encountered grain sizes of 100 to 200 μm, at temperatures from 300°C to 10,000°C, spanning the probable range of temperatures realized in an impact target region. At temperatures of 1000°C or greater, which might easily be reached for short periods of time throughout the central near-surface target region, even the most retentive of these minerals (zircon) would be fully reset in, at most, a few tens of minutes. However, at temperatures significantly lower than 1000°C, which might be encountered at deeper levels beneath the impact structure or near the crater's margins, complete resetting takes significantly longer, up to thousands of years for zircon and titanite at 300°C. These calculations suggest that (U-Th)/He geochronology of zircon and apatite from impact materials may yield impact ages for some samples but not for others, depending on the specific impact-related thermal history experienced by the sample.

Table 1. (U-Th)/He Thermochronometer Resettinga
MineralTemperature (°C)Radius (μm)Resetting Time
  • a

    Calculations for resetting time (the time necessary for 100% 4He loss at a constant temperature) were based on Crank [1975, equations 5.23 and 6.20], with an assumed cylindrical diffusion geometry for apatite and a spherical geometry for zircon and titanite. Diffusion parameters were from Reiners and Farley [1999], Farley [2000], and Reiners et al. [2004] for titanite, apatite, and zircon, respectively. Values were calculated for grain sizes based on the average grain half widths analyzed during this study and 100 μm as a reasonable upper bound for grain half widths that might be encountered in impactite rocks.

Zircon10,000500.00028 s
 1,000505.63 min
 600505.90 days
 300503.19 kyr
 10,0001000.00114 s
 1,00010022.53 min
 60010023.58 days
 30010012.76 kyr
Titanite10,000500.0000027 s
 1,0005013.86 s
 600500.52 days
 300500.99 kyr
 10,0001000.000011 s
 1,00010055.45 s
 6001002.07 days
 3001003.98 kyr
Apatite10,000500.0000050 s
 1,000500.044 s
 600502.85 min
 3005040.56 days
 10,0001000.000020 s
 1,0001001.76 s
 60010011.38 min
 300100162.24 days

Even if the (U-Th)/He chronometers are reset by impact, the accuracy of the dates depends on the duration of elevated thermal conditions related to impact. Impact-related thermal anomalies may persist for tens of thousands or perhaps even up to a million years, depending on the size and velocity of the impactor and whether or not large hydrothermal systems were developed in response to the impact [Ivanov, 2004; Abramov and Kring, 2007]. Under such circumstances 4He might continue to be lost from the minerals for a significant time after the impact event, and (U-Th)/He chronometers may yield postimpact cooling ages that do not match the impact age. The cooling ages may vary from crystal to crystal because of the specific 4He diffusive characteristics in each grain; those characteristics are not only dependent on the intrinsic diffusivity of 4He in a mineral but also on any enhanced 4He diffusivity that might be related to shock-induced structural damage.

3. Dated Samples

We have explored the viability of (U-Th)/He impact dating by investigating materials collected at the well-studied ∼90 km diameter Manicouagan impact structure located in northern Quebec, Canada [e.g., Onorato et al., 1978; Simonds et al., 1978; Grieve and Head, 1983; Spray et al., 2010] (Figures 1a and 1b). The Manicouagan structure features a well-preserved, up to 1.4 km thick, crystalline impact melt sheet that has been subdivided into three lithologic units based on mineralogy and chemistry [Floran et al., 1978; O'Connell-Cooper and Spray, 2011]. The melt sheet has been breached by a central, horseshoe-shaped uplift, exposing Precambrian target rocks (Figure 1b) that are predominantly meta-anorthosites [Spray and Thompson, 2008]. Evidence of impact shock effects are common, including the local transformation of plagioclase to maskelynite (indicative of shock pressures of 25–35 GPa [e.g., Kieffer et al., 1976]) in heterogeneously developed shock veins that also contain stishovite, which limits the maximum temperature that the bulk of the central uplift rocks have experienced to <400°C [Biren and Spray, 2011].

Figure 1.

Location maps of the impact structure and dated samples (modified after Spray and Thompson [2008]). (a) Regional setting of the Manicouagan impact structure in Quebec, Canada. (b) Simplified geological map of the Manicouagan impact structure. The melt sheet sample location, MANIC-12, is at 51°21.114′N, 68°32.982′W, and the shocked basement sample, DS-16, is at 51°24.853′N, 68°42.399′W.

Our work focuses on accessory phases from both the crystalline impact melt sheet and central uplift. By dating neoblastic minerals that have crystallized from the impact melt, we compare (U-Th)/He results to previously published constraints on the age of impact obtained using other geochronometers. By dating shocked, preimpact minerals from the central uplift, we examine the capacity of impact-related processes to reset (U-Th)/He thermochronometers.

Sample MANIC-12, a quartz monzodiorite from the crystalline impact melt sheet, was collected from the eastern inlet of the reservoir (Figure 1b). It consists of pyroxene, plagioclase, K-feldspar, quartz, Fe-Ti oxides, and olivine pseudomorphs. This sample displays a hypidiomorphic granular texture with a typical grain size in excess of 1 mm. Both zircon and apatite are relatively abundant accessory minerals in the quartz monzodiorite (with grain sizes up to 200 μm). Zircons from this unit have yielded easily interpretable U-Pb Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) dates that serve as the best available constraint on the age of impact. Hodych and Dunning [1992] reported two 206Pb/238U dates of 214.5 ± 0.75 (2σ – unless otherwise noted all errors are 2σ) and 213.6 ± 0.63 Ma for multigrain fractions of air-abraded zircons collected from the melt sheet. These results form the basis of the commonly accepted 214 ± 1 Ma age of the Manicouagan impact event [Hodych and Dunning, 1992; Jourdan et al., 2009]. Refinement of the age through single-zircon, chemical abrasion ID-TIMS U-Pb geochronology is in progress, with preliminary results converging at a circa 215.5 Ma age [Ramezani et al., 2005].

The central uplift sample (DS-16 (Figure 1b)) is a gneissose, medium- to coarse-grained meta-anorthosite, comprising plagioclase, pyroxene, hornblende, and garnet. No zircon was found in this rock, and apatite is rare. As a consequence, we focused our (U-Th)/He studies on titanite, an abundant accessory mineral in the sample. It is estimated that the rocks exposed in the central uplift were exhumed within minutes from 8 to 10 km depth during impact. Assuming a reasonable preimpact geothermal gradient, these rocks were likely to have been at ambient temperatures close to the bulk closure temperature of the (U-Th)/He titanite chronometer (∼190–220°C [Reiners and Farley, 1999]). The central uplift is estimated to have experienced ∼100°C of bulk shock-related heating as a consequence of impact, with much higher temperature excursions being realized in localized shock veins [Biren and Spray, 2011].

4. (U-Th)/He Methodology and Results

We prepared nine zircon and five apatite crystals from the MANIC-12 sample and four titanite crystals from DS-16 for (U-Th)/He geochronology in the NG3L (Noble Gas Geochronology and Geochemistry Laboratory) at Arizona State University. Figures 2a and 2b illustrate the typical appearance of the euhedral and apparently inclusion-free apatite and zircon grains. To allow for correction of the measured (U-Th)/He dates for 4He loss due to alpha ejection [Farley et al., 1996; Hourigan et al., 2005], grain dimensions were measured for each apatite and zircon crystal prior to analysis following procedures described in Text S1 in the auxiliary material. Because no euhedral titanites could be procured from DS-16, we abraded the four separated titanite crystal shards prior to analysis (using compressed air and a granular pyrite abrasive) in order to remove the outer layer of each shard, and thus eliminated the need for an alpha ejection correction (Figures 2c and 2d). All analytical data are presented in Table S1 in the auxiliary material.

Figure 2.

Photos of the typical appearance of analyzed grains. (a) Photo of a typical euhedral zircon grain from MANIC-12 that was analyzed showing the measurements taken for the alpha ejection correction. (b) Photo of a typical euhedral apatite grain from MANIC-12, which is missing one of its terminations, also showing the measurements typically taken for the alpha ejection correction. In both photos dimensions are in μm. (c) Photo of a typical titanite shard from DS-16, prior to abrasion. (d) Photo of a typical titanite grain from DS-16 after abrasion.

The nine zircon crystals yield ejection-corrected (U-Th)/He dates ranging from 201.1 ± 6.6 to 222.1 ± 6.9 Ma (Figure 3), with a weighted mean of 213.2 ± 2.3 Ma (2 Standard Error [SE], n = 9) calculated with ISOPLOT 3.7 [Ludwig, 2008]. These data collectively yield a Mean Square of Weighted Deviates (MSWD) of 4.1, a value that is in excess of the expected value considering the assumed analytical imprecision of each analysis [Wendt and Carl, 1991]. Such dispersion is a common characteristic of (U-Th)/He data sets, and in this case is most likely caused by grain-to-grain U and Th zoning that complicate the schema typically employed for the alpha ejection correction [cf. Hourigan et al., 2005]. Following convention, a more realistic estimation of the uncertainty introduced by the excessive dispersion of the dates can be calculated by multiplying the calculated error in the weighted mean date by the square root of the MSWD and Student's t test for N-1 (8) degrees of freedom [Ludwig, 2008], yielding a preferred date and error of 213.2 ± 5.4 Ma (the standard error at the 95% confidence level). An alternative approach to establishing the average of multiple, single-grain (U-Th)/He dates is to apply the “central age” method of Vermeesch [2008]. The slightly different “central age” for the Manicouagan zircon data set is 213.5 ± 5.2 Ma (2 SE). Both the mean and central estimates are statistically indistinguishable from the accepted U-Pb zircon constraint on the age of the impact [Hodych and Dunning, 1992], as well as the U-Pb zircon age obtained through chemical abrasion analysis [Ramezani et al., 2005]. We conclude that the MANIC-12 zircons have quantitatively retained 4He since their crystallization following the Manicouagan impact event, and that the (U-Th)/He zircon weighted mean date represents the age of impact.

Figure 3.

Individual zircon, titanite, and apatite (U-Th)/He age distribution plot. A log ratio plot showing the (U-Th)/He age data as 2σ error envelopes for the individual zircon (white ellipses), titanite (light gray ellipses), and apatite (dark gray ellipses) crystals. The zircon (U-Th)/He central age is also shown (black ellipse). The plot was made using Helioplot [Vermeesch, 2010].

The five apatite crystals yield a scattering of (U-Th)/He dates ranging from 205.9 ± 6.5 Ma to 162.0 ± 5.3 Ma (Figure 3), all of which are significantly younger than the age of the impact. As can be seen in Figure 2b, these crystals were missing one or both terminations (two missed both terminations and three missed one termination). However, given a length-to-radius aspect ratio of greater than five for all crystals (Table S1 in the auxiliary material), the absence of terminations should have negligible influence on the alpha ejection correction [Farley et al., 1996]. Thus we have no reason to attribute the young apatite dates to error associated with the alpha ejection correction. The anomalously young and variable apatite dates are reasonably interpreted as reflecting partial 4He loss related to one or more postimpact thermal events. Variations in partial 4He loss from the apatites can be caused by (1) variation in 4He diffusivity from grain to grain as a consequence of structural damage related to radiation [e.g., Shuster et al., 2006] and/or (2) U and Th zoning variations from grain to grain [cf. Farley et al., 1996]. Without more information regarding the actual 4He diffusivity in each grain, it is impossible to assign specific age significance to each date.

All four preimpact titanites from the central uplift sample have low U+Th concentrations and thus yielded comparatively imprecise (U-Th)/He dates. Two crystals yielded dates of 212 ± 27 Ma and 214 ± 13 Ma, which are within error of the impact age. The other two dates are significantly younger, at 189.6 ± 6.9 Ma and 192.2 ± 9.8 Ma. Our tentative interpretation of this bimodal distribution is that the 4He diffusion kinetics are slightly different in the two populations. Results from the first two titanites suggest that the central uplift rocks were effectively quenched during impact-related exhumation and record the age of that process. Like the younger apatite dates from Manic-12, the two younger titanite dates probably reflect partial 4He loss due to one or more postimpact thermal events. One reason why these titanites were more susceptible to postimpact 4He loss may be physical damage of the crystal structure and the development of intracrystalline fast diffusion pathways due to shock metamorphism. A broader study of how shock damage might affect (U-Th)/He titanite geochronology at Manicouagan is currently in progress, which should help evaluate this hypothesis.

5. Discussion

Our results demonstrate that the (U-Th)/He zircon chronometer is a viable method for dating neoblastic zircons from impact melt sheets, although the intrinsic analytical precision for this method is poorer than that of the U-Pb and 40Ar/39Ar methods. However, the most valuable future application of the technique is in dating preimpact target minerals, found in many impact-related rocks such as suevites and other associated breccias, that were strained and heated significantly (Table 1), but not melted or recrystallized by the impact event. In addition, central uplift rocks in larger impact craters may be rapidly exhumed from depths at which they resided, above their (U-Th)/He closure temperatures, prior to the impact event. Based on the total resetting time estimates presented in Table 1 for apatite, titanite and zircon, the helium isotopic systematics of such preimpact crystals with grain sizes up to 200 μm, and possibly greater, are likely to be reset in target materials collected at terrestrial craters with diameters of more than a few kilometers. Along with international collaborators, NG3L researchers are exploring the suitability of the (U-Th)/He method for dating even smaller terrestrial impacts with promising results (e.g., recent work on the 350 m diameter Monturaqui impact crater in southern Chile [Ukstins-Peate et al., 2010], and the 7.6 km Wetumpka marine impact structure [Wartho et al., 2011]). Other accessory minerals that can be used for (U-Th)/He dating (e.g., xenotime, magnetite, epidote, and monazite [Reiners, 2009]) might also be appropriate for such studies.

Despite the viability of this method, the interpretation of (U-Th)/He dates for impact materials is not always straightforward. Thermal anomalies associated with large impact events may persist for tens of thousands up to a million years [Ivanov, 2004; Abramov and Kring, 2007], such that the dated minerals may continue to experience diffusive 4He loss after the impact event. Fortunately, typical precision bounds for (U-Th)/He dates are on the same order or greater as the decay time scales for many postimpact thermal anomalies. In that sense, significant postimpact heating might actually be instrumental in completely resetting (U-Th)/He systematics of grains that did not experience significant heating during the impact itself. (U-Th)/He impactite zircon and titanite dates are likely to correspond to impact ages within error, unless the zircons and titanites have experienced a radiogenic 4He loss event associated with subsequent thermal processes unrelated to impact, or unless the impact-related heating persisted for an unusually long time.

The apatite and younger titanite (U-Th)/He dates for Manicouagan underscore the importance of careful evaluation of the possibility of postimpact thermal events when interpreting (U-Th)/He data from impactites. Such results also demonstrate the need for a better understanding of the influence of impact-related shock and thermal effects on the helium diffusivity in minerals used for (U-Th)/He dating. Postimpact helium loss can be especially problematic for apatites, which have higher intrinsic 4He diffusivity than zircons [Farley, 2000; Reiners et al., 2004; Reiners, 2009].

Acknowledgments

We are grateful to B. Adams for help in the Noble Gas Geochemistry and Geochronology Laboratory at ASU. This work was funded from startup funds to K.V.H. by ASU and in part by U.S. National Science Foundation grant EAR-0948143 to J.-A.W., M.v.S., and K.V.H. M.B.B., J.G.S., and L.M.T. are funded via NSERC, Canada Research Chair, and Canadian Space Agency grants in support of the Manicouagan Impact Research Program, with additional support for M.B.B. from the Barringer Family Fund for Meteorite Impact Research. We thank Editor L. Derry, D. Shuster, and two anonymous reviewers for their thoughtful comments and suggestions that helped improve an earlier version of this paper.

Ancillary