& Planetary Science 44, Nr 5, 747–762 (2009) Abstract available online at http://meteoritics.org

40Ar-39Ar ages of H-chondrite impact melt breccias

Timothy D. SWINDLE1, 2*, Clark E. ISACHSEN2, John R. WEIRICH1, and David A. KRING3

1Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona 85721–0092, USA 2Department of Geosciences, The University of Arizona, Tucson, Arizona 85721–0077, USA 3Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, Texas 77058, USA *Corresponding author. E-mail: [email protected] (Received 19 April 2008; revision accepted 06 February 2009)

Abstract–40Ar-39Ar analyses of a total of 26 samples from eight shock-darkened impact melt breccias of H- affinity (Gao-Guenie, LAP 02240, LAP 03922, LAP 031125, LAP 031173, LAP 031308, NWA 2058, and ) are reported. These appear to record impacts ranging in time from 303 ± 56 Ma (Gao-Guenie) to 4360 ± 120 Ma (Ourique) ago. Three record impacts 300–400 Ma ago, while two others record impacts 3900–4000 Ma ago. Combining these with other impact ages from H in the literature, it appears that H chondrites record impacts in the first 100 Ma of solar system history, during the era of the “lunar cataclysm” and shortly thereafter (3500–4000 Ma ago), one or more impacts ∼300 Ma ago, and perhaps an impact ∼500 Ma ago (near the time of the disruption). Records of impacts on the parent body are rare or absent between the era of planetary accretion and the “lunar cataclysm” (4400–4050 Ma), during the long stretch between heavy bombardment and recent breakup events (3500–1000 Ma), or at the time of final breakup into -sized bodies (<50 Ma).

INTRODUCTION The first two of these topics are closely related to the idea of a “lunar cataclysm.” When the Apollo samples were Impacts have been occurring since the earliest history of returned from the Moon, two groups, working with different the solar system. For and many planets and moons, geochronology systems (K-Ar and U,Th-Pb) both noticed impact is the dominant geological process occurring on the evidence for widespread resetting of isotopic chronometers surface. However, we have samples to study from a relatively (Turner et al. 1973; Tera et al. 1974). Several studies in recent small number of known craters, only a few hundred (mostly years have addressed this topic either by studying rocks from recent) on Earth and a few on the Moon from which the Earth-Moon system (Dalrymple and Ryder 1991, 1993, astronauts returned samples. 1996; Cohen et al. 2000, 2005; Norman et al. 2005; Trail et al. As a result, some aspects of cratering are rather poorly 2006; Trail et al. 2007) or by considering possible causes for known, in particular, the general evolution of the flux of a large, but short-lived, increase in impact rates 500–600 Ma impactors through the inner solar system. We know that there after the start of the solar system (Gomes et al. 2005; Strom must have been a huge number of impacts as the planets et al. 2005), and have generally appeared to support the idea. accreted, and we know that the flux of impactors was much However, there are also ways in which the data could seem to higher at about 3.8–3.9 Ga, when the most recent of the basins suggest a “cataclysm” that did not really happen (Hartmann formed on the Moon, than it is now. In fact, the surfaces of the 1975, 2003; Haskin et al. 1998; Baldwin 2006). maria that fill the Near-Side basins on the Moon have far The third topic, the impact rate over the last 3.5 Ga, has fewer craters than surfaces that are not much older, so the flux also received increased attention, in part because of its must have dropped substantially by ∼3.5 Ga, when the maria implications for Earth’s geological and biological history were largely filled. However, we do not know what the flux (Culler et al. 2000; Kring 2000, 2003; Levine et al. 2005) and was between the era of accretion and the end of basin in part because of its implications for crater density-based formation, whether basin formation all occurred in a relative chronologies of planetary surfaces (Quantin et al. 2004). brief time interval (50 Ma has been suggested [Ryder 2002, are a promising place to look for evidence that 2003]) or was spread over several hundred Ma (Baldwin can help answer these questions. Most of the meteorites we 1974, 2006), and how the flux has varied over the last 3.5 Ga. have are ordinary chondrites, which come from (unknown)

747 © The Meteoritical Society, 2009. Printed in USA. 748 T. D. Swindle et al. asteroids in the Main Belt, bodies too small to have been portions of the meteorite that have been completely outgassed affected by virtually any process other than impact for more and hence give the time of the impact. Although we do not than 4.4 Ga. believe that our accuracy equals the precision of our Unfortunately, the impact process itself limits the measurements, we will show that it is common for multiple usefulness of chondrites for studying the process. Rocks’ subsamples of a single meteorite to have a consistent isotopic chronometers are much more likely to be reset in minimum apparent age, often expressed in more than one larger impacts, since the amount of energy deposited as heat extraction, while another sample may yield a confusing (which, along with time, is what it takes to reset a diffusion- pattern with a higher minimum age. In this case, we believe based radiometric chronometer such as K-Ar) increases that the minimum age, when seen in multiple samples, is substantially as the size of the impact increases. On relatively likely to represent the age of a degassing event. There is one small asteroids, though, impacts as large as those that created case where an independent measure of the age is available, the lunar basins will cause total disruption, and while assuming the Ordovician fossil meteorites date the L fragments may survive for tens or even hundreds of millions chondrite parent body event (Schmitz et al. 2001). In this of years, we would be unlikely to find and study any material case, using the common minimum age in multiple samples from such a disruption in the first 1000 Ma of solar system would give reasonably accurate ages (cf. Bogard et al. 1995). history. However, variations in the flux of large impactors are H chondrites, along with the comparably abundant L likely to be accompanied by comparable variations in the flux chondrites, make up the vast majority of meteorites falling on of impactors of a size that can reset portions of meteorite the Earth. Many L chondrites show the effects of a major parent bodies without destroying the bodies. collision that has now been dated at about 480 Ma ago In addition, it is frequently very difficult to determine the (Schmitz et al. 2003; Heck et al. 2004; Korochantseva et al. time of an , even if it was substantial enough to 2007), but there is nothing so obvious among the H cause obvious petrographic change. The K-Ar system, based chondrites. Many H chondrites are brecciated or show other on the decay of 40K to 40Ar, is generally considered the most signs of an impact-dominated history, but few have the kind easily reset of the commonly used extraterrestrial of shock features seen in the shock-blackened L chondrites. geochronometers. However, this system dates 40Ar loss, and This lack of a strong 480 Ma overprint makes H chondrites the simplest version of this system will give an accurate age intriguing because they may allow us to see an older period of only if all of the 40Ar in a mineral phase or lattice site is lost. solar system history. As it turns out, this hope is partially This, in turn, requires heating for a longer duration than the fulfilled. pulse of a shock wave, since it takes a finite amount of time In the present study, we have analyzed a total of for Ar to diffuse out of a rock. 26 samples from eight H chondrites that show obvious The K-Ar system can be improved upon by using the evidence of shock melting that may have been sufficient to 40Ar-39Ar technique. A typical 40Ar-39Ar analysis involves cause degassing and, thus, record the ages of collisional first irradiating the sample with neutrons, to convert a fraction events. By combining our data with data from previous of the 39K to 39Ar. At that point, the 39Ar becomes a proxy for studies, we now have a database of nearly 30 H chondrites, so the element potassium. The sample is then heated to more trends begin to emerge. Preliminary data on these progressively higher temperatures, and the Ar released in meteorites have been presented in several abstracts (Scherer each step is analyzed. The 40Ar/39Ar ratio then reflects an et al. 1996; Kring et al. 2000; Swindle et al. 2006a, 2006b; apparent age, and since different minerals and different sites Kring and Swindle 2008). We note that this work builds on within minerals degas at different temperatures, a spectrum of the synthesis of Bogard (1995), still the best review of the apparent ages is obtained (an alternative approach, which we general field of impact ages in meteorites. always tested, and will also make use of from time to time, is to use a three-isotope isochron). In a sample that is SAMPLES completely outgassed at a single time, all apparent ages will be the same. But this is seldom the case for shocked We examined eight impact melt breccias with H- chondrites. In these analyses, the K-bearing mineral that chondrite affinities (refer to the JSC Antarctic Meteorite degasses first is feldspar (or feldspar-composition glass), so it Newsletter and the Meteoritical Bulletin). These specimens is typically most strongly degassed by a shock event (i.e., have shock-darkened appearances, similar to those of comes closest to the true “age”) and degasses at the lowest previously described shocked (e.g., Britt and Pieters 1994) temperatures during the experiment. and shock-melted (e.g., Kring et al. 1996; Grier et al. 2004) Naively, one might hope that the lowest apparent age ordinary chondrites. Examples of views of thin sections are would be the time of the impact. In fact, in a sample that has given in Fig. 1. One sample, Gao-Guenie, is from a strewn been incompletely degassed, the lowest apparent age is only field of multiple stones in Burkina Faso (Africa) that fell in an upper limit to the age of the most recent thermal event. We 1960 (Grady 2000). Most of the stones are unshocked H now routinely analyze several samples from every shocked chondrites of thermal metamorphic type 5, but several are meteorite we study, maximizing our chances of finding shock-melted specimens. We selected a stone that is 40Ar- 39 Ar ages of H-chondrite impact melt breccias 749 dominated by a microcrystalline impact melt, but still contains surviving millimeter to centimeter clasts of the host H-chondrite target. For the purpose of 40Ar-39Ar analyses, we isolated melt and clast fractions so that they could be analyzed separately. Past studies suggest that clasts and melt may be differentially degassed (McConville et al. 1988; Bogard et al. 1995; Grier et al. 2004). One of our meteorites has a poorly known provenance from Northwest Africa and has, thus, been assigned the name Northwest Africa (NWA) 2058. It was recovered in 2001. The type specimen is dominated by melt (>90%; T. Bunch, personal communication), as is our specimen. We did not find any significant clastic material, so separation of clast and melt components was not necessary. Five of our eight meteorites were collected during the 2002–2003 and 2003–2004 field seasons from the LaPaz (LAP) ice field, Antarctica: LAP 02240, LAP 03922, LAP 031125, LAP 031173, and LAP 031308. The possibility of pairing of these meteorites is discussed in the appendix, where we conclude, based on our results, that pairing of any two is unlikely. LAP 02240 and LAP 031125 are solidified impact melts with only a few surviving clastic materials. Like Gao-Guenie, the melt matrix is a microcrystalline assemblage of olivine and pyroxene with interstitial feldspathic material. The melt- matrix texture in all three specimens is similar to that seen in the Orvinio H-chondrite impact melt breccia (Grier et al. 2004) and several L-chondrite impact melt breccias (e.g., Fig. 1. Examples of textures of the meteorites analyzed in this study. At one end, texturally, are impact melt breccias, containing (Kring et al. 1996). Once-molten orbicular metal rimmed with significant amounts of both melt and clast, such as Gao-Guenie (a). troilite is also entrained in the silicate melt, similar to that Others, like LAP 02240 (b) are dominated by melt, with few or no seen in other impact melt breccias (e.g., Smith and Goldstein unmelted clasts visible. 1977; Kring et al. 1996; Grier et al. 2004). LAP 03922, LAP 031173, and LAP 031308 are more fractions were separated from melt breccias, the dominant K- complicated melt breccias. They contain larger - bearing phase is the residual melt that is interstitial to fine- bearing relict clasts and mixtures of silicate melts with grained olivine and pyroxene. In LAP 03922, LAP 031173, different size distributions of metal and sulfide entrained and LAP 031308, however, K may be sequestered in both the within them. Normally, we would separate these types of melt matrix and in coarser feldspar crystals in unmelted clasts and melt fractions for subsequent 40Ar-39Ar analyses, chondrule-bearing relict clasts. because the fractions may have different diffusional None of the shock-melted metal particles in the characteristics and, thus, have degassed to different degrees. meteorites cooled slowly enough to either develop kamacite Unfortunately, the meteorite samples were too small for that rims (e.g., Kring et al., 1996) or Widmanstätten patterns (e.g., type of separation, so the 40Ar-39Ar analyses for these Kring et al. 1999), suggesting all of the samples cooled in near meteorites represent bulk analyses. surface environments (e.g., Smith and Goldstein 1977; Kring In addition to the above impact melt breccias, we also et al. 1996). This implies deposition in melt breccia crater extracted melt clasts from the Ourique meteorite (Kring et al. lenses, in relatively small injection dikes, and impact ejecta 2000), which is a gas-rich H-chondrite regolith breccia blankets. These samples differ from some other H-chondrite (Franchi et al. 1999) that fell in 1998 (Galopim de Carvalho impact lithologies like Rose City (deposited ∼100 m beneath and Fernando Monteiro 1999). The melt clasts sometimes the surface [Smith and Goldstein 1977]) and Portales Valley exceed 1 cm in size. They are composed of skeletal olivine (representing the deep-seated floor of a >20 km diameter that grew quickly from a melt that became feldspathic and crater [Kring et al. 1999]). remains glassy to cryptocrystalline. Whereas we used bulk splits for 40Ar-39Ar analyses of the other meteorites, in this PROCEDURES case we were constrained to 0.5 mg microcores that we extracted from one of the melt clasts in a petrographic section. Samples were processed using routine procedures for the Among the meteorites that are bulk melts and where melt University of Arizona noble gas mass spectrometry 750 T. D. Swindle et al. laboratory. We typically analyzed three splits of each meteorite), there were a total of 12 steps (at least one in each meteorite, unless stated otherwise. Two Ourique samples of the six samples) whose 1σ uncertainty overlapped that of were extracted from a ∼100 µm thick thin section, irradiated the adopted value, 0.0072 ± 0.0006. There were two steps with neutrons and step-heated using an Ar ion laser (Cohen with lower values than this, but they both overlapped this at et al. 2005). Gao-Guenie (three splits each of clast and melt), the 2σ level, statistically reasonable if the adopted value were NWA 2058, and the five LaPaz meteorites were split using the true minimum. The overall result of the spallation clean tools, and the samples irradiated with neutrons and step- correction was typically to raise the apparent age by 30–100 Ma heated using a resistance heated furnace (Barra et al. 2006). in cases where a correction for terrestrial atmosphere was All irradiations were at the University of Michigan Ford made. If the trapped 36Ar was assumed to be Q-type, whose Reactor. J factors were (7.61 ± 0.06) × 10−2 for Ourique, 40Ar/36Ar ratio is <1 (Wieler et al. 1992; Busemann et al. (1.968 ± 0.024) × 10−2 for Gao-Guenie, NWA 2058, and LAP 2000; Korochantseva et al. 2007), the resulting correction to 02240, and (2.144 ± 0.004) × 10−3 for the other LaPaz 40Ar would be <<1σ, so no correction to 40Ar was made. meteorites. In all irradiation packages, samples of MMhb-1 After making a spallation correction, we found that the hornblende were included as the primary irradiation monitor, 36Ar/39Ar ratio always decreased to a minimum at ∼ ° while samples of CaF2 and K2SO4 were included to monitor intermediate temperatures ( 900–1000 C), before rising interfering reactions producing various Ar isotopes from Ca slightly. Unless otherwise stated, we assumed that the 36Ar at and K, respectively. low temperatures came from terrestrial contamination Data corrected for blanks, decay, and interfering (compare to, e.g., Korochantseva [2005]), and made an reactions on Ca and K are available online as supplemental appropriate correction for 40Ar. At and above the minimum, material. However, from this stage on, the path is less we assumed that the 36Ar came from indigenous Q-type gas, straightforward, since the remainder consists of a and made no correction to 40Ar. This approach is also justified mathematically underconstrained system. As well as the by the fact that three-isotope plots never gave a single linear radiogenic 40Ar in which we are interested, contamination correlation across the whole temperature spectrum, but linear from terrestrial atmosphere (which is particularly problematic segments frequently occurred at low temperatures, with for hot desert meteorites) can inject 36Ar, 38Ar, and 40Ar. In apparent 40Ar/36Ar ratios of 250–320 (i.e., terrestrial terrestrial applications, it is possible to use the 36Ar to atmosphere), while at high temperatures, apparent 40Ar/36Ar constrain the amount of atmospheric contamination, but in ratios were basically not defined (if there was a correlation, meteorites cosmic-ray spallation reactions produce 36Ar and the 36Ar/39Ar ratios were low enough that the 40Ar/36Ar ratio 38Ar, neutron capture in the reactor produces 38Ar from could not be established). Note that while all our samples chlorine (Garrison et al. 2000), and chondrites typically experienced incomplete degassing, as evidenced by the lack contain some Q-type 36Ar and 38Ar (Ott 2002). In addition, of perfectly flat plateau plots, there is no evidence in the partial degassing could lead to a trapped component that three-isotope plots of an injected non-atmospheric trapped includes “excess” 40Ar and/or some of the Q-type 36Ar and component containing excess 40Ar (see Results for more 38Ar. discussion of three-isotope plots). Since the Cl-derived 38Ar makes it impossible to use the This technique does not take into account spallation from 38Ar/36Ar ratio to correct for spallation, our solution is to elements other than Ca. However, we can calculate the assume that 1) spallation production of Ar is dominated by equivalent 36Ar/39Ar ratio for K-produced spallation, using Ca, and 2) at least one temperature extraction in one split of a the 36Ar/37Ar for Ca and the relative production rates from given meteorite contains no 36Ar except from spallation from spallation and from the reactor. We find that the ratio is as Ca. If so, then we can use the minimum measured 36Ar/37Ar large as the 1σ uncertainty in the 36Ar/39Ar ratio in only a few ratio in a meteorite as the ratio of Ca spallation-produced 36Ar rare cases, so we have not made a correction for this in Table 1. to reactor-produced 37Ar (Garrison et al. 2000). Since both are The meteorites where it would make the most difference are produced from Ca, the ratio should be a constant for all the Antarctic (LaPaz) samples, which have far lower 36Ar/ temperature steps. The first assumption, though not strictly 39Ar ratios, presumably because they have less atmospheric true, is a good approximation, since Ca and K have contamination. We did try a correction on these meteorites, approximately equal Ar production rates in spallation and found that it reduced the final calculated ages by ∼10 Ma (Hohenberg et al. 1978), and H chondrites typically have Ca/ per step, much less than 1σ on any single step, and ∼0.7σ of K ratios of about 15 (Hutchison 2004). Although we cannot our best plateau. be sure about the second assumption, high-temperature extractions typically release the bulk of the spallation- RESULTS produced Ar, and little trapped 36Ar, so that might be expected to be true; furthermore, the minimum value was typically the Our best estimates for the times of significant thermal same for each sample of a given meteorite. For example, in events involving these samples are summarized in Table 1 the six Gao-Guenie samples (the most we ran from a single and discussed below. For the reader’s convenience, 40Ar- 39 Ar ages of H-chondrite impact melt breccias 751

Table 1. Estimated ages of events recorded by H chondrites from this study. Meteorite No. of samples Age (Ma) Comments Ourique 2 4360 ± 120 Sample G (sample F consistent) NWA 2058 3 509 ± 105 4 steps in two samples Gao-Guenie 6 303 ± 56 Low-T steps in three samples 4000–4300? Possible plateaus LAP 02240 3 <1085 Lowest apparent age 3939 ± 62 Average of high-T plateaus LAP 03922 3 399 ± 27 4 steps in two samples LAP 031125 3 350 ± 26 Minima in all three samples 3942 ± 23 Average of high-T plateaus LAP 031173 3 ≤1075 Lowest apparent ages in one sample LAP 031308 3 752 ± 47 4 steps in two samples meteorites are discussed in alphabetical order. Our An alternative approach to the data analysis is to use discussions are based on “plateau” plots. In most cases, three- three-isotope plots (Fig. 3a), which would give the same isotope plots appear to be scatter plots, with no more than results. A1-1 and A1-2 data yield isochrons with 10 and five three consecutive steps appearing to be collinear, and the points, corresponding to ages of 305 ± 46 Ma and 305 ± 35 Ma, apparent lines differing from sample to sample within a given respectively, virtually identical to our plateau-plot-based meteorite. However, in the few cases where three-isotope results, although they do give different initial 40Ar/36Ar ratios plots appear to contain significant information (Gao-Guenie (298 and 252). Data from A1-3 never give any well-defined A1-1 and A1-2, NWA 2058, and Ourique F), we will briefly lines among sets of adjacent extractions, but they scatter discuss those analyses. Details of the apparent ages for every around the isochrons for A1-1 and A1-2. step are available online as supplemental material. The other interesting question about Gao-Guenie is Gao-Guenie: For Gao-Guenie, we analyzed six samples, whether there is a plateau at high temperatures in the melt (A2) three each of clast and melt material from a single slice of the samples (Fig. 2b). For sample A2-1, the eight points from meteorite. All of the clast (A1) samples yield apparent ages of 1275 °C through 1450 °C, representing 67% of the total 39Ar, ∼300 Ma at low extraction temperatures (Fig. 2a). In sample appear at first glance to be a good plateau, with an age of 4285 A1-1, three steps from 650–850°C, comprising 14.5% of the ± 67 Ma. However, there are several disquieting things about total 39Ar, sum to 315 ± 84 Ma, and the two steps before that this plateau, after looking at the other two samples from A2 agree with 0.2 σ, suggesting the uncertainties may have been and inspecting A2-1 itself in more detail. First, although the overestimated. A1-2 shows nothing resembling a plateau at other two samples are similar, neither agrees, to within a few low temperatures, but the single largest extraction, 600 °C sigma. Unfortunately, the highest-temperature extractions (12% of the 39Ar), gives 286 ± 46 Ma. In A1-3, the second were lost (due to loss of the heating furnace) for A2-2, but the through sixth steps, representing 30% of the 39Ar, are all last four ages (15% of the 39Ar, in the middle of the plateau, within 1σ of 300 Ma, although the first three have large assuming identical release pattern to A2-3) were hovering uncertainties. The last two of these five steps (representing around 4000 Ma, not 4300 Ma. For A2-3, the apparent ages 20.4%) sum to 304 ± 28 Ma. Summing the three best-defined rose throughout the experiment, and only at the highest from A1-1, one from A1-2 and two from A1-3 gives 303 ± 56 Ma, temperatures did they begin to overlap those of A2-1. our best estimate of the most recent thermal event affecting Furthermore, when viewed in detail, the apparent ages in A2- Gao-Guenie. 1 are actually increasing across the plateau, the classic sign of Except for very small (<1% of the 39Ar) extractions, none partial degassing. Finally, the K/Ca systematics of both A2-1 of the melt (A2) samples (Fig. 2b) give any apparent ages and A2-3 (Fig. 2c) suggest that the sloping plateaus are not a <980 Ma (although some 1σ error bars extend to ∼800 Ma), matter of mixing of an undegassed 4300 Ma phase with a indicating less diffusive loss in the melt than in the clast, as is partially degassed lower-temperature-release phase. In both frequently seen (Bogard et al. 1995; Kunz et al. 1997; cases, the K/Ca ratio remains steady while apparent ages McConville et al. 1988). This counterintuitive effect has been increase from 2500–3000 Ma, more consistent with partial attributed to changes in either the diffusion parameters or, degassing of the phase in which the apparent plateau is more likely, the diffusion path length. The differential observed. This is an important point, because impact ages degassing could have occurred either in the event that created between 4050 Ma and 4400 Ma are rare, both in meteorites and the melt (Bogard et al. 1995) or in a later event that partially lunar samples. This meteorite at first appears to represent that degassed the host while leaving the melt largely unaffected unique case, but the identification is much less secure on (Kunz et al. 1997). However, the low-temperature release further inspection, and we suspect it represents diffusion patterns, Arrhenius plots and derived diffusion parameters all during incomplete degassing at some later time. The high- overlap between the clast and melt samples for Gao-Guenie. temperature steps do represent a lower limit to the time of the 752 T. D. Swindle et al.

Fig. 3. Three-isotope plots for Gao-Guenie. Data has been corrected for instrumental effects (blanks, mass fractionation, etc.), radioactive decay, reactor interferences, and spallation, as discussion in the Procedures section. a) Data from all samples, along with least- squares fit lines for A1-1 and A1-2, which have the same x-intercept (apparent age), but different y-intercepts (different initial 40Ar/36Ar ratios). Error bars are given only on the points included in one of the fits to the low temperature points from A1-1 and A1-2, but are Fig. 2. 40Ar-39Ar results for Gao-Guenie. a) Apparent age spectra for generally no larger than those shown. b) Expanded view of lower left samples from A1 location, with the ages calculated from spallation- portion of Fig. 3a, showing that the high-temperature data for the and (in the case of low-temperature extractions) air-corrected data, as Gao-Guenie A2 samples show very little evidence for any component discussed in Procedures. Width of boxes is 1σ uncertainty. Dashed associated with 36Ar. horizontal lines represent our estimates of ages of impact events, as discussed in Results and tabulated in Table 1. Upper portion of figure earlier thermal event (the best limit is 4380 ± 61 Ma, in the plots K/Ca ratios, using the same line darkness and thickness for 1450 °C step of A2-1), but we cannot tell whether that event corresponding samples. b) Same as (a), but for samples from A2 was the metamorphism to Type 5 or was an impact very shortly location. Extraction furnace burned out during analysis of sample A2-2, so fractions of 39Ar are scaled assuming ∼60% of 39Ar had been thereafter. released at this point, the fraction in A2-3, which was heated with a In this case, as in the vast majority of the samples we similar heating schedule. Note that for samples A2-1 and A2-3, the analyzed, three-isotope plots are of little use. The low- K/Ca ratios are relatively constant during extractions that show temperature points are scattered in Fig. 3a. The high- increasing ages with increasing extraction temperatures, as would be temperature points cluster near the abscissa (Fig. 3b), but expected for a diffusion profile, but unlike what would be expected from a mixture of two phases with different K/Ca ratios and different there are no linear arrays of the kind that would indicate the 36 40Ar-39Ar ages. Ar remaining after the spallation correction is part of any 40Ar- 39 Ar ages of H-chondrite impact melt breccias 753

Fig. 4. 40Ar-39Ar apparent age spectra for samples of LAP 02240. See Fig. 5. 40Ar-39Ar apparent age spectra for samples of LAP 03922. See caption to Fig. 2a for details. caption to Fig. 2a for details.

component that contains 40Ar. In this sort of situation, we Unlike the case of Gao-Guenie, the high-temperature believe our analysis based on plateau plots is preferable. data in LAP 02240 behaves exactly as we would expect if the LAP 02240: Meteorite LAP 02240 (Fig. 4) has clearly meteorite had been completely reset at the time the high- undergone partial resetting “relatively” recently. However, temperature steps record. LAP 02240 did experience partial the timing of that is very poorly constrained, although we ran resetting more recently, but that resetting was apparently mild three samples. Any possible terrestrial corrections are of little enough that older plateaus survive. These are by far the best consequence on this Antarctic meteorite, largely because “old” plateaus surviving among the meteorites we analyzed, there is much less contamination and partly because most of and we believe they do represent the timing of an impact the K-produced Ar is released at rather high temperatures. event. In addition, the temperatures at which the plateaus are The lowest apparent age was ∼1100 Ma in sample 3, and there defined (see above) are by far the highest temperatures of any were no apparent low-temperature plateaus. of the possible plateaus we considered, consistent with the At higher temperatures, each of the three samples shows idea that the LAP 02240 plateaus represent Ar sites that ages that rise to 3500–4000 Ma at 20–30% 39Ar release, then would be more difficult to degas, consistent with the rather drop slightly before rising to ∼4000 Ma for the last 50%. On old apparent age. closer inspection, it appears that this may represent two LAP 03922: The first two extractions of C, representing separate phases (or sets of diffusion domains), since there is a 27.7% of the 39Ar in the sample, yield indistinguishable minimum in the K/Ca ratio associated with the first apparent ages, with a summed apparent age of 401 ± 28 Ma. maximum. However, we did not identify any phase other than The second and third heating steps of A give a summed the ubiquitous feldspar (and feldspathic material) that is apparent age of 394 ± 8, though only with 10% of the 39Ar. typically the carrier of K in ordinary chondrites. Summing all four steps (weighted by fraction of their It is possible to define high-temperature plateaus with the sample’s 39Ar), we get 399 ± 27 Ma, our best estimate of the last 54–67% of the 39Ar of each sample with ages of 3912 ± time of a recent partial degassing event (Fig. 5). Sample B 54 (1287–1375 °C), 3856 ± 39 (1300–1375 °C) and 4050 ± 85 gives no apparent ages under 2000 Ma, except in high- (1260–1365 °C) Ma, respectively. Although these do not temperature steps subject to recoil effects, yet another quite agree with each other, if all the gas in all of those illustration of the value of analyzing multiple samples, plateaus is summed together, it gives an age of 3939 ± 62 Ma, particularly for a petrographically complex meteorite. The which agrees within 1σ with each of the three individual timing of any prior events is only barely constrained—the plateaus (and with individual steps representing >50% of the highest ages seen in B set a lower limit of ∼3200 Ma, but 39Ar release in all three samples). We take this as the age of a the last previous event could have been then, could have been significant thermal event, presumably the impact event that metamorphism shortly after formation, or anything in generated the melt. between. 754 T. D. Swindle et al.

Fig. 6. 40Ar-39Ar apparent age spectra for samples of LAP 031125. Fig. 7. 40Ar-39Ar apparent age spectra for samples of LAP 031173. See caption to Fig. 2a for details. See caption to Fig. 2a for details.

LAP 031125: All three samples have minimum apparent be the time of the most recent partial resetting, but since ages of 300 to 400 Ma in one or two low-temperature neither of the other two samples has any apparent ages <2300 Ma, extractions representing a total of 7–13% of the 39Ar. we would only claim to have determined an upper limit to the Averaging these, we get 350 ± 26 Ma, our best estimate for actual time of this event. The last 80% of the 39Ar released the time of recent resetting (Fig. 6). from B yields apparent ages of 3300–3700 Ma (summed total At high temperatures, the three splits are again quite 3505 ± 21 Ma), but neither of the other two samples has even similar. The last ∼60% of A and B have summed apparent a hint of a plateau at this age, so it may just be a coincidence. ages of 3914 ± 22 and 3927 ± 18 Ma, respectively, while the Sample A has apparent ages up to 4260 Ma, a not-very- last ∼40% of C gives a slightly larger apparent age, 3986 ± interesting lower limit to the time of the earliest event of 27 Ma. Adding these together gives 3942 ± 23 Ma, consistent which the K-Ar system retains an isotopic memory. Adding to with all three samples at the 1σ level, so this is our preferred the confusion, the minimum 36Ar/37Ar ratio, which we use for age. As in the case of Gao-Guenie, the plateaus are not spallation corrections (see section Procedures) is much perfectly flat. But the agreement of the summed ages suggests different in C (0.027) than in the other two (0.065), that this does represent a real event. An alternate suggesting perhaps an unusually high amount of retention of interpretation is that the high-temperature extractions of A trapped 36Ar in the latter, consistent with their higher amount and B represent “saddle-shaped” plateau plots (see of retention of radiogenic 40Ar in the most recent event. (MacDougall and Harrison 1999), in which case the true age LAP 031308: All three samples have the classic partial would be more likely to be represented by the ∼30% of the Ar degassing pattern of increasing apparent age with increasing for each sample from the rather flat high-temperature temperature throughout, although they do not give exactly the minimum. These give ages of 3837 ± 23 and 3828 ± 17 for A same numbers (Fig. 8). In this case, the minimum age is an and B, respectively, ∼100 Ma younger than the age we upper limit to the latest event, and the maximum age is a adopted. We slightly prefer the higher age, largely because it lower limit to the age of a previous event. The maxima is consistent with an interpretation of each of the three disagree widely, but the minima of two of the three samples samples, but this is not crucial to any conclusions. seem to be telling the same story. Three steps between 400 ºC LAP 031173: Unlike LAP 031125, the apparent age and 500 °C in samples B and C all give apparent ages of spectra of LAP 031173 (Fig. 7) are every bit as complicated ~780 Ma, while a lower temperature step in B is consistent as the meteorite’s appearance under the microscope, and we with this, albeit with a much bigger uncertainty. The 550 °C can say little about its chronology. Sample C has three step in C gives a slightly younger apparent age, 697 ± 26 Ma. consecutive steps with 15.6% of the total 39Ar and apparent Summing the four steps together gives an age of 752 ± 47 Ma. ages of 1039–1105 Ma (summed 1075 ± 19 Ma). This could NWA 2058: NWA 2058 gives clear evidence for an 40Ar- 39 Ar ages of H-chondrite impact melt breccias 755

Fig. 8. 40Ar-39Ar apparent age spectra for samples of LAP 031308. Fig. 9. 40Ar-39Ar apparent age spectra for samples of NWA 2058. See See caption to Fig. 2a for details. caption to Fig. 2a for details. impact during the last 1000 Ma, but the exact date is hard to in interpretation, since none of the apparent ages in A3 is as pin down (Fig. 9). Our three samples all give similar results. If low as the lowest steps in either of the other two samples. left uncorrected for terrestrial contamination (“air”), all three If we use three-isotope plots, in each case the first five or yield U-shaped patterns on plateau plots, with the youngest six extractions are roughly collinear, but the best fit (i.e., apparent age only ∼1500 Ma (in A2). However, deciding whether or not to eliminate points based on the χ2 Korochantseva et al. (2005) have shown that hot desert goodness of fit parameter) are not consistent with one another. meteorites contain pervasive terrestrial Ar, released at 400– They give ages and “trapped” 40Ar/36Ar ratios of 381 ± 69 Ma 700 °C, and our data make more sense when viewed in that and 301; 1039 ± 60 Ma and 226; and 913 ± 99 Ma and 299, light. Making our normal correction for air, we then have respectively. This is reasonably consistent with our spectra of rising apparent age with rising extraction interpretation based on the plateau plots. temperature, the classic signature of partial diffusive loss. We see no evidence for any other thermal events At high temperatures, the apparent age reaches maximum affecting NWA 2058—the maximum apparent ages are only values of 2800 Ma to 3750 Ma (at 70 to 90% of the 39Ar lower limits to the time of any earlier event, and the lack of a released, depending on the sample), but the spectra do not plateau makes any age from 3750 to the accretion age of the have plateaus, nor are the highest apparent ages consistent meteorites possible. The lack of a clear age is unexpected from one sample to another. At the very highest extraction from a meteorite that is >90% melt. temperatures, the apparent ages decrease, consistent with Ourique: Our best estimate for the time of the most minor recoil effects. significant impact into the rock that was incorporated into What then can we make of the history of NWA 2058? Ourique is 4360 ± 120 Ma. A plateau plot for sample G yields Assuming that the partial diffusive loss occurred at a single high apparent ages at the lowest temperatures, low apparent time, the youngest apparent ages provide upper limits to the ages at high extraction temperatures, and a near-plateau for time of that event. Samples A1 and A2 each have two steps six steps in the center representing ∼67% of the 39Ar release with apparent ages between 445 and 575 Ma, making up a (Fig. 10a). This is the classic pattern for a sample in which total of more than 10% for each sample. However, A3 recoil effects are important, since recoil during the irradiation bottoms out with several steps at 750–950 Ma. In all cases, tends to move potassium-derived 39Ar from high-K low- fairly large corrections for terrestrial contamination make for temperature sites, to low-K high-T sites. The plateau gives an large uncertainties, typically 100–200 Ma. We believe the age of 4558 ± 15 Ma (1σ statistical uncertainty), the sum of all four extractions consistent with ∼500 Ma provide a the extractions a slightly lower age of 4478 ± 26 Ma. Because reasonable estimate of the degassing age, 509 ± 105 Ma. of the clear presence of recoil, we believe the sum is a better However, A3 provides an example of why caution is needed estimate of the true age. The other Ourique core sample, F, 756 T. D. Swindle et al. does not give a plateau in a standard plateau diagram. However, if the last four or five extractions, likely affected by recoil, and the first extraction, where surface-correlated solar wind Ar from the gas-rich meteorite’s regolith exposure could drive up the amount of 36Ar, are excluded, it does give a linear plot on a three-isotope diagram (Fig. 10b), consistent with an age of 4240 Ma, and a trapped component with 40Ar/36Ar ∼ 250 presumably highly fractionated atmospheric Ar (with perhaps some trapped solar wind Ar mixed in). Since the trapped component dominates the 40Ar budget, we believe this is reasonably consistent with the result for sample G, but the best results of the two differ by enough that we will conservatively quote our best estimate as the average of the two, although we suspect the true age is closer to the older end. There is no evidence in either sample for a more recent impact.

DISCUSSION

As stated in the Introduction, we are interested in trying to determine the overall impact history of the H chondrites, in hopes of connecting the impact of their parent body, presumably a Main Belt , with other asteroids (L- chondrite parent body, Vesta) as well as the Earth and Moon. Toward that end, we have added the data in our Table 1 to that in Table 1 of Grier et al. (2004), which, in turn, was largely derived from the still pertinent review of Bogard (1995). We have put it in the form of an ideogram in Fig. 11 (a complete list of the ages is given in Table 2), in which every age is represented by a Gaussian distribution of the same total area, with a center at the measured age of the meteorite, and a width representative of the stated uncertainty. Before discussing the implications of Fig. 11, we need to ask the question of how much significance we should put in 40Ar-39Ar ages. It is clear that the K-Ar system is frequently only Fig. 10. a) 40Ar-39Ar apparent age spectra for samples of Ourique partially reset in shock events. The classic study of this is one melt clasts. See caption to Fig. 2a for details. b) Three-isotope plot for Ourique melt clast F. Line is fit to solid symbols. Numbers next to 40 39 by Bogard et al. (1995), in which they performed Ar- Ar symbols are settings for laser output power (in watts). studies on 10 different samples of Chico. Chico is an L chondrite, and since its Rb-Sr age (Fujiwara and Nakamura the 1–2 Ma kind of precision desired in some studies, in 1992) is identical, within errors, to the age of the Ordovician trying to map out the basic history of solar system impacts, sediments where fossil L chondrites are found (Schmitz et al. the situation is not that dire. For one thing, although the 10 2003; Heck et al. 2004), it is almost certainly a part of that samples that Bogard et al. (1995) analyzed all gave slightly ∼470 Ma event. However, the 40Ar-39Ar data suggests an age different results, eight of the 10 gave minimum apparent more like ∼530 Ma (Bogard et al. 1995), and the minimum ages of 550 Ma or less, so any random selection of three apparent age for one of the samples analyzed is >700 Ma. samples from Chico, with no outside information (such as Assuming this is the result of partial resetting, it only requires the Rb-Sr age or fossil meteorites) would be likely to retention of 0.4% of the 40Ar that would have built up come within 80 Ma of the correct age. Second, many L between 4500 Ma and 470 Ma to generate the 530 Ma age. chondrites yield results much closer to 470 Ma (Kunz et al. Similarly, an analysis of impact glass from the Tswaing 1997; Korochantseva et al. 2007) than does Chico. Finally, terrestrial impact crater (remember that glass often retains it is worth mentioning that because of the logarithmic nature more 40Ar than clasts in meteorite samples) suggests retention of the age equation, a 0.4% retention of radiogenic 40Ar in of 0.015% to 4.15% of the 40Ar in various samples (Jourdan an impact event early in solar system history would be a et al. 2007). much smaller effect (e.g., a mere 2.3 Ma for an impact at However, while this level of retention would preclude 4000 Ma). 40Ar- 39 Ar ages of H-chondrite impact melt breccias 757

Fig. 11. Ideogram of impact ages of H chondrites. Data from Table 2. Fig. 12. Histogram of U,Th-He ages of H chondrites from Wasson Dashed lines are individual samples, each plotted as a Gaussian and Wang (1991). All H chondrites in their category “m1,” for which % distribution with unit area. Solid line is sum of all data. multiple analyses agree within 10 or 150 Ma (whichever is larger) are included. Unlike Fig. 11, there has been no selection based on any petrographic criteria, so most of the samples in Fig. 12 show little IMPLICATIONS OF H CHONDRITE DATA sign of shock.

There are several points we wish to make, based on the possible that the U,Th-He ages could represent partial ideogram in Fig. 11. resetting of much older (4500 Ma?) ages in younger, 1. It is definitely possible for a severely shocked meteorite even recent, events (final break-up?), or loss of 4He by to survive from the era of the accretion of the planets. recoil during actinide decay, it is nevertheless true that Portales Valley (Garrison and Bogard 2001) and Yamato- the same basic pattern is observed. It is, in fact, the same 75100 (Swindle and Kimura 2008) are heavily shocked basic pattern that was observed in 40Ar-39Ar and U,Th-Pb (or shock-melted) H chondrites with ages >4400 Ma. We ages in the early 1970s and led to the proposal of the idea suspect Ourique, from this study, falls in that category as of a “terminal lunar cataclysm” or “late heavy well, and there are also examples of L chondrites, Shaw bombardment” by (Tera et al. 1974; Turner et al. 1973). (Bogard and Hirsch 1980), PAT 91501 (Garrison et al. 4. Gomes et al. (2005) and Strom et al. (2005) have 2005), and MIL 05029 (Weirich et al. 2008). Arguments proposed that the “lunar cataclysm” was the result of a that there are virtually no impact-affected lunar samples bombardment of the inner solar system brought about by with ages >4000 Ma because impact melts cannot changes in the outer planets’ orbits. If so, evidence survive (Hartmann 2003) need to be revisited in light of would be expected to be present in places other than the this. It is certainly possible that conditions on the Moon Moon. Bogard (1995) discussed the fact that this is so in could lead to effects not seen on asteroids, however. HED meteorites, but noted the paucity of “cataclysm” 2. There are several H chondrites with impact ages 3600– ages in ordinary chondrites. He suggested that it could be 4100 Ma (seven, including LAP 02240 and LAP the result of smaller undifferentiated asteroids (i.e., 031125), but none with ages 4100–4400 Ma (unless chondrite parent bodies) being more easily destroyed in highly questionable “plateaus” in Gao-Guenie (this large collisions, and hence we might be seeing only the work) and Orvinio (Grier et al. 2004) really have age ordinary chondrites whose parent asteroids were lucky significance). This is the same pattern seen in HED enough to not get battered. However, while such ages are meteorites (Bogard and Garrison 2003), IIE irons less prevalent in ordinary chondrites than in HED or (Bogard et al. 2000), and, of course, several types of lunar samples, it is clear that a significant subset of the lunar samples (Culler et al. 2000; Cohen et al. 2000, shocked H chondrites, at least, were also shocked in this 2005). interval. This fits with Bogard’s interpretation, since the 3. The same trend is true when U,Th-He ages of H H chondrite parent body would have to have avoided chondrites are considered. Figure 12 shows a histogram complete disruption to survive long enough to deliver of all well-defined U,Th-He ages of H chondrites meteorites today. The reason such a pattern was not calculated by (Wasson and Wang 1991). The most evident in the data available in 1995 is probably the common ages are 3500–4000 Ma. While it is quite result of the data available. Shock-blackened L 758 T. D. Swindle et al.

chondrites are much more common than H chondrites the meteorite flux on Earth to be several times as high as with evidence for shock, and the 40Ar-39Ar (Bogard at present (Schmitz et al. 1996), although whether that 1995) and U,Th-He (Wasson and Wang 1991) ages show would translate into large enough impacts in the asteroid clear evidence of an event ∼500 Ma ago. Because they belt to generate the kind of heating required to explain were so visually distinctive, these made up the vast the H chondrite data is not clear. Another possibility is majority of the shocked ordinary chondrites that had that both were impacted by some sort of shower, a been analyzed. Even among the shocked H chondrites, possibility suggested on the basis of a variety of lines of those reset in the last 1000 Ma are more common than evidence (Farley 1995; Hut et al. 1987; Tagle and Claeys those with clear evidence of older shock events, so it 2004). However, it is also possible that the ∼450–500 Ma wasn’t until the database grew sufficiently that the peaks shock ages seen in H chondrites simply represent at >4400 Ma and at 3600–4100 Ma began to appear. It incomplete degassing during an event at ∼300 Ma. will now be interesting to see whether further data makes 7) Perhaps as interesting as the ages that are common those peaks clearer or shows them to have been mirages. among in H chondrites in Fig. 11 are the ages that are not. 5. Although it is clear that many of the impact-melted or • Approximately 45% of the H chondrites have heavily-shocked H chondrites were affected by an event cosmic-ray exposure ages of 5–10 Ma (Graf and more recent than 1000 Ma, it is not clear exactly when Marti 1995). Besides producing a substantial the event or events occurred. In the plots of ages of H fraction of the total meteorites currently falling on chondrites (Figs. 11, 12), the situation is much muddier Earth (H chondrites make up 30–40% of all than in the L chondrites dominated by the ∼500 Ma meteorites that currently fall on Earth), it has been event. The three highest peaks in the H chondrite suggested that this event is the same one that ideogram (Fig. 11), nearly comparable in height, are at produced the Karin subset of the Koronis asteroid ∼280, ∼350, and ∼460 Ma. Charsonville (270 ± 30 Ma family that can be identified dynamically (Nesvorn˝ (Bogard et al. 1976)), Sweetwater (290 ± 100 Ma et al. 2002a). Yet 40Ar-39Ar ages of <10 Ma are rare (Bogard et al. 1976)), and Rose City (380 ± 100 Ma or absent. A Clovis #2 sample has apparently been (Kunz et al. 1997)), along with Gao-Guenie’s 303 ± degassed ≤40 Ma ago (Turner and Cadogan 1973), 56 Ma, LAP 031125’s 350 ± 26, and LAP 03922’s 399 ± but the only other candidate is Ucera, which has 27 from this work, are all in the 250–400 Ma range. It is been interpreted as having been reset at 40 ± 30 Ma also worth noting that the lowest single extraction from (Bogard et al. 1976). The common 500-Ma 40Ar- six Orvinio samples (Grier et al. 2004) was ∼325 Ma 39Ar age in L chondrites has also been suggested to (although several extractions were 500–600 Ma, which be the result of a family-forming catastrophic may be a more reasonable estimate of the age of the destruction of an asteroid (Nesvorn et al. 2002b), dominant shock event in that meteorite, so it is plotted in leading to the Flora family. If these are both correct, the ideogram as 550 ± 50 Ma). Also, the ∼500 Ma age for then why did one set of meteorites have their 40Ar- NWA 2058, though rather poorly defined, joins Jilin 39Ar ages reset, and the other did not? One (Wang et al. 1980; Harrison and Sunshang 1981), possibility is the size of the event, since the Flora Kimble County (Bogard et al. 1976), and perhaps family formed via disruption of a ∼150-km asteroid, Orvinio (Grier et al. 2004) with ages of ∼500–600 Ma while the Karin family could be the result of a 15 km with uncertainties of ∼100 Ma, and Tulia has been asteroid. In this scenario, the Flora impact would affected by a shock at 457 ± 24 Ma (Trieloff et al. 1994). have left bodies large enough to retain enough heat Given our concern about the possibility of partial to reset the K-Ar system, while the Karin impact resetting of the K-Ar system (discussed above), it is would not. However, another likely possibility is worth noting that there is a sharp fall-off on the young- that the H chondrites are the direct results of the age side of 280 Ma. This is what we might expect if there disruption event, while the L chondrites are the was an event at ∼280 Ma, but some of the samples are result of later impacts chipping meter-sized still slightly contaminated by incomplete resetting. At fragments off bodies (perhaps even as large as the present, the best estimate for the most recent significant 140-kilometer 8 Flora itself) that were large enough thermal events affecting the parent body (or bodies) of to retain enough heat for the 40Ar-39Ar system to be the current H chondrites may be ∼300 Ma and ∼450– reset. Or, to look at it in another way, it is possible 500 Ma. that L chondrites that fell shortly after that event 6. The latter age, if real, is particularly intriguing, and (e.g., the fossil meteorites from Swedish quarries raises the possibility that the H chondrite parent body (Heck et al. 2004; Schmitz et al. 2001)) would not experienced one or more events related to the 470 Ma have shown 40Ar-39Ar evidence for that event, breakup of the L chondrite parent body. One possibility because the event had left them in meter-sized is a bombardment by fragments of the disrupted L bodies too small to retain any significant amount of chondrite parent body. The L disruption certainly caused heat. In that case, H chondrites falling tens of Ma 40Ar- 39 Ar ages of H-chondrite impact melt breccias 759

Table 2. Ages of heavily-shocked or shock-melted H chondrites. Age (Ma) Meteorite Reported Ideogram Reference Channing ≤3700 Not plotted1) (Bogard et al. 1976) Charsonville 270 ± 30 270 ± 30 (Bogard et al. 1976) Clovis #2 <40 40 ± 1002) (Turner and Cadogan 1973) Dar al Gani 896 3704 ± 35 3704 ± 35 (Folco et al. 2004) ~900 900 ± 1002) (Bogard 1995; Rubin et al. 1983) Gao-Guenie 303 ± 56 303 ± 56 This work Jilin 530, 400, 500 500 ± 1002) (Harrison and Sunshang 1981; Müller et al. 1983; Wang et al. 1980) Jilin 2200 Not plotted3) (Wang et al. 1980) Jilin 3950 3950 ± 1002) (Müller et al. 1983) Kimble County 600 ± 40, 610 ± 50 600 ± 40 (Bogard 1995; Bogard et al. 1976) LAP 02240 3939 ± 62 3939 ± 62 This work LAP 03922 399 ± 27 399 ± 27 This work LAP 031125 350 ± 26 350 ± 26 This work LAP 031125 3942 ± 23 3942 ± 23 This work LAP 031308 752 ± 47 752 ± 47 This work Monroe ~1100, 1100 ± 200 1100 ± 1002) (Bogard 1995; Bogard et al. 1976) NWA 2058 509 ± 105 509 ± 105 This work Ourique 4360 ± 120 4360 ± 120 This work Orvinio 500 – 600 550 ± 50 (Grier et al. 2004) Orvinio 800 Not plotted4) (Bogard and Hirsch 1980) Plainview (clast) 3630 ± 70 3630 ± 70 (Keil et al. 1980) Portales Valley 4477 ± 16, 4460 ± 20 4470 ± 20 (Garrison and Bogard 2001) Rose City (host) 380 380 ± 1005) (Bogard et al. 1976; Kunz et al. 1997) Rose City (melt) 3620 ± 10 3620 ± 10 (Kunz et al. 1997) St. Mesmin (clast) 1360 ± 506) 1360 ± 50 (Schultz and Signer 1977) +20 ± Sweetwater 290 −90 290 55 (Bogard et al. 1976) Travis County 830 ± 30 830 ± 30 (Kunz et al. 1997) Tulia 457 ± 24 457 ± 24 (Trieloff et al. 1994) Ucera 40 ± 30 40 ± 30 (Bogard et al. 1976) Yamato-75100 (melt) >4420 4490 ± 70 (Swindle and Kimura 2008) Yamato-794046 3790 ± 407) 3790 ± 40 (Fujimaki et al. 1994) Yanzhuang (host) 130 ± 40 130 ± 40 (Kunz et al. 1997) Yanzhuang (melt) 4090 ± 40 4090 ± 40 (Kunz et al. 1997) 1K-Ar age is 3800 Ma, minimum apparent age is ∼3700 Ma, but 40Ar-39Ar spectrum is continually increasing with age, and U,Th-He age is ∼2600 Ma, so original authors concluded it is impossible to determine whether degassing occurred at 3700–3800 Ma or some time later. 2Ages for which no uncertainties were reported, or for which upper limits based on minimum apparent ages were reported were arbitrarily given 100 Ma 1σ uncertainties, so that they would be included in plot, but not have large weight at a specific age. 3This age was the minimum apparent age in one extraction in a single sample. Since three other samples of Jilin have now been analyzed, and none shows any evidence for a ∼2000 Ma age, we conclude this sample was incompletely degassed. 4Bogard and Husain (1980) analyzed a single sample of Orvinio that had a minimum apparent age of 800 Ma. Grier et al. (2004) analyzed multiple samples, several of which were consistent with 500–600 Ma and several of which were less degassed. We conclude that the Bogard and Husain (1980) sample was incompletely degassed. 5Kunz et al. (1997) analyzed three samples of Rose City host. All had several low-temperature steps that were the same within (typically large) uncertainties as the minimum observed by Bogard et al. (1976). We adopted the age given by Bogard et al. (1976) with a 100 Ma 1σ uncertainty. 6Concordant classical K-Ar age and U,Th-He age. K-Ar age is frequently higher than degassing age. However, concordance with U,Th-He age, coupled the fact that other components in St. Mesmin have very old K-Ar ages, suggests little degassing of the body as a whole. 7Rb-Sr age; system is disturbed, so event at a later time as well is possible, if not likely.

hence would be more likely to have 40Ar-39Ar ages • Finally, there are no ages between ∼3600 Ma and reset by the event 5–10 Ma ago. ∼1500 Ma. A 2200 Ma age for Jilin (Wang et al. • As mentioned, although there are two meteorites 1980), the only report of an H chondrite age in that clearly reset by shock in the first 100 Ma of solar range of which we are aware, was the minimum system history, and five or six more that were reset apparent age recorded at low temperatures in one of between 4100 and 3600 Ma, there is a gap in three samples, while the other two both had between, as in most other compilations of impact minimum apparent ages of ∼500 Ma, as did a sample ages. analyzed later (Harrison and Sunshang 1981). 760 T. D. Swindle et al.

Hence, we suspect that one sample was only properties, and cooling rate determinations of severely shocked partially degassed, the same effect we have seen in chondrites. Geochimica et Cosmochimica Acta 44:1667–1682. 40 39 some of our meteorites. Bogard D. D., Husain L., and Wright R. J. 1976. Ar- Ar dating of collisional events in chondrite parent bodies. Journal of On the whole, although the numbers are still small, some Geophysical Research 81:5664–5678. patterns in the impact history of H chondrites are beginning to Bogard D. D., Garrison D. H., Norman M., Scott E. R. D., and Keil emerge, and it is consistent with other meteorite groups. Some K. 1995. 39Ar/40Ar age and petrology of Chico: Large-scale material within the H chondrite parent body(ies) was heavily melting on the L chondrite parent body. Geochimica et shocked and had its K-Ar system reset, during the first 100 Ma Cosmochimica Acta 59:1383–1399. Bogard D. D., Garrison D. H., and McCoy T. J. 2000. Chronology of the solar system (the planetary accretion phase). Impact and petrology of silicates from IIE iron meteorites: Evidence of events are also recorded between 3600 Ma and 4100 Ma, when a complex parent body evolution. Geochimica et Cosmochimica the “lunar cataclysm” or “terminal heavy bombardment” was Acta 64:2133–2154. occurring and some large impact events were occurring on the Britt D. T. and Pieters C. M. 1994. Darkening in black and gas-rich HED parent body. With the relatively small number of ordinary chondrites: The spectral effects of opaque morphology and distribution. Geochimica et Cosmochimica Acta 58:3905- meteorites studied, there is certainly not strong evidence yet 3919. for any impacts during the intervals between ∼4400 Ma and Busemann H., Baur H., and Wieler R. 2000. Primordial noble gases ∼4100 Ma or between ∼3500 Ma and ∼1000 Ma. Most of the in “phase Q” in carbonaceous and ordinary chondrites studied by H chondrites that show visible evidence of shock were closed-system stepped etching. Meteoritics & Planetary Science involved in impacts in the last 600 Ma or so, perhaps with a 35:949–973. ∼ Cohen B. A., Swindle T. D., and Kring D. A. 2000. Support for the peak at 300 Ma. The impact rate may have gone up in the last lunar cataclysm hypothesis from impact melt 1000 Ma, but the paucity of ages >1000 Ma also may represent ages. Science 290:1754–1756. a typical lifetime for impact melts or heavily shocked Cohen B. A., Swindle T. D., and Kring D. A. 2005. Geochemistry and meteorites to survive. If the latter scenario is correct, earlier 40Ar-39Ar geochronology of impact-melt clasts in feldspathic peaks (3600–4100 Ma and >4450 Ma) presumably represent lunar meteorites: Implications for lunar bombardment history. Meteoritics & Planetary Science 40:755–777. far higher impact rates, producing enough shocked material to Culler T. S., Becker T. A., Muller R. A., and Renne P. R. 2000. Lunar have some survive to the present day. impact history from 40Ar/39Ar dating of glass spherules. Science 287:1785–1788. Acknowledgments–We are indebted to Ted Bunch for the Dalrymple G. B. and Ryder G. 1991. 40Ar/39Ar ages of six Apollo 15 sample of NWA 2058, Eric Olson for the sample of Gao- impact melt rocks by laser step heating. Geophysical Research Letters 18:1163–1166. Guenie, and the Meteorite Working Group and the hardy Dalrymple G. B. and Ryder G. 1993. 40Ar/39Ar age spectra of Apollo souls who braved the LaPaz Icefield for the Antarctic 15 impact melt rocks by laser step-heating and their bearing on samples. We appreciate thorough reviews by Don Bogard and the history of lunar basin formation. 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APPENDIX—PAIRING not understand, as mentioned above), but the two tentative pairs are distinct from one another. LAP 02240 was irradiated We need to consider whether any of our meteorites were in a different irradiation, with a J factor (fluence) an order of paired, since five of them were found at the LaPaz ice field. In magnitude higher, but its value of 0.013 suggests that it is not fact, LAP 031308 and LAP 031173 were suggested to be paired with any of the other four. paired during the initial classification. If some of these Location is not terribly useful—all five were found meteorites are paired, we may be able to combine the data within 7 km of one another (J. Schutt, personal comm.), so from paired meteorites to achieve a more robust pairing of any two is possible. The closest to one another were interpretation. Conversely, we do not want to find the same LAP 03922 and LAP 031308, about 2 km apart. This is not age from two meteorites and count them as two data points if close enough to make pairing seem inevitable, but combined we are really only measuring a single event in paired samples. with the similar CRE ages, pairing seems more likely. We can address the question of pairing in a variety of ways, As far as petrography goes (see Samples section), LAP including cosmic-ray-exposure age, find location, 031125 is a solidified impact melt, which LAP 031173 is a petrography, and, of course, 40Ar-39Ar systematics. melt breccia, so they would not seem likely to be paired. On We start with cosmic-ray-exposure ages. Distinct CRE the other hand, LAP 03922 and LAP 031308 are both melt ages rule out pairing, although similar ages among unpaired breccias, an observation that once again permits, but does not samples are possible. The minimum 36Ar/37Ar ratio is a require, pairing. All this would seem to leave only LAP measure of the CRE age, although calculating the exact age 03922 and LAP 031308 as a potential pair. from that parameter requires knowledge of both the 37Ar In terms of the 40Ar-39Ar data, LAP 031308 and LAP production rate from Ca in the reactor and the 36Ar production 03922 both produced samples showing a variety of behaviors, rate for Ca during the exposure. LAP 03922 (0.047 ± 0.003) but the most prevalent low-temperature age is ∼400 Ma for and LAP 031308 (0.050 ± 0.003) are indistinguishable from LAP 03922 and ∼750 Ma for LAP 031308, and neither shows each other. LAP 031125 (0.070 ± 0.005) and LAP 031173 A the other’s most common age. Hence the ∼750 Ma in LAP and B samples (0.065 ± 0.003) are indistinguishable 031308 could represent less complete resetting than in LAP (although LAP 031173,C gave a value of 0.027 ± 0.04, the 03922, a possibility that should be kept in mind, but we think only case with a distinctly different value, and one that we do it more likely that we actually have five distinct meteorites.