Meteoritics & Planetary Science 40, Nr 5, 755–777 (2005) Abstract available online at http://meteoritics.org

Geochemistry and 40Ar-39Ar geochronology of impact-melt clasts in feldspathic lunar meteorites: Implications for lunar bombardment history

Barbara Anne COHEN1*, Timothy D. SWINDLE2, and David A. KRING2

1Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA 2Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85722, USA *Corresponding author. E-mail: [email protected] (Received 14 May 2004; revision accepted 05 April 2005)

Abstract–We studied 42 impact-melt clasts from lunar feldspathic regolith breccias MacAlpine Hills (MAC) 88105, Queen Alexandra Range (QUE) 93069, Dar al Gani (DaG) 262, and DaG 400 for texture, chemical composition, and/or chronology. Although the textures are similar to the impact- melt clasts identified in mafic Apollo and Luna samples, the meteorite clasts are chemically distinct from them, having lower Fe, Ti, K, and P, thus representing previously unsampled impacts. The 40Ar- 39Ar ages on 31 of the impact melts, the first ages on impact-melt samples from outside the region of the Apollo and Luna sampling sites, range from ∼4 to ∼2.5 Ga. We interpret these samples to have been created in at least six, and possibly nine or more, different impact events. One inferred impact event may be consistent with the Apollo impact-melt rock age cluster at 3.9 Ga, but the meteorite impact-melt clasts with this age are different in chemistry from the Apollo samples, suggesting that the mechanism responsible for the 3.9 Ga peak in lunar impact-melt clast ages is a lunar-wide phenomenon. No meteorite impact melts have ages more than 1σ older than 4.0 Ga. This observation is consistent with, but does not require, a lunar cataclysm.

INTRODUCTION rocks, created in the impact itself, are the most reliable rocks to record the impact date. impact-melt rocks form in the The bombardment history of the Earth-Moon system has bottoms of craters, where pressure-release melting of target been debated since the first recognition that the circular material occurs (Melosh 1989). Craters of all sizes create features on the Moon may be formed by impact processes some melt, from glass soil coatings to impact glass spherules, (Baldwin 1949). In the early 1970s, 40Ar-39Ar and U-Pb glassy breccias, and crystalline melt rocks. Large lunar craters isotopic analyses of Apollo 15, 16, and 17 highland rocks create enough melt to be lined by a melt sheet, where a slow (Turner et al. 1973; Tera et al. 1974) revealed surprising, cooling rate facilitates recrystallization and degassing, widespread isotopic disturbances at 3.9 Ga. Argon and lead producing crystalline, clast-free melt rocks. The minimum losses (and correlated disturbances in the Rb-Sr system) were size at which lunar craters produce clast-free, crystalline attributed to metamorphism of the lunar crust by an enormous impact-melt rocks appears to be 5–7 km, based on cooling number of collisions with asteroids and/or comets in a brief rate considerations (see Deutsch and Stˆffler 1987); ejected pulse of time (<200 Myr) in what was called the lunar melt samples cool more quickly and require a larger parent cataclysm. This event occurred after the saturation of the crater to belong to a large enough melt pool or ejecta blanket highlands and the creation of some 25 large, old lunar basins to cool slowly enough to crystallize. Thus, crystalline impact- (Wilhelms 1987). However, this single pulse was postulated melt clasts can be assumed to be products of impact events to have created >15 large basin structures and resurfaced larger than ∼5 km on the Moon. ∼80% of the Moon. Because impact-melt rocks are fine-grained, remelted Lunar impact history has been recently revived as a topic mixtures of the target materials, they are rarely suitable for of debate (Ryder 1990; Hartmann et al. 2000), largely fueled dating methods that rely on mineral separation, such as U-Pb by the application of laser 40Ar-39Ar dating to impact-melt or Rb-Sr, but are well suited for the 40Ar-39Ar method. The samples (Dalrymple and Ryder 1993, 1996; Cohen et al. 40Ar-39Ar system is only fully reset by a combination of 2000; Culler et al. 2000; Fernandes et al. 2000). Impact-melt temperature and time. Glassy splashes, veins, and agglutinates

755 © The Meteoritical Society, 2005. Printed in USA. 756 B. A. Cohen et al. are so rapidly quenched that 40Ar often does not have time to thorium levels and, by inference, other incompatible elements escape, leading to irregular degassing profiles and such as K, P, and rare earths (KREEP) (Jolliff et al. 2000). The uninterpretable ages (McConville et al. 1988; Cohen et al. impact-melt rocks that have been dated from these collections 2001). Passage of the shock front and subsequent cooling is are predominantly mafic, KREEP-rich samples, which afford also rapid, preventing total degassing in shocked samples enough radiogenic elements to be feasibly dated, but which (Deutsch and Sch‰rer 1994; Bogard et al. 1995). At the other may be dominated by the large amount of melt created in the extreme, rocks exposed to a small elevation in temperature for nearside basins Nectaris, Crisium, Serenitatis, and Imbrium. a long time often show characteristic signs of 40Ar diffusive Haskin et al. (1998) and Korotev (2000) have argued that the loss without complete resetting, such as granulitic impactites mafic, KREEP-bearing impact-melt rocks in the Apollo 14–17 (Cushing et al. 1999) and rocks exposed to daily thermal samples are a special product of impacts into the PKT and not cycling on the lunar surface (Turner 1971; Turner et al. 1971). produced by impact melting of the lower crust under typical Crystalline impact-melt rocks, which have been completely feldspathic highlands. These samples may not include melted and slowly cooled, are the most useful in revealing evidence of earlier events and thus the age distribution absolute impact dates—critical information in determining the obtained from these samples may not reflect the global impact large-scale bombardment history of the Moon. record on the Moon. Lunar meteorites, which were only Crystalline, clast-free impact-melt rocks occur in the recognized after the Apollo missions, provide a new returned Apollo and Luna sample collections as rocks, clasts opportunity to test the hypothesis. These are samples of the in breccias, and soil fragments. They can be distinguished Moon randomly ejected from the lunar surface (Warren and from lunar igneous rocks by their unusual chemical Kallemeyn 1991) without enough velocity to escape the Earth- compositions (combinations of target materials unable to be Moon system. They experienced only mild shock during derived from igneous fractionation processes) and unique launch (Warren 1994) and typically landed on Earth after textures (haystack, poikilitic). The largest example, 14310 ≤1 Ma in space (Warren 1994; Gladman et al. 1995). (3439 g), has nearly identical 40Ar-39Ar and internal Rb-Sr Meteorites that are chemically distinct from the Apollo mafic isochron ages of 3.88 Ga (York et al. 1972; Mark et al. 1974; impact-melt samples may not have been affected by nearside McKay et al. 1979). Melt rocks 68415/6 (544 g total) also have equatorial basin impacts. Instead, impact-melt clasts within a well-constrained Rb-Sr internal isochron of 3.84 ± 0.01 Ga these meteorites may have been formed in large impact events (Papanastassiou and Wasserburg 1971) and a concurrent 40Ar- in other regions of the Moon, as suggested by Taylor (1991) 39Ar age (Stettler et al. 1973), demonstrating the ability of the for MacAlpine Hills (MAC) 88105. 40Ar-39Ar technique to accurately date impact-melt rocks. There are two objectives in this study. The first is to Smaller rocks and breccia clasts are not large enough to permit identify impact-melt clasts in lunar highland meteorites that mineral separation and the majority of impact ages have been are chemically different from the mafic, KREEP-rich Apollo obtained using only 40Ar-39Ar systematics. The availability of and Luna impact-melt rocks. Candidate samples have textures laser heating in 40Ar-39Ar work inspired new, very precise age similar to well-known rocks of impact origin, establishing determinations of Apollo 15 and 17 impact-melt samples their origins in slowly cooled impact-melt sheets, but are (Dalrymple and Ryder 1993, 1996). These samples were feldspathic and lacking in basaltic or KREEPy contributions, inferred to have been formed in the Serenitatis basin at 3.89 Ga indicating their formation either before the formation of the and give an upper limit on the age of the Imbrium basin of 3.85 PKT, or far from the Apollo sampling sites. The second Ga. Formation of the Crisium basin at 3.89 Ga has been objective is to determine the ages of the impact-melt clasts inferred by dating melt rocks in the Luna 20 collection using 40Ar-39Ar techniques. The impact-melt samples occur as (Podosek et al. 1973; Swindle et al. 1991). Together with the small (generally <500 µm) clasts in the meteorite breccias, so inferred radiometric age of Nectaris at ∼3.92 Ga and crater special handling techniques for extracting and analyzing the density age of Orientale at ≤3.75 Ga (Wilhelms 1987; Stˆffler samples had to be developed for this task. These techniques and Ryder 2001), five large basins were formed on the Moon allow us to date a large number of clasts within a single within 200 Ma, though subtle variations in age and trace meteorite, looking for multiple impact events in a single rock element chemistry among the dated samples may argue for and contributing 31 new impact-melt sample ages, a more or fewer impact events creating all the samples statistically significant number, to test the lunar cataclysm (Dalrymple and Ryder 1993; Korotev 1994; Haskin et al. hypothesis. If the Apollo and Luna samples are biased because 1998). of their proximity to one or more large nearside basins, we The preponderance of 3.9 Ga impact-melt rock ages and would expect to see a more random distribution of lunar the lack of older samples of unequivocal impact origin are impact-melt clast ages from the feldspathic breccia meteorites. highly suggestive of an extraordinary bombardment event at We gave a brief report on these sample ages in Cohen et al. ∼3.9 Ga. However, the Apollo and Luna sample sites were all (2000). In this paper, we give a full treatment of the in or near an area called the Procellarum KREEP terrain geochemical and geochronological data that led to the (PKT), identified by the surface expression of elevated conclusions in that paper. For more details, see Cohen (2000). Impact melt clasts from lunar feldspathic regolith breccias 757

METEORITES DaG 400 is not well constrained, solar wind abundance differences between it and DaG 262 (Scherer et al. 1998) rule Four meteorites were obtained for this study: Antarctic out pairing of these two meteorites. Thus, at least three finds MacAlpine Hills (MAC) 88105 and Queen Alexandra distinct areas on the Moon are sampled by these four Range (QUE) 93069, and Libyan desert finds Dar al Gani meteorites, each containing a variety of clasts from a few km (DaG) 262 and DaG 400. All are lunar feldspathic regolith area (Warren 1994). However, clasts from a single event breccias containing abundant impact melt (15–50% by could have found their way to more than one of the breccias volume). Figure 1 shows the studied clasts in being studied. photomicrographs and backscattered electron image mosaics; Table 1 gives details of our thin sections. GEOCHEMISTRY MAC 88105 (Lindstrom 1989) is a fine-grained regolith breccia with abundant angular feldspathic clasts and vesicular Technique melt veins. In thin section, MAC 88105 is a microbreccia of small mineral grains and rock clasts in a brown, glassy matrix. We analyzed one 100-µm thick section of each meteorite; impact-melt and glass clasts make up ∼50% of the rock by this thickness ensures enough material can be extracted for volume, with a range of compositions including Al-rich and 40Ar-39Ar work (tens of micrograms from each clast), but basaltic types (Jolliff et al. 1991; Koeberl et al. 1991; transmitted light still penetrates the section and allows us to Lindstrom et al. 1991; Neal et al. 1991; Taylor 1991). see that the extracted samples were uniform throughout the Regolith components are rare and solar-wind abundance is section depth. Crystalline, clast-poor impact-melt samples low (Eugster et al. 1991), indicating an origin in an immature were identified on the basis of their textures. Petrographically, regolith. they are very fine-grained to cryptocrystalline and milky (not QUE 93069 (Lindstrom 1994; Bischoff 1996) has a light isotropic glasses) and range in color from bright white to gray matrix with abundant millimeter-sized white-to-gray shades of tan and dark brown. We used backscattered-electron clasts. In thin section, QUE 93069 is a microbreccia from a (BSE) imaging to texturally classify the melt clasts using the mature, anorthositic regolith (Kring et al. 1995) containing impact-melt rock nomenclature of Stˆffler et al. (1985) as plagioclase grains, granular clasts, impact-melt clasts, and much as possible (Fig. 2). Table 2 shows the characteristics of glass spherules in a brown, glassy matrix. Pale brown clasts studied by each technique. Most of the clasts in this devitrified glass clasts were identified by Koeberl et al. study are microporphyritic, a few are poikilitic, and (1996), Korotev et al. (1996), and Grier et al. (1995). approximately five clasts have a striated or “haystack” DaG 262 (Bischoff et al. 1998; Jolliff et al. 1999; Floss texture, believed to be an impact-derived texture (Lofgren and Crozaz, 2001; Cahill et al. 2004) is a well-consolidated 1977). While poikiloblastic textures can arise during thermal breccia, consisting of a fine-grained matrix with abundant metamorphism, the Apollo collection contains poikilitic clasts of granular anorthosite and crystalline impact-melt clasts that are unequivocally impact-melt rocks (Dalrymple clasts, melt veins, and metal grains. The breccia is moderately and Ryder 1996). Therefore, poikilitic clasts were also weathered and contains terrestrial weathering products such included in the study despite the possibility that they might as calcite-filled veins. not be impact-melt samples. Their inclusion did not change Dar al Gani 400 (Zipfel et al. 1998; Bukovanska et al. any of our conclusions. 1999; Semenova et al. 2000; Floss and Crozaz 2001; Cahill A Cameca SX-50 microprobe at the University of et al. 2004) also contains terrestrial weathering products. The Arizona was calibrated with mineral standards and operated bulk meteorite is a well-consolidated dark-gray breccia with with a beam current of 20 nA, voltage of 15kV, and mineral fragments, granular rock fragments, and impact-melt integration time of 20 seconds per element (Na was the first clasts. element analyzed). Two olivine and two plagioclase feldspar The cosmic ray exposure (CRE) histories of MAC 88105 standards were analyzed after every calibration and (Nishiizumi et al. 1991; Warren 1994) and QUE 93069 intermittently during data collection. A focused beam was (Thalmann et al. 1996) are sufficiently different to rule out used for point analyses of feldspathic phenocrysts (generally source-crater pairing, with each other or with feldspathic 5–30 µm in size); the beam was defocused to 10 µm for bulk breccia meteorites Y-82192/82193/86032, ALHA 81005, and analyses. Both beam sizes gave identical analyses of standard Y-791197, though QUE 93069 is paired with mixed highland/ minerals. In addition, rhyolitic glass and tektite glass mare breccia QUE 94269 (Nishiizumi et al. 1996; Polnau and standards were analyzed to show that no significant Eugster 1998). QUE 93069 is also similar to DaG 262 in volatilization (<10%) of K or Na occurs with a 10 µm beam chemistry and CRE age (Eugster et al. 2000), but glass under the microprobe conditions used. The analytical spherules are abundant in QUE 93609 and lacking in DaG uncertainty for each oxide, based on counting statistics, is 262 (Bischoff et al. 1998), possibly indicating their origins in reported in Table 3. different places on the Moon. Though the CRE history of A set of points (5–50) set up in a grid pattern over a single 758 B. A. Cohen et al.

Fig. 1. Grayscale, transmitted-light photomicrographs (left) and BSE mosaics (right) of the thin sections in this study. The photomicrographs are all at the same scale indicated by the 1 cm scale bar in (c). The microcore locations are shown in the photomicrographs; each site has a bright ring where material was etched away to define a sample the size of the darker middle circle, as shown in the example in (a). The BSE mosaics are enlarged to show the impact-melt clasts identified in each section (outlined areas).

Table 1. Details of the thin sections studied. Meteorite Area of thin section (cm2) Number of melt clasts identified % Impact melt by volume MAC 88105 0.75 9 43 QUE 93069 ∼11114 DaG 262 0.25 16 45 DaG 400 1.5 >40 44 Impact melt clasts from lunar feldspathic regolith breccias 759

Fig. 2. BSE images of impact-melt clast textures: (a) and (b) are microporphyritic (400 K, 400 I), (c) is striated (262 A), and (d) is poikilitic (262 D). Dark gray is plagioclase, light gray is pyroxene, and white is olivine. The scale bar in all panels is 20 µm. clast was analyzed with the 10 µm beam and all good analyses feldspathic, containing >80% normative feldspar (∼An96), (95% < total <105%) were averaged together to arrive at the and all are olivine- and pyroxene-normative. Figure 3 shows bulk composition. Table 4 shows the average composition and major-element relationships among the meteorite clasts. standard deviation within each clast. The standard deviation is Lunar feldspathic breccias can be described, to first order, in generally not an accurate measurement of analytical terms of three compositional parameters (Korotev et al. uncertainty because each impact-melt clast is heterogeneous 2003): the Al2O3 concentration (anticorrelated with FeO + on varying scales, but it does give an estimate of the sample Mg), which reflects the ratio of plagioclase to iron-bearing heterogeneity. The defocused beam technique yields absolute minerals, the concentrations of incompatible elements (for elemental abundances that are different from true abundances which K2O is a proxy in Fig. 3a), and the MgO/FeO ratio because of the interaction of excited elements from multiple (Fig. 3b), which increases with increasing olivine/pyroxene in phases (Warren 1997). We applied only standard ZAF the mafic mineral fraction. corrections to our defocused beam analyses; thus, there may Figure 3 shows that, by and large, the impact-melt clasts be a systematic uncertainty of a few percent. However, we are within each meteorite are compositionally similar to the bulk interested in comparing clasts with each other and identifying composition of the meteorite. As noted by Delano (1991), the clusters, so systematic errors of a few wt% are not range of compositions of impact-melt clasts within any single meaningful. meteorite is smaller than the range of Apollo 16 regolith breccia compositions (Fig. 3), implying that the Results compositional diversity within the few-kilometer area of meteorite assembly is much less than the Apollo 16 site. The A CIPW norm was calculated from the bulk composition compositional diversity of the Apollo 16 breccias arises from of each clast and the normative feldspar content and their proximity to the PKT and to nearside maria, where mafic compositions are shown in Table 2. All identified clasts are and KREEP-rich material was distributed by the nearside 760 B. A. Cohen et al.

Table 2. impact-melt clast characteristics. Clast size Norm. wt% Name of clast (µm) Texture feldspar Microprobe 40Ar-39Ar MAC A 400 × 2000 Poikilitic 79 × × MAC B 3000 × 4000 Microporphyritic 91 × × MAC C 700 × 600 Microporphyritic 96 × × MAC D 500 × 1000 Microporphyritic 92 × × MAC E 500 × 1000 Microporphyritic 95 × × MAC F 2000 × 2000 Microporphyritic 89 × × MAC F2 1000 × 1000 Microporphyritic – – × MAC G 1000 × 1000 Microporphyritic 86 × × MAC H 2000 × 3000 Microporphyritic 93 × × MAC I – – – – × QUE A 150 × 100 Microporphyritic 83 × – QUE B 800 × 500 Microporphyritic to glassy 87 × – QUE D 1000 × 750 Microporphyritic 86 × × QUE D2 – – – – × QUE E 600 × 600 Poikilitic – – × QUE F 500 × 500 Microporphyritic 77 × × QUE F2 200 × 300 Poikilitic 86 × – QUE G 400 × 800 Poikilitic 72 × × QUE I 500 × 500 Microporphyritic 90 × × QUE K – – – – × 262 A 500 × 400 Striated 61 × × 262 D 350 × 200 Poikilitic – – × 262 E 200 × 200 Poikilitic 69 × – 262 F 1000 × 700 Microporphyritic 82 × × 262 G – Microporphyritic – – × 262 H 150 × 250 Microporphyritic 87 × × 262 I 200 × 100 Striated 82 × × 262 J 500 × 500 Microporphyritic 89 × – 262 M – Microporphyritic 64 × – 262 N 1000 × 500 Microporphyritic – × – 262 O 400 × 300 Poikilitic – × × 262 P 1500 × 500 Glassy 72 × × 262 Q2 5000 × 5000 Microporphyritic 80 × × 262 R 1000 × 1000 Microporphyritic 87 × × 400 A1 – Microporphyritic 80 × × 400 A2 – Microporphyritic 78 × – 400 B – Microporphyritic 76 × – 400 C1 – Microporphyritic 92 × × 400 C3 – Microporphyritic – – × 400 D – Microporphyritic 73 × × 400 E – Microporphyritic 82 × – 400 G – – 79 × – 400 H – Microporphyritic 84 × – 400 I – Microporphyritic 82 × – 400 J – Microporphyritic 87 × – 400 K – Microporphyritic 88 × – 400 L – Microporphyritic 81 × – 400 L1 – Microporphyritic 98 × – 400 L9 – Crystalline plagioclase – – × 400 L15 – Poikilitic – – × 400 P – Microporphyritic 82 × – 400 Q – Microporphyritic 84 × × 400 R – Microporphyritic – × – 400 T – Microporphyritic 82 × × 400 T2 – Microporphyritic 85 × × Impact melt clasts from lunar feldspathic regolith breccias 761

Table 2. Continued. impact-melt clast characteristics. Clast size Norm. wt% Name of clast (µm) Texture feldspar Microprobe 40Ar-39Ar 400 U – Microporphyritic 84 × – 400 V – Microporphyritic 81 × × 400 V2 – Microporphyritic 79 × 400 W – Microporphyritic – – × 400 AA – Microporphyritic – × × 400 BB – Microporphyritic 80 × × 400 DD – Striated-porphyritic – – × 400 FF – – – – × 400 HH – Microporphyritic 79 × – 400 JJ – Microporphyritic 81 × – basins and worked into the regolith. In contrast, the clasts in meteorites in this study, crushing is not an effective way to this study have extremely low K2O and P2O5 contents (both extract impact-melt clasts because the breccia may not break <0.05 wt%), ruling out their origin in the PKT (Fig. 3a). Mafic cleanly along clast boundaries. Therefore, we employed a impact-melt clasts have been reported in both MAC 88105 microcorer to extract individual melt clasts from the thick and QUE 93069 (Jolliff et al. 1991; Koeberl et al. 1991; section for dating (Cohen 2000). We extracted core samples Lindstrom et al. 1991; Neal et al. 1991; Taylor 1991; Koeberl from the 100-µm thick sections with diameters ≥100 µm in et al. 1996; Korotev et al. 1996); few mafic clasts have been order to obtain enough material. When impact-melt clasts reported in DaG 262 or DaG 400 (Bischoff et al. 1998; contained fragments of possibly relict materials, the selected Fernandes et al. 2000; Semenova et al. 2000). No clasts in this microcore avoided these fragments. Each microcore was study show evidence of containing any significant proportion drilled and removed from the section using distilled water. of basaltic material, having low FeO and MgO (Fig. 3b) and Within a few hours of being cored, each sample was baked in TiO2 (<0.5 wt%). On the other hand, the diversity of Mg’ a 200 °C oven for approximately one hour to dry and degas (molar Mg/Mg + Fe) among the clasts (Fig. 3c) suggests that any epoxy that might have remained, though the epoxy still multiple impacts may be responsible for creating the impact- produced problems, as discussed later. Table 2 compares melt samples, because the impact melting process might be some characteristics of the extracted impact-melt clasts. expected to very effectively homogenize Mg’ in the melt Microcored samples were placed into wells in pure products. (99.999%) aluminum metal discs and irradiated for 500 hr in The major element chemistry of the impact-melt clasts in location L-67 of the Phoenix-Ford Memorial Reactor at the this study implies either that the source terrain for each University of Michigan. The irradiation took place over six breccia is well within the feldspathic highlands, i.e., far from weeks in five separate doses to produce a J-factor of 7.33 × the incompatible-rich PKT or mafic South Pole–Aitken basin 10−2. The standards Ga1550 biotite (97.9 ± 0.9 Ma) terrane, or that the melt rocks formed before the widespread (McDougall and Roksandic 1974) and MMhb-1 hornblende distribution of material from these terranes. Additionally, the (523.1 ± 2.6 Ma) (Renne et al. 1998), as well as pure CaF and differences among both meteorites and clasts within each K2SO4 salts, were simultaneously irradiated. meteorite implies that these clasts were not all created in a The sample discs were placed into a laser port covered by single impact event, but rather formed in several different an optically transparent silica window. A Liconix 5000-series events. However, within the uncertainties of our analytical continuous Ar ion (514 nm) laser step-heated each sample for techniques, we could not discern groups of chemically 30 sec at each step. Heating steps were achieved by varying identical clasts; thus, we were unable to use major element the laser current between 10 and 17 A, rastering the beam chemistry and texture to distinguish families of feldspathic over the sample surface, and, for the lowest-temperature step, impact-melt samples created in common impact events. by defocusing the 10 A laser beam over the sample. Though the actual temperature achieved at each step was not GEOCHRONOLOGY measured, it is not necessary to the experiment. The minimum temperature achieved (with the 10 A defocused beam) is Technique ∼800–900 °C, known because the samples began to glow; samples generally fused between 13 A and 17 A. Thus, all Because the impact-melt clasts can be quite small steps in these experiments are high-temperature steps relative (∼100 µm) and fine-grained, mineral separates cannot be used to conventional 40Ar-39Ar experiments. The small size and for dating. On the other hand, contribution of Ar from low potassium content of the samples limited the number of adjacent clasts or matrix is a concern for in situ laser heating. heating steps that could be performed per sample. The sample In well-consolidated, glass-bound breccias such as the gas was incrementally expanded into a volume of ∼850 cm3 762 B. A. Cohen et al. Total 5 O 2 OP 2 OK 2 Na NiO FeO MnO MgO CaO 3 O 2 Cr 3 O 2 Al ition of impact-melt clasts. 2 TiO 2 SiO a 29 42.9 0.14 29.4 0.0617 43.1 0.02 0.27 4.34 30.1 0.07 0.11 4.68 0.03 16.8 3.49 0.35 0.05 0.04 4.12 0.05 17.1 98.9 0.35 0.05 0.09 98.9 b b 400 F400 G400 H 6400 I 5 43.5400 J 3 42.9400 K 5 43.3400 L 3 0.18400 L1 43.5 6 0.17400 P 43.5 8 0.20 43.5 5400 Q 29.6 0.19 44.6400 Q 3 28.3 43.4 0.12 6 0.13 30.1 43.6 0.16 0.09 29.5 43.1 0.03 0.06 31.3 31.6 0.06 0.20 0.13 29.1 0.04 0.04 35.4 0.02 0.04 0.05 n.d. 29.4 30.5 0.09 0.02 4.09 0.04 5.31 n.d. 0.02 0.08 3.48 0.08 0.05 3.91 0.06 0.04 0.07 2.89 n.d. 2.56 0.06 n.d. 3.72 0.08 0.26 4.62 5.74 0.07 0.02 3.85 4.21 3.63 0.06 16.4 4.51 n.d. 15.8 3.36 0.06 3.09 17.4 0.05 0.40 4.54 17.0 0.41 0.66 17.8 0.35 18.0 4.83 0.06 0.42 3.88 17.4 0.06 19.1 0.39 0.36 0.03 16.6 0.38 0.07 0.04 17.4 0.35 0.08 0.05 0.04 0.29 0.09 0.05 0.39 0.11 99.0 0.03 99.0 0.07 0.06 0.06 99.4 0.05 0.06 99.3 0.14 99.5 0.07 99.3 100.2 0.07 99.4 99.1 99.4 MAC AMAC B 12MAC C 33 44.6MAC D 20 44.5MAC E 15 44.0MAC F 11 0.15 44.4MAC G 11 0.14 44.0MAC H 0.08 7 44.6QUE A 43 0.12 28.5QUE B 44.7 15 0.06 32.7 44.3QUE D 34 34.7 0.13 44.0QUE F2 46 33.3 0.07 0.19QUE F 44.1 12 0.12 34.2 0.06 43.7QUE G 28 0.04 0.21 32.3 44.3QUE I 15 0.04 n.d. 0.20 30.9262 A 44.2 33.4 0.15 n.d. 0.03 13 44.7262 E 0.07 0.03 29.7 0.03262 F 3 43.9 0.03 31.5 0.30 0.04262 H 4.82 5 0.03 0.20 31.0 0.03 44.3262 I 2.22 3 31.0 0.02 0.09 1.21 0.09 46.9262 J 4 0.06 27.7 1.37 0.03 43.9262 M 0.07 0.43 4 0.03 25.9 0.07 45.4 1.32262 N 0.04 4 0.07 0.35 0.02 2.67 32.4262 O 0.03 4 44.4 0.18 0.02 0.12262 P 0.02 43.5 5 3.32 0.26 21.7 4.90 48.2 0.09 0.02 1.89262 Q2 0.02 4 n.d. 24.8 2.20 0.24 43.9 4.68262 R 0.04 0.05 5 10 1.09 29.4 0.16 41.6 0.03400 A 3.33 0.52 1.56 0.05 31.2 0.20 16.5 0.02 44.3 3.63400 A2 0.03 44.7 5 0.06 1.59 0.14 18.3 29.3 2.88400 B 0.02 0.07 8 0.04 2.10 18.9 0.08 32.2 4 44.2400 C 5.61 0.06 0.38 22.7 18.7 0.10 0.21 0.02 0.32 3.17 43.4 6.17400 D 0.06 8 0.35 1.59 44.0 32.2 18.9 0.04 14 0.03400 E 0.07 0.33 2.40 0.17 26.7 3.47 18.2 0.03 0.05 43.4400 E 0.35 3 0.09 0.04 0.21 0.18 43.8 n.d. 3.35 25.9 28.9 9.15 17.9 0.33 0.11 0.36 5 0.02 18.6 3.11 0.02 43.7 0.02 5.50 n.d. 0.34 0.03 0.17 2.07 31.4 0.03 17.3 0.03 43.9 4.68 0.08 0.06 0.02 28.8 0.03 0.35 5.33 0.13 17.7 0.09 0.13 2.62 28.2 0.38 0.02 0.19 0.04 5.54 0.03 17.5 0.11 3.67 0.42 n.d. 0.12 27.2 0.03 0.03 0.07 18.5 2.38 1.76 33.3 0.07 0.04 0.33 100.1 0.10 0.02 7.88 0.18 0.02 15.8 10.32 0.07 0.03 0.05 0.32 0.03 26.1 100.6 17.1 2.94 0.39 0.05 n.d. 7.81 100.4 32.3 0.12 0.04 0.02 5.51 18.0 0.04 0.04 100.0 0.02 0.38 5.88 0.05 0.04 0.13 7.59 4.06 13.2 n.d. 0.05 0.24 0.02 100.6 0.07 3.02 0.04 0.02 0.35 14.9 0.04 0.08 100.5 0.03 4.14 2.47 0.07 0.04 0.05 0.03 16.0 2.36 100.7 0.29 4.60 0.10 0.05 5.22 100.5 n.d. 0.03 0.05 4.97 17.8 0.31 2.84 0.03 n.d. 0.02 100.0 16.9 6.04 11.49 0.06 0.33 1.87 100.7 0.12 0.07 18.4 0.07 0.40 14.1 7.57 4.24 0.03 5.90 0.07 99.6 0.04 0.43 18.3 99.4 2.48 0.02 0.07 14.0 0.05 0.36 3.35 0.04 0.08 0.45 99.9 0.06 5.39 14.9 100.1 16.9 0.09 0.34 5.98 0.05 0.09 0.26 0.02 99.7 6.05 0.10 0.05 17.8 2.11 0.16 0.38 0.39 99.9 0.06 16.2 0.03 15.6 6.36 101.0 0.10 0.02 0.39 100.6 2.96 15.8 0.08 0.38 18.4 0.11 0.04 0.06 100.9 0.34 0.06 16.4 0.04 99.3 0.36 0.04 17.8 0.36 99.0 0.03 0.05 0.13 99.6 0.05 0.39 100.7 0.38 0.07 99.8 0.09 0.03 0.11 101.3 99.9 0.04 0.06 0.04 0.05 100.1 0.05 99.2 0.11 99.8 0.05 99.4 100.2 99.5 100.1 Clast No Table 3.Table Major element compos Impact melt clasts from lunar feldspathic regolith breccias 763 Total 5 O 2 OP 2 OK 2 Na 0.1–5.7 0.01–0.16 0.01– 0.07 0.01–0.34 01–0.49 0.03–10.8 0.01–0.14 0.05 –8.88 NiO FeO MnO MgO CaO ) averaged together. ) averaged together. % 105 3 ibility of technique. < O 2 Cr total <

% 3 O 2 Al to ascertain reproduc with each element (counting statistics). 2 ion among analyses of each clast. TiO 2 Major element composition of impact-melt clasts. 0.21.0 0.020.2–4.4 0.12 0.01–0.39 0.2–15.8 0.2 0.01–0.19 3.7 0. 0.07 0.06 0.08 0.03 0.12 2.46 0.00 0.04 2.80 2.71 0.2 1.8 0.02 0.06 0.06 0.02 0.13 0.06 SiO a 34 43.3 0.1825 43.2 29.7 0.17 0.0819 28.4 43.4 0.04 0.21 0.08 3.79 29.1 0.12 0.05 0.11 4.61 4.46 0.03 0.06 16.9 4.12 6.08 0.34 0.07 16.2 0.04 0.34 4.81 0.14 0.06 16.7 99.1 0.36 0.05 0.05 99.4 0.10 99.0 Continued. b b b not detected. = d d c Mean s.d. Range s.d. 400 T2400 U 9400 V400 V2 7 43.9400 V2 4 3 44.2400 AA 43.8400 BB 0.14 44.1 4400 EE 0.20 4400 HH 43.4 0.23 3400 JJ 0.14 30.7 10 43.6400 JJ 42.5 30.1 43.3 0.17σ 3 28.9 0.20 31.3 0.04 43.5 n.d. 0.25 0.07 31.7 0.10 28.6 0.08 n.d. 0.25 35.4 28.5 0.03 0.07 0.03 0.11 n.d. 27.6 3.15 0.02 0.10 3.56 n.d. 3.75 n.d. 0.09 3.21 0.04 0.03 0.03 0.07 2.47 0.07 4.03 n.d. 0.05 3.91 0.19 4.40 3.97 0.03 4.53 0.06 4.22 3.80 17.6 n.d. 0.07 17.1 3.02 17.3 0.36 5.04 0.06 17.4 0.45 0.32 5.08 0.41 18.0 0.36 0.05 16.5 5.64 0.05 20.4 16.3 0.40 0.08 0.42 0.05 0.07 16.7 0.29 0.39 0.05 0.07 0.06 0.06 0.08 100.0 0.42 n.d. 0.08 99.8 0.05 99.3 0.12 100.5 0.09 0.25 0.02 99.4 98.7 0.12 98.7 99.2 98.7 Clast400 R No 400 T400 T 4 5 43.7 43.1 0.11 0.14 34.0 28.6 0.03 0.11 0.03 0.03 1.16 4.45 0.03 0.08 1.65 5.50 18.5 16.7 0.40 0.35 0.05 0.05 0.03 0.30 99.7 99.5 n. d. Two different grids analyzed on these clasts different Two Mean and range in standard deviat The number of separate, defocused-beam analyses (95 Absolute analytical uncertainty associated Table 3. Table a b c d 764 B. A. Cohen et al.

Table 4. 40Ar-39Ar dating results. Weight % of 39Ar Uncorrected a 40 36 b c Sample (µg) in plateau plateau age ±1σ ( Ar/ Ar)i Isochron age ±1σ Best-estimate age ±1σ MAC A 42 64 2531 ± 1502 2474 ± 1549 MAC B1 55 12 3012 ± 173 2944 ± 189 MAC C 25 82 3347 ± 209 3298 ± 217 MAC D 22 93 3597 ± 269 3248 ± 251 MAC F 121 29 3807 ± 60 1.8 ± 1.9 3730 ± 103 MAC F2 100 47 3942 ± 92 3905 ± 96 MAC G 35 81 3176 ± 263 3415 ± 272 MAC H 44 77 3910 ± 149 5.2 ± 4.2 3360 ± 575 MAC I 39 72 4038 ± 102 3.9 ± 1.9 3763 ± 108 QUE E 19 441 3850 ± 225 3341 ± 663 QUE F 16 35 3015 ± 423 2993 ± 430 QUE G 4 521 2516 ± 2593 1838 ± 2488 QUE I 24 63 2727 ± 1019 2685 ± 1035 QUE K 27 57 3814 ± 437 3727 ± 459 262 H 3 61 4123 ± 475 4120 ± 474 262 Q2 25 51 3550 ± 169 3520 ± 174 262 R 27 54 2429 ± 171 2429 ± 170 400 A1 13 37 2577 ± 352 2556 ± 354 400 C1 37 25 2622 ± 613 2619 ± 611 400 C3 12 61 3407 ± 85 4.1 ± 5.1 3312 ± 152 400 D 24 52 3235 ± 169 3235 ± 129 400 L9 3 100d 2453 ± 532 2455 ± 519 400 L15 34 61 2924 ± 191 2914 ± 190 400 Q 61 64 3467 ± 69 3395 ± 84 400 T 31 64 3335 ± 213 3335 ± 213 400 T2 21 57 2744 ± 369 2697 ± 381 400 W 80 70 3610 ± 35 1.7 ± 0.7 3390 ± 95 400 AA 48 51 3074 ± 104 0.9 ± 5.1 3070 ± 197 400 BB 50 82 2967 ± 84 2967 ± 84 400 DD 48 48 3111 ± 141 1.6 ± 7.5 3100 ± 178 400 FF 30 56 2788 ± 271 2788 ± 234 aAges uncorrected for trapped Ar, representing the upper limit to sample ages. b 40 36 After the isochron-defined ( Ar/ Ar)i was subtracted. cAfter the best-estimate ratio (40Ar/36Ar = 3 ± 3) was subtracted. dPlateau comprises a single heating step. including an SAES SORB-AC getter. The getter was off salts, monitors, and samples were irradiated together and during the DaG 262 runs, leading to a low signal-to-noise corrected to the same date, corrections based on measured ratio and larger uncertainties. ratios will still yield all isotopes in the correct ratios. Sample analysis took place in several batches over two Therefore, the relative K/Ca ratios from one step to another or months. System blanks were measured each day, averaged one sample to another are accurate. Sample K2O contents over each analysis batch, and subtracted from the data. The calculated using 39Ar content are similar to, but sometimes variation in blank levels over the entire data collection period higher than, the K2O contents found using the microprobe. was ≤50%. The average of all blank runs at masses 36 to 40 is, We are not sure what causes the discrepancies; however, in no in order, 1.37 × 10−13, 1.49 × 10−13, 3.10 × 10−14, 1.89 × 10−13, cases are the calculated values more than a factor of and 3.63 × 10−11 ccSTP. The contribution from HCl at masses approximately 3 higher than the measured values, and in no 36 and 38 was ∼0.03%; thus, this negligible correction was case is the apparent K2O content >0.1 wt%. not considered further. Cosmic ray contributions of 38Ar and 36Ar were All isotopic corrections in the samples and standards subtracted by deconvolving the 38Ar/36Ar signatures from depending on the time since irradiation were based on the last cosmic-ray-spallation (36Ar/38Ar = 0.66; Hohenberg et al. day of irradiation; this method has no effect on the amount of 1978) and terrestrial atmosphere (36Ar/38Ar = 0.19). This 39Ar, but systematically underestimates the amount of 37Ar by correction completely subtracted all 36Ar from five samples ∼50% (irradiation took place over approximately one half-life (400D, 400L9, 400T, 400BB, and 400FF). Note that, in these of 37Ar) for all samples and standards. However, because all cases, 36Ar = 0, but still has an associated uncertainty; this Impact melt clasts from lunar feldspathic regolith breccias 765

uncertainty was propagated when any further corrections involving isotopes ratioed to 36Ar were attempted (such as the trapped-argon corrections discussed below). Figure 4 shows argon release data for each sample. A plateau in a sample (steps shown in bold) consisted of consecutive steps with apparent ages within 2σ of each other, except when a step contained no measurable 40Ar above the background and less than 2% of 39Ar released (e.g., the third degassing step in MACC). We calculated sample ages by adding together the gas released in each plateau step and using this total amount of 40Ar and 39Ar to calculate a single integrated age and associated uncertainty, rather than calculating some other weighted average of the individual heating step ages. Including the heating steps where the apparent age was zero lowers the age by less than 1% in any case. Most samples showed a high apparent age in the first heating step. We attribute this to degassing of atmospheric argon from bubbles in epoxy clinging to the microcore and discount the first heating step in all cases, including those samples in which the first heating step was within 2σ of the following steps (sample 400 L9 had three total degassing steps; the first two released 40Ar and 36Ar but no 39Ar above the background and so they do not appear in Fig. 4v. We discarded these steps in the age calculation.). It should be noted that we took the most conservative approach with regard to individual data points within each sample; some heating steps, as well as some entire samples, were excluded from the data set when the data were not robust for any reason. However, the ages calculated using these questionable data did not differ substantially from the ages reported and would not affect our conclusions. In samples with remaining excess 36Ar after the cosmic ray correction, a three-isotope plot was constructed to determine the amount of trapped Ar. A modified York fit accounting for uncertainty in both isotope ratios was calculated based on the same temperature steps used in the plateau. Not every individual temperature step released 36Ar above background, thus, the same number of degassing points is not always available in the isochron. We were able to fit meaningful isochrons to seven samples (MACF, MACH, MACI, 400C3, 400W, 400AA, and 400DD) and deduce an age based on the slope of the data (Table 4). In these cases, the plateau age (after subtracting the initial 40Ar/36Ar) is exactly the same, within uncertainty, as the age calculated from the isochron. Due to the relatively large uncertainties in the remaining samples, the (40Ar/36Ar) was largely unconstrained. To Fig. 3. Major element chemistry of the impact-melt clasts in this i address this, we developed three possible scenarios: a study, shown as a function of Al2O3 content: a) K2O (wt%), b) MgO + FeO (wt%), and c) Mg’ (molar Mg/[Mg + Fe]). Each point terrestrial atmosphere correction (40Ar/36Ar = 295.5), a recent represents the average composition of a single melt clast. Shown for solar wind exposure correction (40Ar/36Ar ≤ 1.0; Eugster et al. comparison are fields encompassing whole-rock analyses of 2001), and a conservative best estimate correction based on feldspathic lunar meteorites (Al2O3 = 25–31 wt%; [Korotev et al. the average of the isochron intercepts, which propagates an 2003]), Apollo 16 feldspathic regolith breccias (Al2O3 = 20–34 wt%; 40 36 [McKay et al. 1986]), and Apollo mafic impact-melt breccias uncertainty through the subtraction ( Ar/ Ar = 3 ± 3). The (Al2O3=12–22 wt%; [Korotev 2000]). best estimate corresponds to surface exposure <3 Ga ago 766 B. A. Cohen et al.

Fig. 4. Argon release patterns and K/Ca ratios for impact-melt samples in MAC 88105 (a–i), QUE 93069 (j–n), DaG 262 (o–q), and DaG 400 (r–ee). Impact melt clasts from lunar feldspathic regolith breccias 767

Fig. 4. Continued. Argon release patterns and K/Ca ratios for impact-melt samples in QUE 93069 (j–n), DaG 262 (o–q), and DaG 400 (r–ee). 768 B. A. Cohen et al.

Fig. 4. Continued. Argon release patterns and K/Ca ratios for impact-melt samples in DaG 400 (r–ee).

(Eugster et al. 2001), a conservatively wide range. Allowing a sample age. Ages calculated using this correction are thus higher ratio would mean an earlier exposure but would push reasonable estimates of the sample ages (Table 4) that take the corrected ages younger, sometimes requiring the into account propagation of uncertainty in the ages. physically unrealistic case of exposure before formation. The Five samples had no 36Ar contribution to subtract; seven terrestrial atmosphere correction was usually a gross had data with which we were able to construct an isochron overcorrection, subtracting all 40Ar and causing apparent ages and calculate the 40Ar/36Ar ratio to be subtracted. We could to become zero. The recent solar wind correction did not not deconvolve the relative contributions of the various change the apparent age in any sample by more than 0.01%. trapped-argon components in the remaining 30 samples. The The best estimate correction can be considered a reasonable ages reported in Table 4 reflect the different techniques we guess of the amount of trapped lunar Ar present in samples were able to use to arrive at the sample ages. However, one of with no terrestrial atmosphere Ar. The magnitude of this the goals of the study is to be able to compare clast ages correction depends on the amount of 36Ar remaining in the within and among the meteorites. For this purpose, we must sample, but generally caused less than a 10% reduction in use a sample age data set that is internally self-consistent. Impact melt clasts from lunar feldspathic regolith breccias 769

Therefore, we approach this problem twice: first, using all chemically. The QUE 93069 samples fall into a single normal 40 36 data without correcting for ( Ar/ Ar)i, and second, using the distribution with a large uncertainty, 2.38 ± 0.69 Ga, ages corrected using the derived isochron 40Ar/36Ar ratios for indicating that any textural or compositional differences seen the seven samples and the ages calculated using the best- in the major element analyses have no correlation with the age estimate correction (40Ar/36Ar = 3 ± 3) for all other samples. of the samples. Ages derived from the uncorrected data must be considered Because of a technical problem with the SAES getter, upper limits to the true sample ages with smaller uncertainties only three clasts from DaG 262 had interpretable ages, and than may be warranted. Any corrections would lower the two of the three do not overlap until the 3σ level. The oldest apparent sample ages, an effect that only strengthens our main clast is 4.1 Ga, but with a large uncertainty of 0.5 Ga. The conclusions that no impact-melt rocks older than 4.0 have yet other two clasts are younger at 3.5 Ga and 2.4 Ga. There was been found and that many impact-melt clasts from lunar no obvious difference in chemistry or texture among these meteorites have ages much younger than the canonical three clasts. The thirteen impact-melt clasts in DaG 400 range cataclysm. more widely in age than do the ten in MAC 88105. Many different ages are recorded, although some of these ages Results overlap at the 1σ level. The youngest group (2.59 ± 0.45 Ga) contains four samples, including one crystalline-plagioclase The data set reported in Cohen et al. (2000) consisted of clast, the oldest and youngest ages of which overlap at the 1σ only the uncorrected sample ages; in this paper, we report level, but form a broad distribution. Better defined groups both data sets but, for consistency, refer to the best estimate occur at 3.04 ± 0.21 Ga (six samples) and 3.36 ± 0.14 Ga (40Ar/36Ar corrected) sample ages in our discussion unless (three samples). otherwise indicated. We also explore the effect on our results The fact that MAC 88105, QUE 93069, and DaG 400 are of using two data subsets: those samples with large plateaus not paired with each other, and probably come from different and those samples with the smallest uncertainties. places on the Moon, implies that each of the impact events Table 4 lists ages derived from the data before and after represented in these meteorites is a different event on the 40 36 the ( Ar/ Ar)i corrections (determined by the isochron or lunar surface. Thus, in these three meteorites, we find best estimate correction) were applied. Each sample age can evidence for six or seven different impact events (Table 5). be represented by a Gaussian distribution having a width Including the two well-dated clasts from DaG 262 as proportional to the 1σ uncertainty and a unit area under the representing two more impact events raises this number to curve. As the uncertainty in age decreases, the individual eight or nine, though as mentioned in the Meteorites section, sample’s curve becomes narrower and higher at the peak. The DaG 262 and QUE 93069 may be from a similar source area individual Gaussian curves for the samples add together to on the Moon, but not paired. In addition, there are several produce an ideogram (Fig. 5), the advantage of which over a samples in MAC 88105 and DaG 400 that do not fall into any traditional age histogram is that the ideogram accounts for the of the groups, which may represent distinct events. Therefore, uncertainty associated with each sample. A normal we conclude that these four meteorites contain evidence for at distribution fitted to each peak in the ideogram identifies the least six to nine different impact events on the Moon. age (and associated uncertainty) of a single impact event that Figure 6a shows an ideogram of all the impact-melt could have created the group of samples (Table 5). In this samples dated in this study along with a histogram of the approach, we disregarded peaks made up of only one sample, inferred impact events (from the corrected ages) that were or peaks with very low probabilities (wide distributions), sampled in each meteorite. The peak of the meteorite impact- though these could represent still other impact events. The melt clast ideogram is much younger than the 3.9 Ga peak same samples make up the age clusters in both the observed in the Apollo samples. We considered the possibility uncorrected and corrected data sets, with the exception of of a systematic error in the data collection or reduction, QUE 93069, the samples of which make two distinct groups however, the simultaneous analysis of two different flux in the uncorrected data but one in the corrected data. monitors argues against this possibility. In addition, during Two clear peaks arise in the age distribution of impact- the analysis of the lunar samples, a clast from the H chondrite melt clasts within MAC 88105: a group of four clasts at Ourique was dated using the same techniques and reduction, 3.79 ± 0.14 Ga, within 1σ of the presumed age of the yielding an age of 4.45 Ga (Kring et al. 2000). Some melt Orientale basin (Stˆffler and Ryder 2001), and three clasts at samples contained mineral fragments that may have been 3.23 Ga. This supports Taylor’s (1991) suggestion that either phenocrysts or incompletely degassed relic minerals, multiple impact-melt populations exist within MAC 88105, but fragments of older rock entrained in the melt would yield though chemically the two groups could not be distinguished older apparent sample ages, not younger. in this study. The one poikilitic clast is included in the We also considered shock, exposure, and recoil effects younger group and is not statistically different in age from the when interpreting the 40Ar-39Ar data. Shock and solar heating other impact-melt clasts, though it is slightly different on the lunar surface can partially degas or rearrange 40Ar 770 B. A. Cohen et al.

Fig. 5. Age ideograms for impact-melt clasts in each meteorite using corrected sample ages. Each ideogram (solid curve) is a sum of all the Gaussian curves, representing clast ages. The dashed curves are normal distributions fit to the ideogram peaks, yielding an average age for each group of clasts. The y-axis scale (relative probability) is the same in each panel.

Table 5. Inferred impact event ages based on normal distributions fit to ideograms. Meteorite Clasts Event (Ga)a Event (Ga)b MAC 88105 C, D, G 3226 ± 244 3364 ± 294 MAC 88105 F, F2, H, I 3794 ± 140 3915 ± 144 QUE 93069 E, F, G, I, K 3275 ± 692 2969 ± 548; 3841 ± 288 DaG 400 A1, C1, L9, T2 2590 ± 447 2614 ± 448 DaG 400 D, L15, AA, BB, DD, FF 3035 ± 214 3037 ± 174 DaG 400 C3, Q, T 3358 ± 143 3433 ± 98 aUsing isochron- or proxy-corrected ages. bUsing uncorrected ages; two age clusters are apparent in QUE 93069 uncorrected data. within a rock, causing it to lose radiogenic 40Ar from its least further. Samples that have experienced recoil show retentive sites (Deutsch and Sch‰rer 1994) and producing a characteristically high ages in their low temperature release disturbed step-heat profile where low temperature sites show steps, where fine-grained, less retentive sites have lost their a young apparent age. The most retentive sites may remain 39Ar (Turner and Cadogan 1974; McDougall and Harrison undisturbed, yielding a plateau in the high temperature steps. 1999), and low ages in the high temperature steps, reflecting This profile is commonly seen in lunar rocks (Turner 1971; 39Ar that was implanted into more retentive sites. Epoxy Turner et al. 1971; Turner 1972; McDougall and Harrison degassing dominated the first gas release step in almost every 1999). However, it was rarely observed in the samples studied sample in this study, overwhelming any possible signature of here, for which resolvable low temperature data are lacking recoil, but a complementary downturn in the highest and often overwhelmed by atmospheric Ar outgassing from temperature steps was not observed. In a single phase, single epoxy. The high temperature steps in these experiments yield grain-size sample, recoil can cause a net volume loss of 39Ar plateaus; therefore, shock degassing was not considered from the outside surfaces to a mean depth of 0.082 µm Impact melt clasts from lunar feldspathic regolith breccias 771

Fig. 6. Ideogram summing ages from samples in all meteorites; solid curve is corrected data; dashed curve shows uncorrected data for comparison. Two subsets of the data are also shown: b) only samples with plateaus consisting of more than one step and/or more than 50% of 39Ar released; c) only samples with uncertainties ≤200 Myr. The number of discrete impact events (Table 5; binned in 150-Myr intervals) is shown in the histogram beneath the curve. Though the data subsets in (b) and (c) preclude identification of some inferred events, the spread in ages is similar in all representations.

(Turner and Cadogan 1974; Huneke and Smith 1976); the impossible. The plateaus in our samples average ∼50% of the affected volume in a typical microcore is less than 1% of the 39Ar released, comparable to plateaus in other studies, but the sample and not considered significant. heating steps in this study are much coarser and fewer steps The impact-melt clasts in this study have similar textures make up the plateaus. Fernandes et al. (2000) also found to clasts dated earlier (Turner et al. 1971; Podosek et al. 1973; relatively young (2.9–3.5 Ga) ages for DaG 262 components, Cadogan and Turner 1977; Swindle et al. 1991; Dalrymple including one definite impact-melt clast. and Ryder 1993, 1996), which had ages around 3.9 Ga, but are We also considered two subsets of our results (Figs. 6b much finer-grained. The major differences between this and and 6c). Excluding samples whose plateau encompasses previous studies are the sample composition and mass. Our fewer than two steps and/or less than 50% of all the 39Ar technique allows us to conduct step-heating experiments on released (Fig. 6b) leaves 22 samples remaining, with an age smaller, more feldspathic samples than in previous studies distribution from 2.4 to 4.0 Ga. Including only those samples (factors of 10–100 less total K2O), greatly expanding the with age uncertainties less than 200 Myr (Fig. 6c) produces a number of datable samples within a single meteorite, but distribution of 15 samples with ages between 2.4 and 3.9 Ga. making detailed interpretation of the argon spectra Using these subsets of data decreases the certainty of 772 B. A. Cohen et al. assigning impact events within each meteorite; using the data the surface. In this scenario, impact-melt rocks are in fact set in Fig. 6b allows identification of four events and the data preferentially pulverized because they always reside at the set in Fig. 6c shows three events. However, the overall lunar surface at the time of their creation, whereas plutons and distribution is similar in all three treatments of the data set, primary crust can be stored at depth and excavated at a later from the peak of Apollo sample ages at 3.9 Ga to distinctly time. In this case, melt rocks from large, old impact basins younger ages. We are able to clearly identify two impact either exist now as very small clasts in the regolith (smaller events at 3.9 Ga, represented by multiple samples using the than ∼200 µm), or as larger rocks buried under deep regolith, uncorrected data. This is unlikely to be simply coincidental and impact-melt rocks at the surface reflect only the most with the Apollo sample age spike, though we cannot rule out recent impacts. For this scenario to work, the volume of this possibility. We next consider interpretations of the sample surviving impact melt must be much less than the volume of ages and their implications for lunar bombardment history. igneous plutons and primary anorthositic crust at the same level in the crust, so that, for a given number of old igneous DISCUSSION rocks at the surface, a lower number of impact-melt fragments would be expected. The relative volume of plutons and the The lunar meteorite impact-melt clasts in this study are mechanics of large scale regolith development are largely probably derived from the feldspathic highlands of the Moon unconstrained, and more work needs to be done on this and thus represent an aspect of lunar impact history that is not scenario (Chapman et al. 2002; Chapman et al. 2004). available using the Apollo and Luna samples. These Another possible reason that the Apollo-site impact-melt feldspathic impact-melt clasts and bulk meteorite have FeO rocks only date a few basins could be that the sheer volume of contents ≤4.5 wt%, the remotely sensed lunar highlands mean impact melt created in basin forming events overwhelms the (Lucey et al. 1995). No distinct populations of impact-melt volume created in smaller craters. To address this possibility, clasts within a single meteorite could be distinguished using we calculated the impact-melt volume produced from basins major element chemistry or texture, but diversity in the clasts’ and smaller craters over the surface of the Moon (Fig. 7). The Mg’ suggest that each meteorite samples more than one number of craters with diameter 1–300 km was calculated impact event. This suggestion is borne out in the clast ages, using the present day flux curve scaled by a factor of 50, to which range from 2.4 to 4.1 Ga. However, there is no apparent approximate the inferred flux at 4.0 Ga (Neukum et al. 2001) correlation of age with any macroscopic characteristic, and scaled to the entire lunar surface. Craters >300 km are the including texture, grain size, or normative mineralogy. 42 known or suspected lunar basins listed by Wilhelms Siderophile elements (Korotev 1994; Norman et al. 2001) (1987), all of which formed earlier than ∼3.8 Ga. This may be better able to distinguish groups of impact-melt calculation thus compares the instantaneous number of samples from the same event. ≤300 km craters created at 4.0 Ga to the basins as if they were The oldest impact event recorded in these lunar all instantaneously created at the same time, which is meant to meteorites (MAC 88105, 3.79 ± 0.14 Ga) occurred at roughly be illustrative rather than a realistic scenario. Each transient the same time as the events represented by the peak in the crater diameter was calculated from the final crater diameter Apollo sample age distribution. These impact events are (Croft 1985; Melosh 1989; Kring 1995), using scaling laws consistent with the basin age for Imbrium at 3.92–3.94 Ga for simple craters up to a crater diameter of 18 km and for (Stˆffler and Ryder 2001). The meteoritic evidence thus complex craters for larger craters (including multiringed appears to support Ryder’s (1990) argument that a low flux of basins). Using alternative scaling laws (e.g., Kring 1995) has impactors existed prior to ∼3.9 Ga, since no impact-melt a negligible effect on this calculation. The volume of impact rocks can be found anywhere on the Moon from that era. In an melt was calculated based on the transient crater diameter alternative explanation for the lack of old impact-melt rocks, (Cintala and Grieve 1998), the calculation of which is similar Hartmann’s (1975) stone wall hypothesis and Grinspoon’s for a wide range of projectile types and impact velocities. The (1989) model postulate that the lunar crust is completely reset volume calculated in this way is comparable to that modeled and/or destroyed by multiple generations of crater saturation by Pierazzo et al. (1997) for typical asteroid impact velocities prior to 3.9 Ga. However, this argument cannot be completely (20 km/sec), thought to be most important in the lunar case valid because rocks are resistant to shock resetting (Deutsch (Kring and Cohen 2001; Swindle and Kring 2001). The and Sch‰rer 1994) and impact-melt rocks are not volume of melt created by each crater was multiplied by the mechanically weaker than other rocks, so are not likely to be number of craters of that size to arrive at the total amount of preferentially pulverized. impact melt created in each size bin. Hartmann (2003) modified his “stone wall” hypothesis, This calculation illustrates that basin-generated impact postulating that material in the upper layers of the melt should be dominant in and near the basins themselves, as megaregolith does become pulverized, but material stored they may eject a sizeable fraction of their impact melt out into deeper in the crust is occasionally excavated by large impacts, ejecta blankets (Warren 1996). The extent and thickness of providing old anorthositic and plutonic rocks to be sampled at the Imbrium ejecta blanket has been offered as an explanation Impact melt clasts from lunar feldspathic regolith breccias 773

Fig. 7. The relative impact-melt volume contributions from lunar basins and smaller craters. Filled squares show the volume of impact melt generated by all craters in each size bin (D); open circles show the cumulative amount of impact melt created by all craters of diameter D and less. The number of craters up to 300 km diameter is calculated from the crater curve (see text); roughness in the trends among the larger basins reflects the small numbers of currently visible basins at these sizes. In particular, there are no visible basins with diameters between 1200 km (Imbrium) and 2500 km (South Pole–Aitken). While basins generate the most impact melt, smaller craters create appreciable volumes of impact melt. for the dominant 3.9 Ga age in the Apollo impact-melt rocks, spherules in the Apollo 14 soil, implies that the meteorite assuming most or all the mafic impact-melt rocks in the breccias were closed to input of new material at around the Apollo collection were created in the Imbrium event and time of the youngest clast within them (∼2.5 Ga). The lunar distributed to collection sites in its ejecta blanket (Haskin ejection time, based on cosmic ray exposure (0.16 Ma for 1998). However, it is improbable that Imbrium or other QUE 93069, 0.27 Ma for MAC 88105, 0.15 Ma for DaG 262) impacts into the PKT created the feldspathic, KREEP-poor (Warren 1994; Thalmann et al. 1996; Bischoff et al. 1998), is clasts in the feldspathic meteorites, particularly the 3.9 Ga much later than the apparent breccia closure time. This clasts in MAC 88105. In fact, more than half the identifiable implies that either the breccia was lithified at 2.5 Ga and lunar basins formed in the feldspathic highlands, and so launched by a separate impact event, or that the breccia should have created a large amount of feldspathic, KREEP- components were buried deeply and effectively closed to later poor impact melt in and near these basins. The feldspathic material at around 2.5 Ga, possibly allowing for a single highlands are not currently covered by the Th-rich ejecta impact event to cause both lithification and launch. Because blanket, meaning the feldspathic breccia meteorites with the the breccia site remained open to new materials until well lowest Th contents, such as those used in this study, stand the after the near-side basin forming impacts, and yet contain no best chance of incorporating feldspathic basin melt into them. KREEPy clasts derived from the PKT, these meteorites must Because the youngest basin, Orientale, is ∼3.8 Ga have formed in locations well removed from the nearside (Wilhelms 1987; Stˆffler and Ryder 2001), impact-melt basins, which ejected Th-rich material hundreds of kilometers samples with younger ages necessarily come from smaller over the lunar surface. craters. Smaller craters are widespread over the lunar surface This work intended to test the lunar cataclysm hypothesis and generate an appreciable volume of impact melt, only an by measuring the ages of a large number of impact-melt order of magnitude less than all the basins combined (Fig. 7). samples. Out of 31 different samples, representing at least Some 20–30 craters >100 km and more than 60 smaller, fresh seven to nine different impact events, no impact-melt clast craters can be geologically identified as post-Imbrian in age with an age more than 1σ older than 3.9 Ga was found. The (Wilhelms 1987; Grier et al. 2001). These smaller craters’ ages continued absence of impact-melt samples older than 3.9 Ga are reflected in the lunar spherules from Apollo 14 soils (Culler supports, though does not prove, the existence of a low et al. 2000). A cluster of well-defined spherule ages at 3.9 Ga impactor flux prior to 3.9 Ga. The original “lunar cataclysm,” is attributable to the proximity of the Apollo 14 soil sample to as proposed by Tera et al. (1974), envisioned an increase in the Imbrium created Fra Mauro formation, but the spherule the lunar cratering flux over the time span of ∼200 Myr. Ryder ages reflect smaller impacts occurring throughout time. (1990) has argued that, based on the stratigraphy and apparent The lack of very recent impact-melted material in these ages of the lunar basins, the cataclysmic event lasted for as meteorites, compared with the ubiquity of recent melt little as 10–20 Myr. However, the peak in the lunar meteorite 774 B. A. Cohen et al. impact-melt clast age distribution at 3.5 Ga is 400 Myr and Neptune (Levison et al. 2001), effects of a short-lived, younger than the basins’ age of 3.9 Ga. Neither the smooth fifth terrestrial planet (Chambers and Lissauer 2002), and decline or terminal cataclysm hypothesis, as we currently interaction of our solar system with galactic objects (Napier understand them, predict the peak of ages of impact-melt and Clube 1979) may be able to produce a cataclysmic samples from lunar meteorites and spherules at 3.5 Ga. If bombardment of consistent magnitude and timing, but require these impact-melt clasts were created in the same cataclysmic further work and tests related to their viability. cratering event as the lunar basins, a number of smaller The lunar cataclysm hypothesis, if true, has far-reaching impactors must have been available until 3.5 Ga, possibly as consequences. Because the Earth has a larger gravitational a tailing off of the major cataclysmic impactor flux, though cross-section than the Moon, the number of impacts occurring crater density observations appear to preclude this possibility on Earth would have been at least an order of magnitude (Stˆffler and Ryder 2001). It is interesting to note, however, larger than on the Moon. The impact cataclysm is also nearly that evidence may exist for several impact events on the Earth coincident with the earliest isotopic evidence of life, 3.85 Ga at this time (Byerly et al. 2002). (Mojzsis and Harrison 2000), suggesting that the bombarding It may be that assembly of the meteorites in the upper asteroids may have affected the biologic evolution of Earth. layers of the lunar regolith skews the clast population toward The effect may have been detrimental, by destroying existing small, young, local impact events, whereas hand sample life or organic fragments (Maher and Stevenson 1988; Sleep collection may favor large chunks of melt rocks derived from and Zahnle 1998), or beneficial, by delivering precursor the largest nearby impact. More analyses of impact-melt molecules (Pierazzo and Chyba 1999) and providing suitable clasts in breccias or soil samples identified using the same environments for evolution (Kring 2000). Either way, a criteria as in the meteorites, along with modeling of impact- catastrophic bombardment of the Earth-Moon system must melt distribution in the lunar regolith, may be able to clarify have affected the origin and evolution of life. this discrepancy. Another test would be to directly sample the impact-melt sheet of a large lunar basin. The South Pole– Acknowledgments–This work was partially supported by a Aitken basin, with a diameter of 2500 km, probably created NASA Space Grant Fellowship (B.A.C.), NASA grants more impact melt than all other lunar craters combined NAG5-4767 (T. Swindle) and NAG5-4944 (D. Kring), and (Fig. 7). Though no South Pole–Aitken basin impact-melt NASA’s astrobiology program via a subcontract from rock has yet been identified in the Apollo, Luna, or meteorite Arizona State University to the University of Arizona collections, a large amount of melt probably still resides on (D. Kring). We appreciate sample loans from A. Bischoff at the basin floor (Pieters et al. 2001) and could be directly the Institute of Planetology, University of M¸nster, Germany sampled by a robotic mission (Duke 2003). and J. Zipfel at Max-Planck-Institut f¸r Chemie, Germany. Impact ages in ordinary chondrites and HED meteorites This manuscript benefited from reviews by Don Bogard, (Bogard 1995) and the martian meteorite ALH 84001 (Ash Randy Korotev, Ernst Zinner, Marc Caffee, and an et al. 1996), tungsten isotopic anomalies in the Isua anonymous reviewer, and from discussions with Suzanne metasediments on the Earth (Schoenberg et al. 2002), and the Baldwin, Bill Hartmann, and Graham Ryder. We used the inferred age of Mercury’s Caloris basin (from crater counting NASA Astrophysical Data System Abstract Service. based on the lunar flux; Neukum et al. 2001), suggest that the 3.9 Ga event affected the entire inner solar system (Kring and Editorial Handling—Dr. Marc Caffee Cohen 2001). 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