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.