Meteoritics & 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-chondrite affinity (Gao-Guenie, LAP 02240, LAP 03922, LAP 031125, LAP 031173, LAP 031308, NWA 2058, and Ourique) 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 chondrites 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 L chondrite parent body disruption). Records of impacts on the H chondrite 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 meteorite-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 asteroids 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, Meteorites 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 impact event, 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).