Meteoritics & Planetary Science 38, Nr 10, 1485–1497 (2003) Abstract available online at http://meteoritics.org New noble gas data of primitive and differentiated achondrites including Northwest Africa 011 and Tafassasset Andrea PATZER,1* Ludolf SCHULTZ,2 and Luitgard FRANKE2 1Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85712, USA 2Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany *Corresponding author. E-mail: [email protected] (Received 19 October 2002; revision accepted 12 February 2003) Abstract–This work reports on the noble gas inventory of 3 new acapulcoites, 3 brachinites, 2 new eucrites from the Dar al Gani region in Libya, the unique achondrite Dar al Gani (DaG) 896 from the same locality, the new eucrite-like achondrite Northwest Africa (NWA) 011, and the controversial sample Tafassasset. We determined cosmic ray exposure and gas retention ages, evaluated shielding conditions, and discuss the trapped noble gas component of the specimens. All exposure ages are within the known range of stony meteorites and partly confirm previously established age clusters. Shielding conditions vary, suggesting substantial shielding for all 3 brachinites and Tafassasset. We cannot exclude, however, that the Mg-rich composition of brachinites simply simulates heavy shielding. Regarding the trapped component, we found Q-like compositions only for the acapulcoite Thiel Mountains (TIL) 99002. The brachinite Elephant Moraine (EET) 99402 yields a high, subsolar 36Ar/ 132Xe ratio of ~400 along with a slightly elevated 84Kr/132Xe ratio, indicating minor atmospheric contamination. All the other samples, particularly the eucrite DaG 983, are characterized by clearly elevated Ar/Kr/Xe ratios due to significant terrestrial alteration. Tafassasset exhibits noble gas parameters that are different from those of CR chondrites, including a relatively high cosmic ray exposure age, the absence of a solar component, low 132Xe concentrations, a low trapped 36Ar/132Xe ratio of ~30, and a noticeable amount of radiogenic 129Xe. Similar attributes have been observed for some primitive achondrites. These attributes are also consistent with the metamorphic character of the sample. We, therefore, consider Tafassasset’s noble gas record to be inconclusive as to its classification (primitive achondrite versus metamorphosed CR chondrite). INTRODUCTION irradiation. Galactic cosmic rays trigger diverse nuclear reactions that, among others, open up the possibility to The investigation of the noble gas inventory of meteorites determine cosmic ray exposure ages (CREA). The statistical serves various purposes. First of all, the noble gas record can evaluation of CREA calculated for samples of the same confirm the extraterrestrial provenance of the studied rock. meteorite group, in turn, allows us to identify common break- The specific isotopic composition and elemental ratios of rare up events (e.g., Patzer and Schultz 2001; Wieler and Graf gases found in meteorites, i.e., their individual noble gas 2001; Wieler 2002a). In this fashion, Eugster and Michel signatures, may also yield information about the thermal and (1995) and Welten et al. (1997) found convincing support for collisional history of the given sample and its parent body as the hypothesis that the HED meteorites (howardites, eucrites, well as constraints on the relationship to other types of and diogenites) originate from the same planetary body. meteorites (e.g., Ott 2002; Swindle 2002; Wieler 2002a, b). Based on clusters in the exposure age distribution pattern, Another aspect of research that involves the noble gas they also concluded that the HED parent body experienced 5 inventory of a meteorite focuses on the delivery times and significant impact events, 2 of which built up major clusters at related dynamical parameters of meteoroids. While traveling about 22 and 38 Ma. Analogously, Weigel et al. (1999) report through space, a meteoroid is exposed to cosmic ray the simultaneous ejection of most acapulcoites and lodranites 1485 © Meteoritical Society, 2003. Printed in USA. 1486 A. Patzer et al. ~6 Ma ago and confirm the derivation of both meteorite types EET 99402 from a common asteroid. This work reports the noble gas inventory of 11 recently This brachinite has been listed in Grossman and Zipfel recovered achondrites including 3 acapulcoites, 3 brachinites, (2001) and briefly described by McCoy (2000). The 2 eucrites, the unique achondrites NWA 011 and DaG 896, classification of EET 99402 has also been based on the and a new Saharan sample called Tafassasset. In particular, isotopic composition of its oxygen isotopes (Clayton and we will address shielding conditions and the CREA of these Mayeda quoted in McCoy [2000]). meteorites and how they compare to CREA of other samples of the respective meteorite groups. In addition, we will Hughes 026 investigate the radiogenic and trapped noble gas components that are present in each sample and elucidate their Hughes 026 was among 21 meteorites that were significance. recovered from different regions in western and southern Australia between 1993 and 1995. M. Prinz grouped this rock SAMPLES AND METHODS with the brachinites (Grossman 1998). The classification received support from the oxygen isotopic composition as Dho 125 determined by Clayton and Mayeda (1996; under the synonym Australia 1). Found in Oman (Dho = Dhofar) in 2000, this meteorite has turned out to be an acapulcoite with a smaller average Reid 013 grain size than and slight compositional differences from other members of this subgroup (Greshake et al. 2001). The Reid 013 is another brachinite from Australia and was bulk chemical composition of Dho 125 and its comparison to found in the Nullarbor desert in 1991 (Wlotzka 1993; for other acapulcoites have been studied at the Lunar and oxygen isotopes see Clayton and Mayeda [1996], under the Planetary Laboratory in Tucson, USA by A. Patzer and synonym Window Butte). collaborators (Patzer et al., unpublished data). Dar al Gani 872 NWA 1058 DaG 872 was found in 2001 in the Libyan Sahara Northwest Africa 1058 (provisional name) was weighing 885 g and being completely covered with fusion purchased in Morocco in 2001. The total mass comprises crust. It has been classified as a monomict noncumulate 0.18 kg. It was originally thought to be an acapulcoite (A. (basaltic) eucrite (Russell et al. 2002; for a detailed Bischoff 2002, private communication). An analysis of its petrographic description and constraints on its evolution, see isotopic composition of oxygen, however, yielded data that Patzer et al. [2003a]). deviate significantly from those of other acapulcoites and lodranites and plot close to the TFL (terrestrial fractionation Dar al Gani 896 line; J. Zipfel 2003, private communication). It has been categorized now as a primitive achondrite (Russell et al. This meteorite was classified as a unique picritic 2003). Another investigation of the petrography and achondrite (Folco et al. 2002; see also Russell et al. 2003). It mineralogical composition of NWA 1058 has been possibly represents the rapidly cooled high temperature melt of performed by Patzer et al. (unpublished data). That study the silicate fraction of H chondritic material (Folco et al. 2002). shows that the meteorite is texturally and chemically similar to the primitive acapulcoite Graves Nunataks (GRA) 98028. Dar al Gani 983 The difference in oxygen records from other acapulcoites may be due to terrestrial alteration as the sample of NWA DaG 983 was found in March 2002 and is classified as a 1058 examined by Patzer et al. (unpublished data) is polymict eucrite (Russell et al. 2003). Its bulk composition is substantially weathered (W3). similar to Sioux County or Juvinas (J. Schlüter, private communication). An ongoing comprehensive investigation of TIL 99002 this meteorite describes it as a complex eucritic impact breccia (Patzer et al., unpublished data; see also Patzer et al. Preliminary work grouped TIL 99002 with the 2003b). acapulcoites (P. Sipiera, private communication; see also Russell et al. 2002). A comprehensive classification Tafassasset confirming this sample to be an acapulcoite has been conducted by Patzer et al. (unpublished data). The classifications of this meteorite by 2 different New noble gas data of primitive and differentiated achondrites 1487 workgroups (Bourot-Denise et al. 2002; Zipfel et al. 2002) are that resemble those of main group eucrites. Only the isotopic discordant. One contributing factor to the ambiguity of its composition of oxygen in NWA 011 differs considerably from taxonomical position is the fact that Tafassasset apparently that of other eucrites and led the authors to suggest that this represents a multiple fall comprising 26 pieces with a total meteorite represents a new basaltic parent body similar to weight of ~110 kg (Russell et al. 2002). It was recovered from Vesta (see also Promprated et al. 2003). H. Palme (2002) the Ténéré desert in Niger in the springs of 2000 and 2001. discusses the possibility that this meteorite may be a basaltic One piece of the strewn field was analyzed and classified by sample from the planet Mercury. Bourot-Denise et al. (2002). These authors suggest an affinity The concentrations and isotopic compositions of the to CR chondrites due to the appearance of a relict chondrule noble gases were determined in an all-metal mass and oxygen isotopic data. Their sample also showed clear spectrometer by one-step pyrolysis at ~1700°C. Before the signs of recrystallization leading them to label Tafassasset an extraction of the rare gases, the bulk material was pre-heated equilibrated CR chondrite. Another fragment of, most likely, in vacuo to 120°C for at least 24 hr to reduce adsorbed the same meteoroid was studied by Zipfel et al. (2002) who atmospheric noble gases. For the analysis, a fusion crust-free report remarkable similarities to the rock investigated in Paris chip weighing between ~40 and ~100 mg was broken off of and, therefore, promote pairing of the samples. However, each sample and wrapped into Ni foil. Multiple measurements based on bulk chemical data, they advocate a general were carried out for DaG 872, 896, 983, and Tafassasset.