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Plagioclase-Bearing Monomict Ureilite Or Ungrouped Achondrite?

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Plagioclase-Bearing Monomict Ureilite Or Ungrouped Achondrite? Meteoritics & Planetary Science 41, Nr 6, 925–952 (2006) Abstract available online at http://meteoritics.org Northwest Africa 1500: Plagioclase-bearing monomict ureilite or ungrouped achondrite? Cyrena Anne GOODRICH1*, Frank WLOTZKA2, D. Kent ROSS3, and Rainer BARTOSCHEWITZ4 1Department of Physical Sciences, Kingsborough Community College, 2001 Oriental Boulevard, Brooklyn, New York 11235, USA 2Max Planck Institute for Chemistry, PO 3060, D-55020 Mainz, Germany 3School of Ocean and Earth Sciences and Technology, University of Hawai’i at Manoa, Honolulu, Hawai’i 96822, USA 4Meteorite Laboratory Lehmweg 53 D-38518 Gifhorn, Germany *Corresponding author. E-mail: [email protected] (Received 03 October 2005; revision accepted 03 March 2006) Abstract–Northwest Africa (NWA) 1500 is an ultramafic meteorite dominated by coarse (∼100– 500 μm) olivine (95–96%), augite (2–3%), and chromite (0.6–1.6%) in an equilibrated texture. Plagioclase (0.7–1.8%) occurs as poikilitic grains (up to ∼3 mm) in vein-like areas that have concentrations of augite and minor orthopyroxene. Other phases are Cl-apatite, metal, sulfide, and graphite. Olivine ranges from Fo 65–73, with a strong peak at Fo 68–69. Most grains are reverse- zoned, and also have ∼10–30 μm reduction rims. In terms of its dominant mineralogy and texture, NWA 1500 resembles the majority of monomict ureilites. However, it is more ferroan than known ureilites (Fo ≥75) and other mineral compositional parameters are out of the ureilite range as well. Furthermore, neither apatite nor plagioclase have ever been observed, and chromite is rare in monomict ureilites. Nevertheless, this meteorite may be petrologically related to the rare augite-bearing ureilites and represent a previously unsampled part of the ureilite parent body (UPB). The Mn/Mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation. However, its petrogenesis differs from that of known augite- bearing ureilites in that: 1) it formed under more oxidized conditions; 2) plagioclase appeared before orthopyroxene in its crystallization sequence; and 3) it equilibrated to significantly lower temperatures (800–1000 °C, from two-pyroxene and olivine-chromite thermometry). Formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the UPB than previously sampled. However, its significantly different thermal history (compared to ureilites) may more plausibly be explained if it formed on a different parent body. This conclusion is consistent with its oxygen isotopic composition, which suggests that it is an ungrouped achondrite. Nevertheless, the parent body of NWA 1500 may have been compositionally and petrologically similar to the UPB, and may have had a similar differentiation history. INTRODUCTION the presence of reduction rims (highly magnesian compositions riddled with tiny grains of low-Ni metal) on The Northwest Africa (NWA) 1500 meteorite, a single silicates (Goodrich 1992; Mittlefehldt et al. 1998). However, stone weighing ∼3.3 kg, was bought by meteorite hunters in other characteristics described by Bartoschewitz et al. (2003) Zagora in 2000 and was traded to R. Bartoschewitz in 2002. It are either rare or previously unknown in ureilites. These was classified by F. Wlotzka and R. Bartoschewitz as an include the presence of augite and absence of pigeonite, anomalous ureilite (Russell et al. 2003). Two initial studies of which would place NWA 1500 among the small group NWA 1500 have been reported in abstracts (Bartoschewitz (<10%) of augite-bearing ureilites (Goodrich et al. 2004), and et al. 2003; Mittlefehldt and Hudon 2004). As described by the presence of primary chromite, which has previously been Bartoschewitz et al. (2003), the dominant characteristics of observed in only two monomict ureilites (Prinz et al. 1994; NWA 1500 are those typical of ureilites: a preponderance of Warren and Kallemeyn 1994; Goodrich 1999b; Sikirdji and olivine in a highly equilibrated texture, the presence of dark Warren 2001). In addition, the olivine composition (Fo ∼72) matrix and vein material containing graphite and metal, and reported by Bartoschewitz et al. (2003) is more ferroan than 925 © The Meteoritical Society, 2006. Printed in USA. 926 C. A. Goodrich et al. that of any previously known monomict ureilite (the most Instrumental neutron activation analysis (INAA) of one ferroan of which is Fo ∼75). However, the most notable sample (0.15 g) was performed by B. Spettel of Max-Planck- characteristic of NWA 1500 is that it contains plagioclase, a Insitute f¸r Chemie in Mainz. The sample was irradiated for phase that has not been observed in any monomict ureilite. 6 h in a TRIGA reactor at the Institut f¸r Kernchemie of the Bartoschewitz et al. (2003) suggested that NWA 1500 was the University of Mainz with a flux of 7 × 1011 n/cm2 s−1. After first member of the “missing” basaltic ureilites, although they irradiation the sample was counted several times on small and also note that the oxygen isotopic composition of this large Ge detectors, using procedures described in Wänke et al. meteorite does not fall within the range of oxygen isotopic (1977). A second sample was analyzed by M.I. Prudêncio at compositions of known ureilites. Instituto Tecnológico e Nuclear, Portugal. The sample, along In contrast, Mittlefehldt and Hudon (2004) suggested that with reference materials (USGS standards PCC-1 and DTS-1 the differences between NWA 1500 and known ureilites are for Cr and Ni and IGGE standards GSS-1 and GSD-9 for so great that it is unlikely to belong to this group. They other elements: Govindaraju 1994), was ground in an agate observed that olivine compositions range to even more mortar, dried at 110 °C for 24 h, and stored in a silica gel ferroan values (Fo 67) than those reported by Bartoschewitz desiccator prior to weighing. Powder portions of 0.2–0.3 g et al. (2003), and furthermore fall significantly off the well- were weighed into polyethylene vials. The vials, together established ureilite Fe/Mn-Fe/Mg trend (e.g., Goodrich and with Fe flux monitors (long irradiation) or 0.1% Au-Al alloy Righter 2000). In addition, they found the carbon content of flux monitors (short irradiation), were placed into appropriate their sample to be very low (below their detection limits), plastic containers for irradiation. Short irradiation (1 min) was whereas most ureilites contain significant amounts (up to carried out in a pneumatic system of the Portuguese Research ∼7 wt%) of carbon. Based on these differences, and the Reactor (ITN) at a thermal flux of 2.8 × 1012 n cm−2 s−1. A anomalous (relative to ureilites) oxygen isotopic composition long irradiation (6 h) was carried out in the core grid of of NWA 1500, they concluded that this meteorite is a unique the Portuguese Research Reactor at a thermal flux of 3.34 × 12 −2 −1 φ φ φ φ γ achondrite. 10 n/m s ; epi/ th = 1.4%; th / fast = 12.1. A -ray Here we report a detailed petrologic study of NWA 1500, spectrometer consisting of a 150 cm3 coaxial Ge detector and with particular emphasis on comparing this meteorite to the a low energy photon detector (LEPD), connected through augite-bearing ureilites, examining its petrogenesis in the Canberra 2020 amplifiers to Accuspec B (Canberra) context of a model for the differentiation history of the ureilite multichannel analyzer were used. This system had a FWHM parent body (UPB), and determining whether it could have of 1.9 keV at 1.33 MeV (coaxial Ge detector), of 300 eV at formed on this body. 5.9 keV and of 550 eV at 122 keV (LEPD). The spectra were processed by using the appropriate software. Data for ANALYTICAL PROCEDURES multiple aliquots were averaged. Carbon and nitrogen were analyzed by N. Lahajnar at the We studied three thin sections of NWA 1500. Section #1 Universit‰t Hamburg Institut f¸r Biogeochemie und (the rectangular section shown in Fig. 1b) is the section that Meereschemie using high temperature oxidation in a NA- was described by Russell et al. (2003) and Bartoschewitz 1500 Carlo Erba elemental analyzer (Nieuwenhuize et al. et al. (2003). In addition, we prepared two new 1” round thin 1994; Verardo et al. 1990). One bulk sample weighing 0.3 g sections (designated #r1 and #r2), representing serial sections was crushed in an porcelain mortar and homogenized. Two parallel to the cut face of a single sample. The face of this subsamples of 27.145 mg and 44.089 mg were completely sample was not parallel to that of section #1. oxidized by controlled instantaneous flash-combustion at Electron microprobe (EMP) analyses, X-ray mapping, 1020 °C with pure oxygen (Air Liquide O2 5.6) in helium (Air and backscattered electron imaging were carried out using the Liquide He 5.0) as the carrier gas. The resulting gas mixture Cameca SX-50 microprobe and the JEOL JSM-LV5900 was eluted in a gaschromatographic column, from which scanning electron microscope at the University of Hawaii. nitrogen (as N2) and carbon (as CO2) emerge purified. The Conditions for standard EMP analyses were 15 KeV, with 10– separated nitrogen and carbon were then passed over a 30 nA beam current (10 nA for analysis of plagioclase and thermo-conductivity detector for quantification. For each 20–30 nA for all other phases) and 20–40 s counting times. analytical run, acetanilide standards and blank tin capsules High-precision analyses of olivine in NWA 1500 and various were used for calibration. ureilites were carried out at 15 keV and 60 nA beam current, with 400 s counting times for Mn, Cr, and Ca. Several PETROGRAPHY ureilites that were analyzed under similar conditions by Goodrich et al. (1987, 2001) and Goodrich and Righter (2000) General were included to ensure consistency with existing data.
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