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. Appropriate natural and synthetic silicates, oxides, and metals The three thin sections we studied showed that the were used as standards in all analyses, and PAP φ–pz majority of NWA 1500 is an ultramafic rock with a highly corrections were applied to the analyses. equilibrated texture characterized by gently rounded grain Northwest Africa 1500 927

Fig. 1. Combined 3-element X-ray maps (in RBG coordinates red = Al, green = Ca, and blue = Mg) of thin sections of NWA 1500. Olivine appears blue, augite appears green, plagioclase appears yellow, and chromite appears red. a) Section #r1. Plagioclase and augite are concentrated in a vein-like area. Large plagioclase grains poikilitically enclose rounded olivine and augite grains. Plagioclase also occurs as patches of small, intergranular grains (just up and right of center). b) Section #1. In the left half of section, poikilitic plagioclase grains occur in two vein-like areas, one of which also has a concentration of augite. Elsewhere in this half of the section, both augite and plagioclase are rare. In contrast, the right half of this section contains plagioclase only as a few patches of small interstitial grains and augite occurs as individual grains heterogeneously dispersed among the olivine. The white outlined boxes show the locations of the images in Fig. 2b (right) and Fig. 2c (left). 928 C. A. Goodrich et al.

Fig. 2. a) A transmitted light photomicrograph of NWA 1500 in crossed polars. This area shows mostly olivine grains, which exhibit grain boundary darkening because their rims (∼10–30 μm) are reduced (relative to core compositions) and contain tiny grains of metal. In such images, NWA 1500 bears a strong resemblance to typical ureilites, which are characterized by similar reduction rims. b) Backscattered electron image (BEI) of NWA 1500, showing mostly olivine grains, with some augite (darker grey). Note that augites tend to have convex boundaries with olivine, and partially enclose rounded olivines poikilitically. White grains are chromite. c) Similar to (b), but illustrating slight fabric (parallel alignment of elongate grains). d) BEI of one of the most ferroan olivine grains in section #1. Reverse zonation from Fo ∼65 in center, to Fo ∼72 where in contact with narrow reduction rims (very dark rims with inclusions of metal). Spots show traverse of electron microprobe analyses (see Fig. 5b). boundaries and abundant triple junctions (Figs. 1 and 2). (Figs. 1 and 4). It also occurs in a few patches of much smaller Modal abundances are given in Table 1. Olivine is the most (∼24–150 μm) interstitial grains (Fig. 1). Other minor phases abundant phase (95–96%), and shows grain sizes in the range present are orthopyroxene, apatite, metal, sulfide, and of ∼100–500 (mostly 200–300) μm. Augite occurs as a minor graphite. phase (2–3%), with grain sizes comparable to or larger than The distribution and mode of occurrence of both (up to ∼925 μm) those of olivine. Many augite grains have plagioclase and augite are heterogeneous. For example, the intergranular forms (convex boundaries with olivine), and left half of section #1 (Fig. 1b) contains two areas of poikilitic some partially enclose rounded olivine grains poikilitically plagioclase, one of which is within a vein-like area in which (Figs. 2b and 2c). Most show fine twin bands similar to those there is also a large concentration of augite grains forming a observed in other meteoritic augites and believed to have mass, as well as a concentration of orthopyroxene. Elsewhere been induced by low-level shock (Tribaudino et al. 1997; in this half of the section, both augite and plagioclase are very Goodrich et al. 2001). rare. In contrast, the right half of this section contains Chromite also occurs as a minor phase (0.6–1.6%) with plagioclase only as a few patches of small interstitial grains grains sizes up to ∼300 μm (Fig. 3). Plagioclase (0.7–1.8%) and augite occurs as individual grains heterogeneously occurs principally as large (up to ~3 mm in size), dispersed among the olivine. Olivine-rich areas appear to polysynthetically twinned, poikilitic grains (enclosing have a weak fabric (parallel alignment of elongated grains) rounded olivine and augite), concentrated in vein-like areas that is subparallel to the plagioclase- and augite-rich “vein” Northwest Africa 1500 929

Table 1. Modal abundances of major phases in NWA 1500. Section #1 Section #1 left Section #1 right Section r #1 Olivine 96.1 96.0 95.9 94.8 Augite 2.6 2.2 3.1 1.7 Plagioclase 0.7 1.2 0.3 1.8 Chromite 0.6 0.6 0.7 1.6

Fig. 3. One of the largest chromite grains in section #1, surrounded by olivine. a) BEI. b) Reflected light (note grain boundary darkening of olivine due to reduction rims). Olivine surrounding chromite is reduced, forming a rim around the grain. Chromites themselves do not show reduction rims.

(compare Figs. 2a and 2c with Fig. 1b). The other two across one of these is shown in Fig. 5b. These core sections we studied also show a vein-like area (the same area compositions are more ferroan than previously reported in both sections, since they are serial) in which poikilitic (Bartoschewitz et al. 2003; Mittlefehldt et al. 2004) and are plagioclase and most of the augite in the section is significantly more ferroan than those observed in any concentrated (Fig. 1a). monomict ureilite (the most ferroan of which is Fo ∼75). Although the appearance of the reduction rims is quite Olivine dramatically different, in contrast to the metal-free cores, these rims are only slightly more magnesian (Fo of olivine up All olivine grains have narrow (∼10–30 mm) rims that to only ∼73, mg of orthopyroxene up to only ∼76–77) than the are more magnesian than grain cores (in some cases they most magnesian core compositions. consist of orthopyroxene rather than olivine) and riddled with Cr2O3 and CaO contents of the olivine are relatively tiny inclusions of metal (Fig. 2), and thus are similar to the homogeneous: 0.04 ± 0.01 wt% and 0.09 ± 0.01 wt%, “reduction rims” that are characteristic of ureilites (Wlotzka respectively (226 analyses). Compared to compositions of 1972; Goodrich 1992; Mittlefehldt et al. 1998). Optically, olivine in typical ureilites (Cr2O3 = 0.4–0.9 wt% and CaO = they cause grain boundary darkening and are responsible for a 0.3–0.45 wt%), these values are very low. The Cr2O3 value is strong visual resemblance of NWA 1500 to typical ureilites also lower than that (∼0.35%) in the chromite-bearing ureilite (e.g., Fig. 2a). In addition, high-contrast backscattered Lewis Cliff (LEW) 88774 (Table 2), and the CaO value is electron imaging reveals that, although cores are devoid of lower than values (0.2–0.3%) in augite-bearing ureilites metal inclusions (“core” is used to mean the whole interior of (Table 2; Takeda et al. 1989; Goodrich et al. 2001). NiO a grain except the reduction rim, not just the very middle), a contents of the olivine are below detection limits, as is typical significant fraction of them are reversely zoned (Figs. 2b–d). of ureilites. A survey of olivine core compositions (226 analyses) Fe-Mn-Mg compositions of olivine can be used both to produced a histogram that shows a range from Fo ∼65 to Fo distinguish meteorite groups and to interpret petrogenesis, ∼72, with a strong peak at Fo 68–69 (Fig. 5a). The Fo 68–69 when presented on a plot of molar Fe/Mg versus molar Fe/Mn peak represents the dominant composition of the majority of ratio (Mittlefehldt 1986; Goodrich and Delaney 2000). Fe- grains, which are weakly reverse-zoned to Fo ∼72. A few of Mn-Mg compositions of ureilite olivine have been the larger grains, however, are strongly zoned from Fo 65 in determined to high precision and are shown in Fig. 6. Olivine- their centers to Fo ∼72 where they are in contact with the pigeonite and olivine-orthopyroxene ureilites show a well- metal-rich reduction rims (Fig. 2d). A compositional profile defined trend of near-constant, chondritic Mn/Mg ratio. In 930 C. A. Goodrich et al. contrast, all augite-bearing ureilites are displaced from the trend to higher Mn/Mg ratios. Compositions of olivine in NWA 1500 are likewise displaced to much higher Mn/Mg ratio relative to the trend of the olivine + low-Ca pyroxene ureilites (Fig. 6; Table 2). Overall, they show a trend of pure reduction (as do internal trends for each of the augite-bearing ureilites), extending from Fe/Mg = 0.53 (mg 65) to Fe/Mg = ∼0.39 (mg 72).

Augite

Augite (262 analyses) is homogeneous in mg and Wo, with values of 80.7 ± 0.5 and 45.0 ± 0.5, respectively (Fig. 7a). The Wo content is notably higher than Wo contents (30–37) of augite in augite-bearing ureilites (Fig. 7a). Al2O3, ± Cr2O3, TiO2, and Na2O contents are, respectively: 0.97 0.12, 0.66 ± 0.09, 0.13 ± 0.02, and 0.35 ± 0.04 wt%. Compositional profiles across most grains show slight decreases in Al2O3 and Cr2O3 near edges (Fig. 7b). Both Al2O3 and Cr2O3 contents are lower than those (1.3–3.7% and 1.3–1.9%, respectively) of augite in augite-bearing ureilites (Takeda et al. 1989; Weber et al. 2003; Goodrich et al. 2001). Representative analyses of interior and edge compositions are given in Table 3.

Plagioclase

Poikilitic plagioclase grains are quite homogeneous in composition, except in ∼20 μm rims. A histogram of 674 analyses (from all poikilitic grains in all three sections) shows a strong peak at An 37–38 (Fig. 8a). K2O content is very low (Or 0.1–0.2), and MgO contents are below the detection limit (Table 3). Rims show normal zonation to An 26–32 (Fig. 8b), Fig. 4. Poikilitic plagioclase grains in NWA 1500. a) A transmitted with increasing K2O (to Or 0.7) contents (Table 3). No light photomicrograph in crossed polars of section #r2. Plagioclase reliable FeO values were obtained, due to the presence of tiny encloses rounded grains of olivine, augite, graphite (dark, wiggly metal inclusions. Small, intergranular plagioclase grains are grains), and some metal. homogeneous, and have lower An contents (∼32–33) than the interiors of the poikilitic grains (Fig. 8a). orthopyroxene). However, it differs in occurrence from primary orthopyroxene found in augite-bearing ureilites, Orthopyroxene which forms grains comparable in size to the olivine and augite grains (or even larger grains poikilitically enclosing Orthopyroxene that is texturally and compositionally olivine and augite). It also differs compositionally, in having distinct from the orthopyroxene in reduction rims (which has lower Wo, Al2O3, and Cr2O3 (orthopyroxene in augite ≤ Wo 1 and Al2O3 below detection limit) occurs as a minor, bearing-ureilites has Wo 4.5–5, Al2O3 = 1–2.3% and Cr2O3 = but significant, component, principally concentrated in the 1–1.2%). plagioclase- and augite-rich vein-like areas (Fig. 4b). Here it is found as interstitial grains ∼100–150 μm in size, and as Other Phases overgrowths on olivine and augite (Fig. 9). It commonly contains tiny rounded “islands” of olivine (Fig. 9), suggesting Chromites have 73.0 ± 0.3% chromite, 24.1 ± 0.3% formation in an olivine (+melt) → orthopyroxene reaction. spinel, and 2.6 ± 0.1% ulvˆspinel component (Table 3), and The composition of this orthopyroxene is mg 71.4 ± 0.3, show no significant zonation or variation between large and Wo 2.1 ± 0.1, with 0.33 ± 0.04% Al2O3 and 0.16 ± 0.02% small grains (Fig. 10). Calculated magnetite contents are very Cr2O3 (Table 3). Its texture suggests that it is a late, primary low, and within error of zero (Table 3). Cr# (molar Cr/[Cr + phase (rather than a secondary phase like the reduction-rim Al]) and fe# (molar Fe/[Fe + Mg]) are 0.752 ± 0.005 and Northwest Africa 1500 931

Fig. 4. Continued. b) A vein-like area of poikilitic plagioclase in section #r1 (see also Fig. 1a). A backscattered electron image is on the left; a combined three-element X-ray map (in RGB coordinates red = Ca, blue = Mg, and green = Si) is on the right. In the X-ray map, the plagioclase is purple, augite is pink, olivine is green, and chromite is black. The light blue grains concentrated in the vein are orthopyroxene. The reduction rims around the olivine are also light blue. 932 C. A. Goodrich et al.

chromites in NWA 1500 have Cr# similar to and fe# consistent with those of chromites in the two known chromite-bearing ureilites. Chromites in NWA 1500 are commonly surrounded by narrow rims in which the adjacent silicates are reduced (e.g., Fig. 3), but little reduction of the chromite itself was observed. Apatite was observed as one 150 μm-sized grain (Fig. 12), and in patches of smaller grains. It contains 4–5% Cl and 0.7–1.4% F, with 0.4–3.4% FeO and 0.2–0.4% Na2O (Table 3). Primary phosphates have not been observed in monomict ureilites, although they are abundant in FeO-rich feldspathic lithologies found in polymict ureilites (Ikeda et al. 2000; Cohen et al. 2004; Goodrich et al. 2004). Metal (with 0.9–1.6% Ni) is common as an interstitial phase (as in ureilites), and shows a low degree of terrestrial weathering. Its Cr, P and Si concentrations are near or below detection limits (Table 4), in contrast to metal in ureilites (Goodrich 1992; Mittlefehldt et al. 1998). Sulfide also occurs as an interstitial phase (as in ureilites), but is less abundant than metal. Its Cr content is very low (Table 4), in contrast to sulfides found in many ureilites (Goodrich 1992; Mittlefehldt et al. 1998). Carbon (which is common in most ureilites) occurs as graphite, indicating that shock effects are mild. Its distribution, however, is inhomogeneous. It is mainly concentrated in the plagioclase-rich vein-like areas as inclusions in poikilitic plagioclase grains (Fig. 4a), and is rare in the olivine-rich areas.

CHEMICAL COMPOSITION

INAA data obtained for two samples of NWA 1500 are given in Table 5. Sample 1 had low Ca and Na contents, suggesting that it was from an olivine-rich part of the meteorite and contained little augite or plagioclase. Values for Ca, Na, K, Sc, and Zn contents of this sample are within the Fig. 5. a) A histogram showing the results (226 analyses) of a survey of compositions of olivine cores (interiors of grains free from metal range of those in olivine-pigeonite ureilites (Mittlefehldt et al. inclusions). The most ferroan composition is Fo ∼65 and occurs near 1998). Its Cr content (∼1.5 × CI) is higher than in typical the centers of a few grains (e.g., the grain in Fig. 2d), which are ureilites, but lower than that (∼10 × CI) in the chromite-rich reversely zoned to Fo ∼72 where they are in contact with reduction ureilite LEW 88774. REE concentrations are very low and rims. The strong peak at Fo 68–69 reflects the dominant core only upper limits were obtained for all but Sm (0.035 × CI) composition of most grains, which are weakly reversely zoned to Fo ∼72. b) The compositional profile across one of the most ferroan and Yb (0.170215 × CI). The latter values are within the range olivine grains in section #1 (see Fig. 2d). of those found in olivine-pigeonite ureilites (Fig. 13). Abundances of the siderophile elements Co, Ni, W, Re, and 0.757 ± 0.011, respectively (108 analyses). A comparison Au are within the range found in olivine-pigeonite ureilites, with chromites in the ureilite LEW 88774 is shown in Fig. 11. but Ir is somewhat lower (Mittlefehldt et al. 1998). Chromites in LEW 88774 have experienced a very high In contrast, sample 2 had higher Na (214 ppm versus degree of secondary reduction (Prinz et al. 1994; Warren and 90 ppm), which by simple mass-balance would imply Kallemeyn 1994), which has led to extreme decreases in fe# ∼0.4% plagioclase. It also has significantly higher LREE (∼0.4–0.05), and at high degrees of reduction (very low fe#) concentrations (Table 5) and an LREE-enriched pattern also extreme decreases in Cr# (Fig. 11). Primary chromite (Fig. 13) that resembles the typical REE pattern of must have had Cr# ∼0.75 and fe# >0.45. Chromites in the plagioclase (although the Eu anomaly cannot be seen because ureilite NWA 766 are reported to be similar in composition to data were not obtained for Sm). However, these LREE those in LEW 88774 (Sikirdji and Warren 2001). Thus, concentrations are so high that they would probably imply a Northwest Africa 1500 933 were made with high beam ). The number of analyses is given in al. (2001). The majority ted in the number of significant digits 2 0.005 0.7809 0.030 0.002 0.728 0.252 0.009 0.003 0.281 0.001 ugite-bearing Augite-bearing Augite-bearing (2000) and Goodrich et 0.4550.301 0.003 0.003 0.445 0.388 0.005 0.008 0.427 0.389 0.006 0.004 39.00 0.24 38.00 0.1922.42 39.49 0.28 0.17 47.46 0.42 20.49 0.28 0.0100.130.14 0.495 12.61 46.70 0.0050.002 0.07 0.09 0.444 0.151 20.92 0.007 40.09 0.001 0.10 0.31 0.541 0.293 0.007 11.96 47.21 0.001 0.08 0.19 0.142 0.001 -opx Olivine-opx Olivine-pig Olivine-pig high precision (as reflec ine in Goodrich and Righter .81 0.21 99.63 0.24 100.17 0.45 100.06 0.34 .435.238 0.006 0.004 0.555 0.281 0.007 0.004 0.435 0.241 0.004 0.004 0.576 0.281 0.008 0.004 . a tioned papers) and are of -Mn-Mg data for ureilite oliv 1500 and various ureilites. in NWA 1500 and various ureilites in NWA LEW 88201 EET 90019 Y-791538 EET 96322/262 DaG 340 Kenna LEW 88774 META78008 Y-74130 EET 96314 HaH 064 FRO 90054 Augite-bearing Augite-bearing Augite-bearing A with one another and Fe c times (following procedures described in aforemen b Compositions of olivine in NWA Compositions of olivine in NWA olivine observed. 0.53 0.5040.147 0.004 0.636 0.098 0.009 0.001 0.57 0.124 0.004 0.001 0.64 0.095 0.001 0.17 FRO 90228 39.2 40.90 0.1819.6187.2 39.81 0.18 18.96 91.0 39.74 0.23 0.1 0.41 22.79 38.67 0.27 89.0 18.22 0.28 0.1 0.34 38.78 91.3 31.09 0.20 0.1 0.17 38.55 84.8 0.18 41.83 0.1 0.68 79.9 46.72 0.64 0.2 78.0 0.1 NWA 1500 NWA Augite-bearing Olivine-opx Olivine-pig Olivine Avg. (2)Avg. (42) Avg. dev. St. (34) Avg. dev. St. (22) Avg. dev. St. (10) Avg. dev. St. (26) Avg. dev. St. (66) Avg. dev. St. 0.04 0.01 0.370.53 0.05 0.00 0.386 0.33 0.005 0.00 0.405 0.317 0.001 0.299 36.8 0.1 38.2 0.271.565.2 37.71 1.8 0.1 39.6 0.18 75.0 37.94 1.8 0.2 0.13 49.43 75.9 0.57 0.1 48.02 77.0 0.63 0.1 86.9 0.1 77.3 0.1 87.6 0.1 Avg. (3)Avg. dev. St. (42) Avg. dev. St. (31) Avg. dev. St. (16) Avg. dev. St. (41) Avg. dev. St. (20) Avg. dev. St. (22) Avg. dev. St. Continued. d d d d 3 3 2 2 O O e e 2 2 parentheses. currents and long counting Most ferroan core Molar ratios. All analyses in this table are consistent Probably paired with FRO 90054. 100 * molar Mg/(Mg + Fe). a b c d e Cr mg FeOMgOMnOCaO 12.5 47.7TotalFe/Mg 0.63Fe/Mn 100.94 0.35 8.82 50.27 0.459 101.27 0.08 0.12 0.315 0.004 0.28 10.62 0.002 48.07 0.460 99.93 0.06 0.11 0.331 0.004 0.24 0.006 49.72 8.44 0.457 99.24 0.316 0.10 0.09 0.006 0.005 0.40 44.90 14.32 99.33 0.12 0.13 0.29 42.01 18.84 101.24 0.13 0.15 0.18 40.31 20.22 100.66 0.14 0.12 0.36 SiO SiO Cr mg FeOMgOMnO 31.2 32.8CaOTotal 0.43Fe/Mg 0.1 101.4 0.09 0.1Fe/Mn 0.01 23.1 0.01 38.8 0.2 0.57 0.2 101.3 0.22 0.2 0.03 0.02 22.04 0.5 39.03 0.440 0.255 0.09 99.87 0.12 0.005 0.003 21.14 0.20 39.65 0 0 99 Table 2. Table Table 2. Compositions of olivine Table 934 C. A. Goodrich et al.

Fig. 6. Molar Fe/Mn versus Fe/Mg in monomict ureilites and in NWA 1500. Olivine-pigeonite and olivine-orthopyroxene ureilites plot on a single trend of near-constant, chondritic Mn/Mg ratio, which suggests that they are residues and are related to one another principally by various degrees of reduction rather than different degrees of melting (Goodrich and Delaney 2000). The data are fit by the power law relationship Fe/Mn = 139.7*(Fe/Mg)0.865 (the trend would be linear and pass through the origin if only reduction were involved; the slight curvature is due to partial melting). Augite-bearing ureilites plot to the right of the residue fit (at higher Mn/Mg ratio), suggesting that they contain a melt component (Goodrich and Delaney 2000). Olivine in NWA 1500 is similar to that in augite-bearing ureilites in having higher Mn/Mg than the olivine + low-Ca pyroxene ureilites; internally, it shows a trend of pure reduction (dashed line—linear trend passing through the origin), as does each of the augite-bearing ureilites (not shown). Data are of high precision (both analytical error and internal variation are on the order of the size of the symbols), and represent an internally consistent set that includes previously published (Goodrich et al. 1987; Goodrich and Righter 2000; Goodrich et al. 2001) as well as new data (Table 2). much higher content of plagioclase than 0.4%. In addition, the DISCUSSION K content of this sample is extremely high (239 ppm), and, if due entirely to plagioclase, would imply the unreasonably Comparison of NWA 1500 to Ureilites high amount of ∼26%. It is more likely that this high K content results from terrestrial desert contamination, and that Although NWA 1500 was originally classified as an this also accounts for the high LREE concentrations. anomalous ureilite (Russell et al. 2003; Bartoschewitz et al. The carbon concentration determined for one sample of 2003), Mittlefehldt and Hudon (2004) suggested that it is a NWA 1500 (average of results for two subsamples of ∼27 mg unique achondrite. In addressing the question of whether and 44 mg) was 0.117 wt%. Mittlefehldt and Hudon (2004) NWA 1500 is a ureilite, we begin by summarizing the measured the carbon content of a sample of NWA 1500 and petrographic/compositional similarities and differences found that it was below their detection limits (which were not between this meteorite and known ureilites. In terms of its stated). Considering the macroscopically observable ultramafic mineralogy (with olivine as the most abundant heterogeneity in the distribution of graphite in this meteorite, phase), highly equilibrated texture (including grain sizes), variations in carbon content of bulk samples would not be and slight fabric, NWA 1500 is very similar to the majority of surprising (it seems likely, for example, that carbon mildly to moderately shocked monomict ureilites (Goodrich concentrations in plagioclase-rich areas might be 1992; Mittlefehldt et al. 1998). The presence of reduction significantly higher than either of these determinations on rims on olivine greatly strengthens this similarity. Reduction bulk samples). For comparison, carbon contents measured for rims constitute one of the most characteristic features of all ureilites range from ∼0.24–6.5 wt% (Hudon et al. 2004; ureilites, and have otherwise (among meteorites) been Mittlefehldt et al. 1998 and references therein). Nitrogen observed only in lodranites (McCoy et al. 1997). Although concentration determined for the same sample was 80 ppm, the compositional contrast between cores and reduction rims which is within the range of values (∼10–150 ppm) measured is much less than in some ureilites (in which reduction rims for ureilites (Mittlefehldt et al. 1998 and references therein). may have mg as high as 98–99, independent of core Northwest Africa 1500 935

Fig. 7. a) A pyroxene quadrilateral showing the compositions of primary augite and orthopyroxene in NWA 1500 compared to those in ureilites. b) The compositional profiles across augite grain in NWA 1500. Most grains show slight decreases in Al2O3, Cr2O3, and (to a lesser extent) Na2O near edges. composition), it is similar to that in the augite-bearing ureilite one of the characteristic features of the augite-bearing Hughes 009, which has cores of Fo ∼87 and reduction rims no ureilites (although Mittlefehldt and Hudon [2004] more magnesian than Fo ∼91.5 (Goodrich et al. 2001). emphasized it as a difference between NWA 1500 and The presence of augite (and absence of pigeonite) in ureilites). Finally, the presence of intergranular NWA 1500 implies that if this meteorite is a ureilite, it orthopyroxene emphasizes the similarity between NWA 1500 belongs to the small group of monomict ureilites and the augite-bearing ureilites, in which orthopyroxene is distinguished as augite-bearing (Table 6). Textural features of common as a late, primary phase (Takeda et al. 1989; its augite (grain sizes comparable to or larger than those of Goodrich et al. 2001; Goodrich and Fioretti 2000; Berkley olivine, convex boundaries with olivine, and partial, poikilitic and Goodrich 2001). enclosure of olivine) are indeed similar to those seen in With respect to other phases, the presence of graphite, augite-bearing ureilites (Takeda et al. 1989; Tribaudino et al. metal, and sulfide are also consistent with NWA 1500 being a 1997; Goodrich et al. 2001; Weber et al. 2003). Furthermore, ureilite. Graphite (and other forms of carbon in shocked the displacement of its olivine to higher Mn/Mg ratio relative samples) in particular is a characteristic component of to that in the olivine + low-Ca pyroxene ureilites (Fig. 6) is ureilites (Mittlefehldt et al. 1998), and heterogeneity in its 936 C. A. Goodrich et al. 2.6 0.3 0.757 0.752 Chromite avg. (108) Apatite Apatite 24.1 73.0 Intergranular Plagioclase Poikilitic Plagioclase Edge Poikilitic Plagioclase Interior 37.8 26.8 32.9 Augite Edge lase, chromite, and apatite in NWA 1500. lase, chromite, and apatite in NWA Augite Interior Reduction Rim Opx ). ). ). 3+ 3+ ). 3+ 3+ 2.1 0.9 45.0 44.6 0.050.330.16 0.02 0.04 0.08 0.15 0.97 0.70 0.12 0.7371.4 0.50 na 26.4 0.00 75.8 na 24.3 80.6 0.02 na 25.5 80.2 0.00 11.8 53.4 0.99 na na na na na na l + Ti + Cr Fe l + Ti Intergranular Opx (80) Avg. 53.9na 54.3 53.5 nana 53.1 na na 58.3 na na 61.6 na 59.7 na na 0.03 na na 0.17 na 0.66 0.16 na na na na 40.7 40.4 /(Al + Ti + Cr Fe /(Al + Ti 3+ d g f e 3 3 spinel 3 5 2 2 O 0.02 0.01 0.35 0.3 7.2 8.3 7.7 na 0.31 0.22 ˆ h b O O a h c 2 O O na na na na 0.03 0.11 0.02 na na na 2 2 O 2 2 2 SiO TiO Al Cr V FeOMgOMnOCaO 18.3K 25.7P 0.39Cl 1.09F 16.2Total 28.4mg 0.47Wo na 0.49An 6.7 na 15.6 99.9 0.18 22.0 na 100.0 15.8 6.9 na 0.16 22.0 100.2 na bdl 0.11 na 0.00 7.9 99.6 bdl 0.26 na 0.02 na 99.8 5.56 bdl 0.32 0.00 na na 6.8 100.2 27.1 0.34 4.8 na 100.0 na na 1.15 99.1 na na 0.08 na 54.2 0.63 0.04 na 102.5 na na 54.1 100.6 5.2 0.67 3.6 1.4 Na Spinel Chromite Magnetite fe# Cr# Ulv 100 * molar Ca/(Ca + Mg Fe). + Cr Fe + Ti 100 * molar (2Ti)/(Al 100 * molar Fe fe# = molar Fe/(Fe + Mg); Cr# Cr/(Cr Al). 100 * molar Mg/(Mg + Fe). 100 * molar Ca/(Ca + Na K). 100 * molar Al/((A 100 * molar Cr/(Al + Ti + Cr Fe 100 * molar Cr/(Al + Ti Table 3. Compositions of pyroxenes, plagioc Table a b c d e f g h Northwest Africa 1500 937

Fig. 8. Plagioclase in NWA 1500. a) A histogram showing An content of large poikilitic grains versus small intergranular grains. b) Compositional profiles across poikilitic plagioclase grains. Most profiles show essentially homogeneous cores (An ∼37–38), with normal zonation to An ∼26 in 20–30 μm rims. distribution (as observed in NWA 1500) is not uncommon. There are, however, significant differences between The presence of chromite is notable for a ureilite, but at least NWA 1500 and known ureilites. Olivine compositions, even two chromite-bearing monomict ureilites are now known in reduction rims, are more ferroan than those of any (Prinz et al. 1994; Warren and Kallemeyn 1994; Goodrich monomict ureilite (the most ferroan of which is Fo ∼75). 2001b; Sikirdji and Warren 2001), and in terms of Cr#, NWA Furthermore, extensive reverse zoning of olivine cores (free 1500’s chromite is similar to that in LEW 88774 and distinct of the metal inclusions that define reduction rims) has not from that in other olivine-rich meteorites such as lodranites been observed in ureilites (in some ureilites, areas of reverse and pallasites (see Goodrich and Righter 2000). Finally, with zonation without metal inclusions may extend inward from respect to chemistry, the bulk compositional data we obtained reduction rims for a few microns, but the bulk of the core is (including carbon and nitrogen concentrations) are consistent highly homogeneous). Finally, Cr2O3 and CaO contents of with NWA 1500 being a ureilite. olivine in NWA 1500 are significantly lower than those in 938 C. A. Goodrich et al.

Fig. 9. A BEI of late, primary orthopyroxene in NWA, occurring as small intergranular grains and overgrowths on olivine and augite. Note that some grains contain small “islands” of olivine (marked), suggesting an olivine → orthopyroxene reaction. even chromite-bearing or augite-bearing ureilites, and within limits) compared to those of metal in known ureilites the range of those in lodranites/acapulcoites and pallasites (although highly variable, Cr and P contents of metal in (see Goodrich and Righter 2000). ureilites are usually significant, and range up to 1–1.5%). The Augite is likewise more ferroan than augite in known Cr content of sulfide is likewise low (Cr contents of primary monomict ureilites, and in addition has a higher Wo content sulfides in ureilites range up to 34%). (45 versus 30–37). Furthermore, zonation of augite (even the Finally, neither primary apatite nor plagioclase have ever slight edge zonation observed in NWA 1500) has never been been observed in a monomict ureilite. If NWA 1500 is a reported in a ureilite. The orthopyroxene in NWA 1500 shows ureilite, the presence of the latter would be particularly both textural and compositional differences from significant, since (aside from minor, unrepresentative clasts orthopyroxene in the augite-bearing ureilites. In the latter, it of feldspathic lithologies found in polymict ureilites) the occurs either as very coarse (up to cm-size) poikilitic crystals basaltic component from the UPB is so far completely enclosing rounded olivine and augite grains, or in an missing from the meteorite record. equilibrated texture with sizes and shapes comparable to Thus, although NWA 1500 is in many ways similar to those of augite. Its occurrence as smaller, intergranular grains known augite-bearing ureilites, it is clearly different. and rims, as in NWA 1500, has not been observed in ureilites. However, petrographic identity of a meteorite to the known In addition, its Wo content (2.1) is lower than that (4–5) of members of an established group is not a necessary orthopyroxene in ureilites. requirement for it to belong to that group. Another criterion The compositions of metal in NWA 1500 are notable in that should be considered is whether it can be petrologically that Cr and P contents are very low (near or below detection related to the known members. If so, it may expand our Northwest Africa 1500 939

Fig. 10. Cr# and fe# of chromites in NWA 1500. Grains are essentially homogeneous and show no compositional dependence on size. knowledge of the diversity of rock types found on the parent of ureilites, is characterized by ∼mm-size anhedral olivine body of the group, and enhance our understanding of its and pyroxene grains that meet in triple junctions and have geologic history. In the following sections, we will consider gently curved intergranular boundaries. This texture may also whether this is the case for NWA 1500 and the ureilite parent be described by the more informative (albeit interpretive) body (UPB). We begin by reviewing and clarifying some term “equilibrated.” The poikilitic texture, which occurs in aspects of the classification of ureilites, and further only in a small percentage of all ureilites, is characterized by developing a model that was introduced by Goodrich et al. the presence of very large (up to cm-size) crystals of low-Ca (2004) for the petrogenesis of augite-bearing ureilites. pyroxene (orthopyroxene or pigeonite). These crystals poikilitically enclose areas of olivine ± pyroxene that Classification of Ureilites themselves show the typical (equilibrated) ureilite texture, as well as single rounded grains of olivine and pyroxene. Monomict ureilites were divided by Goodrich (1992) on Ureilites showing the poikilitic texture also commonly have the basis of texture into two types: typical and poikilitic other areas that show only the typical texture and are (neglecting highly shocked ureilites, which have a mosaicized sufficiently heterogeneous that some thin sections show only texture). The typical texture, which occurs in the vast majority one of the two textures. The presence of two textural domains 940 C. A. Goodrich et al.

Fig. 11. Cr# and fe# of chromites in NWA 1500 (solid squares) compared to those of chromite in the chromite-bearing ureilite LEW 88774 (9 grains, each shown with different symbol). Chromites in LEW 88774 have experienced a very high degree of secondary reduction, which has led to extreme decreases in fe#, and at high degrees of reduction (very low fe#) also extreme decreases in Cr#. The primary chromite composition can be inferred to have had Cr# ∼0.75 and fe# >0.45. Chromites in NWA 1500 are consistent with this composition, and show no evidence of secondary reduction. Data for both samples obtained in this study.

Fig. 12. A BEI of the one large apatite grain (light grey surrounded by medium grey olivine; white = metal and dark grey = reduction rim olivine or orthopyroxene) observed in this study. Apatite also occurs in a few patches of very small grains. in these ureilites led to use of the term “bimodal” to describe textured ureilites are notable in that, aside from the low-Ca them (Mittlefehldt et al. 1998). pyroxene oikocrysts, the dominant pyroxene is augite, and As discussed by Goodrich (1992) and Mittlefehldt et al. pigeonite is minor or absent. The oikocrysts are most (1998), the identity of the pyroxene(s) found in ureilites is commonly orthopyroxene, but in one case (LEW 88774) they complex. In the vast majority of ureilites that show only the consist of inverted pigeonite (orthopyroxene with augite typical texture, the sole pyroxene is uninverted pigeonite with exsolution lamellae). An important point here is that we use Wo ∼7–13. In a few, however, augite and/or orthopyroxene the term “orthopyroxene” (as did Goodrich et al. 2004) to occur instead of or in addition to pigeonite. Most poikilitic- designate low-Ca pyroxene with Wo ≤ 5 (this conforms to the Northwest Africa 1500 941

Table 4. Compositions of metal and sulfide in NWA 1500. Table 5. Bulk composition of NWA 1500. Metal Sulfide Sample 1 Sample 2 Avg (17) St. dev (17) Ca <0.100% na Si 0.04 0.02 0.01 Fe 23.41% 23.35% P 0.03 0.01 bdl ppm ppm S 0.00 0.01 36.9 Na 90 214.4 Cr 0.01 0.00 0.06 K 6.2 239 Fe 98.04 0.63 62.7 Sc 5.31 6.64 Co bdl 0.00 bdl Cr 4060 4530 Ni 1.34 0.18 bdl Mn 2870 3034 Zn bdl 0.03 bdl Co 172 167 Mo 0.01 0.01 0.4 Ni 650 502 W bdl 0.01 bdl Cu 9.00 na Total 99.5 100.1 Zn 2116 257 All values in wt%. Ga 1.80 na bdl = below detection limit. As 0.087 1.73 Detection limits: Si = 0.008%; P = 0.03%; S = 0.03%; Cr = 0.03%; Co = Se 1.20 na 0.04%; Ni = 0.05%; Zn = 0.1%; Mo = 0.09%; W = 0.03%. Br 0.063 0.264 Rb <5.0 na definition of orthopyroxene in Deere et al. (1978). In fact, the Mo <0.30 na present structural states of such pyroxenes in ureilites, as In <0.20 na shown by micro-structural investigations (e.g., Takeda 1987; Sb <0.01 na Takeda et al. 1989, 1992; Goodrich et al. 2001; Weber et al. Cs <0.01 na 2003), are complex and reflect shock-induced or other Ba 24.0 na subsolidus transformations (Weber et al. 2003). Nevertheless, La <0.01 0.45 the low Wo content (compared to the Wo ∼7–13 of the Ce na 0.7 common pigeonite in ureilites) of these pyroxenes is very Sm 0.0052 0.7 < likely indicative of primary crystallization as an orthorhombic Eu 0.005 0.7 phase (or, more to the point, of primary crystallization as a Tb na 0.0185 Yb 0.0270 na phase distinct from the pigeonite), and we refer to them as Lu <0.009 na orthopyroxene in order to emphasize this. Note that this use of Hf <0.1 0.195 the term may lead to some confusion in comparing our work Ta <0.015 na with that of other authors. For example, Weber and Bischoff W <0.04 0.641 (2003) refer to the oikocrysts of Wo ∼4.5 pyroxene in Re <0.01 na Hammadah al Hamra (HaH) 064 as pigeonite (they contain Ir 0.030 na antiphase domains characteristic of low-temperature Au 0.00487 na conversion to low pigeonite), whereas we will refer to them as Th <0.09 na orthopyroxene because they most likely crystallized at high U <0.02 na temperature as Pbca orthopyroxene (Putnis 1992; Deere et al. 1978). Also, it is common that in initial descriptions of new dominant (usually the sole) pyroxene, and almost invariably ureilites (in the Antarctic Meteorite Newsletter or Meteoritical have the typical (equilibrated) texture. These are ureilites of Bulletin) pyroxenes with Wo ≤ 5 are reported to be pigeonite, the “classic” Kenna type. Olivine-orthopyroxene ureilites, of whereas we would describe them as orthopyroxene. Primary which only five samples are currently known, contain orthopyroxene in ureilites (regardless of present structural orthopyroxene instead of or in addition to pigeonite (augite state) can often be recognized optically by a fine-scale has also been observed in some, but is minor). Of these, four lineated texture (probably shock-induced twinning: Goodrich have equilibrated textures and one has a largely poikilitic et al. 2001; Tribaudino et al. 1997), orthorhombic cleavage, texture (i.e., the orthopyroxene occurs as large oikocrysts and a tendency to form slightly “more euhedral” grains than enclosing olivine). The augite-bearing ureilites, of which does pigeonite. eight have been well described (Table 6), are those in which It is, in fact, the pyroxenes present in a ureilite that are augite is a major phase and occupies the textural niche filled most informative of its petrogenesis. For this reason, by pigeonite in olivine-pigeonite ureilites. All of them also Goodrich et al. (2004) introduced a new division of monomict contain orthopyroxene as a major phase (Table 6), and all but ureilites into three types: olivine-pigeonite, olivine- two (Hughes 009 and Frontier Mountain [FRO] 90054/93008/ orthopyroxene, and augite-bearing. Olivine-pigeonite 90228) have dominantly poikilitic textures, with ureilites (by far the most abundant) contain pigeonite as the orthopyroxene oikocrysts enclosing typical-textured areas of 942 C. A. Goodrich et al.

Fig. 13. REE concentrations (obtained by INAA) in two samples of NWA 1500 compared to those of olivine + low-Ca pyroxene ureilites (shaded region) and augite-bearing ureilites (open boxes). Sample 1 has very low REE concentrations (within the range of those in olivine- rich ureilites), suggesting that it was from an olivine-rich part of the rock and contained little augite or plagioclase. Sample 2 has high LREE concentrations, which may be due to terrestrial contamination. olivine and augite (these are the ureilites that have been called and pyroxene/olivine ratio of the olivine-pigeonite ureilites bimodal). Pigeonite occurs in a few of them (in a textural (Singletary and Grove 2003). Olivine-orthopyroxene ureilites occurrence similar to that of the augite), but is minor are all notably magnesian, with Fo ∼86–92. Their Fe/Mg-Fe/ (Table 6). We emphasize that the classification of ureilites by Mn compositions fall on the same trend as the olivine- pyroxene type does not strictly correspond to the pigeonite ureilites (Fig. 6), suggesting that they too are classification by texture, and that it is the former which we residues, and their high pyroxene/olivine ratios extend the now consider most important in informing a model for the correlation between mg and pyroxene/olivine ratio observed petrologic structure of the UPB. for the olivine-pigeonite ureilites. In the smelting model, they must have formed at lower pressures (shallow depths) than Structure of the Ureilite Parent Body the olivine-pigeonite ureilites (Fig. 14). Assuming relatively homogeneous starting materials, the Based upon this new classification of ureilites, Goodrich smelting model thus predicts that during high-T igneous et al. (2004) presented a model for the petrologic structure of processing the UPB developed a stratification (or gradient) in the UPB. We briefly summarize that model, and then further mg and pyroxene abundance, due to the pressure dependence develop aspects of it that pertain to the petrogenesis of augite- of carbon redox control (Fig. 15). The occurrence of bearing ureilites. The Fe/Mg versus Fe/Mn trend of the orthopyroxene only among the most magnesian (shallowest) olivine-pigeonite ureilites, which is characterized by near- samples suggests that it was also stratified in pyroxene type, constant, chondritic Mn/Mg ration (Fig. 6) suggests that they which is consistent with phase relations in the system {Ol}- are residues (from <30% melting of material with chondritic Opx-Wo-Plag (Fig. 16). The configuration of the liquidus Mn/Mg ratio) and are related to one another principally by surface in this system (and hence the relative stabilities of various degrees of reduction of common precursor material pigeonite, orthopyroxene, and augite) is strongly dependent (Goodrich and Delaney 2000). This reduction relationship is on compositional parameters, particularly mg (Longhi 1991; thought to be due to carbon redox control, or smelting Longhi and Pan 1988). As mg increases, the stability of (Fig. 14), over pressures ranging from ∼30 bars for the most orthopyroxene is enhanced relative to that of pigeonite, i.e., magnesian sample to ∼100 bars for the most ferroan sample the pigeonite (+olivine) field moves away from the Plag apex (Berkley and Jones 1982; Goodrich et al. 1987; Warren and of the system, and shrinks (shown in Fig. 16 for the range of Kallemeyn 1992; Walker and Grove 1993; Sinha et al. 1997; ureilite mg). In addition, pure reduction (simple removal of Singletary and Grove 2003), thus indicating that they are FeO) moves any composition directly toward the Opx apex of derived from a range of depths. The smelting model is also the system. Thus, a starting composition such as that shown in supported by an observed positive correlation between mg Fig. 16 would move with progressive reduction from the Northwest Africa 1500 943 al. (2003); (8) of augite, we classify it as augite- several recently discovered samples tti (2000); (7) Weber et tti (2000); (7) Weber (bimodal) inclusions Melt 1, 7 ypical Melt inclusions 1, 5, 6 Poikilitic (bimodal) 1, 2, 3 rich (2000); (6) Goodrich and Fiore of the low-Ca pyroxene and presence 5; Yamaguchi et al. 2005) suggest that 5; Yamaguchi to META78008 Poikilitic (bimodal) 1, 2 003); (5) Fioretti and Good 15% Poikilitic (bimodal) Chromite 1, 9, 10 ∼ rkley and Goodrich (2001). t. Recent reports (Russell et al. 2004, 200 70–80% 1% 5–10% Poikilitic (bimodal) 1, 2, 11 ∼ ∼ However, because of the low Wo content (4.5) because of the low Wo However, ) Singletary and Grove (2 z et al. (1994); (11) Be z et al. (1994); (11) Hughes 009 Similar to Hughes 009 T 50% Similar to META78008 Poikilitic ∼ ich adequate descriptions currently exis llemeyn (1994); (10) Prin . a 86.9 40–56% 7% Typical 1, 4 Singletary and Grove (2003) as olivine-pigeonite. b ; (2) Takeda et al. (1989); (3) Berkley (1986); (4 ; (2) Takeda may also belong to this group. Goodrich et al. (2001); (9) Warren and Ka Goodrich et al. (2001); (9) Warren bearing. Includes all augite-bearing ureilites for wh References: (1) This work This ureilite was classified by FRO 90054/93008/90228 87.6 Similar to META78008Y-74130 75.9 47% 76.9 Similar to META78008 Similar EET 96293/96314/96331 Name/PairingsALH 82130/82106/84136 95.1 29–32% Fo Opxa b minor 8% Plg Augite Texture Comments References Hughes 009LEW 88774 87.3 75.0 40–56% 7–30% Typical Melt inclusions 1, 8 HaH 064 77.4 Table 6. Augite-bearing ureilites Table 944 C. A. Goodrich et al.

Fig. 14. Plots of log fO2 versus T, showing curves of graphite-CO + CO2 equilibria (solid lines, labeled in bars) at various total gas pressures, and of olivine-silica-metal equilibria (dashed lines, labelled in Fo) at various Fo values. a) Since ureilites show equilibration temperatures in the range of ∼1200–1275 °C, their olivine compositions (Fo ∼76–94) imply that they formed over pressures of ∼30–100 bars, assuming carbon recox control (smelting). In this temperature range, the compositions of olivine in NWA 1500 (Fo 65–72) would imply higher pressures. b) ∼ NWA 1500 shows equilibration temperatures in the range 835–1030 °C and hence log fO2 in the range shown by the thick line. The interpretation of these values depends on whether cooling was isobaric or not (see text). Graphite-CO + CO2 curves are based on the precise thermodynamical treatment of French and Eugster (1965). Olivine-silica-metal curves are from the equations of Williams (1972) and Nitsan (1974). Northwest Africa 1500 945

Fig. 15. The petrologic structure of the UPB, which must have been stratified (or had a gradient) in mg, pyroxene/olivine ratio, and pyroxene type, due to the pressure dependence of carbon redox control (see Fig. 14). Diagram modified from Goodrich et al. (2004). Augite-bearing ureilites may be cumulates and paracumulates, which formed from melts that originated at greater depths than the olivine + low-Ca pyroxene residual ureilites (within the field of augite stability; see Fig. 16) but ascended to within the same range before becoming entrapped. Their present mg values must be lower than those of their parent magmas (due to reduction during ascent), and represent their final depths of equilibration. In a ureilite model, NWA 1500 must be a cumulate formed at a greater depth (under more oxidized conditions) than any of the known augite-bearing ureilites. augite to the pigeonite to the orthopyroxene stability fields. of pyroxene). Note that the choice of starting FeO content The combination of these two effects (movement of the places a limit on the depth of smelting in the UPB, because composition and movement of the phase boundaries) would there will be some pressure above which carbon redox result in a strong dependence of pyroxene type on mg (depth) reactions buffer mg values lower than those of the starting in the UPB (Fig. 15). material (Fig. 14) and so no smelting can occur. For mg ∼65, The composition shown in Fig. 16 is a model bulk this pressure is ∼200 bars at magmatic temperatures (Figs. 14 composition for the UPB, taken from our recent work and 15). (Goodrich 2006; Goodrich et al. 2006). This composition is generally similar to alkali-poor carbonaceous chondrites such Petrogenesis of Augite-Bearing Ureilites as Allende, but is notable in that it has a superchondritic Ca/ Al ratio (2.5 × CI). The reason for this is that modeling of The consequence of the smelting model that a single Goodrich (1999), which utilized the melting and starting composition on the UPB would move with crystallization program MAGPOX (Longhi 1991 and progressive reduction (decreasing depth) through augite → updates), showed that the trajectory of pure reduction for pigeonite → orthopyroxene stability leads to an apparent strictly chondritic compositions does not significantly problem in the case of the augite-bearing ureilites. It follows intersect the pigeonite field (Fig. 16) and thus does not lead to that if all ureilites are residues (each representing a particular ureilite-like olivine-pigeonite residues; superchondritic Ca/Al depth), the augite-bearing ones should have mg values lower ratios are required. Although this conclusion has been than those of the olivine-pigeonite ureilites, which is not the challenged (Kita et al. 2004), our recent re-evaluation has case. In fact, the augite-bearing ureilites show approximately shown it to be robust. We assume the total iron content of the the same range of mg as the olivine-pigeonite ureilites bulk UPB to have been similar to that of the most Fe-rich CV (Table 6). A clue to resolving this apparent problem comes chondrites, or mg = ∼62 if all Fe is in the form of FeO from the poikilitic textures of most of the augite-bearing (labelled mg 65 in Fig. 16 because this is the Fo of olivine in ureilites. These textures (orthopyroxene oikocrysts enclosing a residue produced from such material after complete removal olivine and augite) are similar to those commonly seen in 946 C. A. Goodrich et al.

Fig. 16. Phase system orthopyroxene–plagioclase–wollastonite (Opx)–(Plag)–(Wo) projected from olivine (Longhi 1991). Phase boundaries shown at mg 70, 76, and 95 (mg of equilibrium olivine) illustrate the strong dependence of relative augite-pigeonite-orthopyroxene stability on mg. Model bulk composition of the UPB shown at various degrees of reduction (labelled with mg 65–95). With progressive reduction, this composition moves from the augite to the pigeonite to the orthopyroxene stability field. Olivine + low-Ca pyroxene ureilites (mg ∼76–92) are interpreted to be residues formed over a range of depths (since the carbon redox reactions that determined their mg are pressure-dependent). The augite-bearing ureilites may have crystallized from magmas that were generated at greater depths, under less reduced conditions in which augite was the stable pyroxene. Melts (∼14–25%) derived from mg 65 source region (inferred bulk UPB) shown by open circles. Dashed arrows illustrate possible crystallization paths of such melts during ascent. Note that these paths do not follow those of normally fractionating magmas, because reduction during ascent (caused by decreasing pressure) pulls them directly toward Opx. Possible crystallization path of NWA 1500 parent magma during ascent shown by solid arrow. This figure is an updated version of Fig. 3 from Goodrich et al. (2004). See Longhi (1991) for projection equations.

Fig. 17. Augite → low-Ca pyroxene reaction texture in poikilitic augite-bearing ureilite LEW 88774 (transmitted light, crossed polars). Image shows isolated, irregularly shaped grains of augite (mainly blue, with striations) enclosed within an oikocryst of low-Ca pyroxene (lamellae in oikocryst are exsolution lamellae). Augite grains are in optical continuity, and have been shown by TEM studies to share a common crystallographic orientation within the oikocryst (Goodrich 1999; Goodrich and Keller 2000). They are interpreted as remnants of once single cumulus grains, that were resorbed by pore liquid from which the oikocryst grew. Northwest Africa 1500 947 heteradcumulates in terrestrial layered igneous complexes 95) seen in the augite-bearing ureilites. Thus, the augite- and thought to have formed by growth of oikocrysts from bearing ureilites are interpreted as the products of melts that pore liquids (Wager and Brown 1967; Hunter 1996). Thus, originated deep on the UPB, but which ascended to depths they suggest that the orthopyroxene in these rocks crystallized within the same range as that represented by the olivine + from melts. In addition, many of the augite-bearing ureilites low-Ca pyroxene ureilites before reaching final equilibration contain melt inclusions (melt inclusions have not been (i.e., becoming entrapped and forming cumulates or observed in any olivine-pigeonite ureilite, although the first paracumulates). author of this paper has searched for them), which occur not This model is also attractive because it can explain the only in the orthopyroxene oikocrysts but also in the olivine appearance of low-Ca pyroxene (typically orthopyroxene) as and augite (Goodrich et al. 2001; Goodrich 2001a; Fioretti the next phase to crystallize in these rocks. The normal and Goodrich 2000; Goodrich and Fioretti 2000), thus fractionation/crystallization path for melts such as those we suggesting that the olivine and augite also crystallized from are considering would be olivine → augite → plagioclase → melts. The textural evidence that these ureilites crystallized low-Ca pyroxene. However, the effect of reduction is to pull from melts is augmented by the Fe/Mn-Fe/Mg compositions the fractionation/crystallization path toward Opx, and in of their olivine, which are invariably displaced to higher Mn/ many cases this will cause it to miss the augite/plagioclase Mg ratio relative to the residue trend of the olivine + low-Ca phase boundary entirely and to pass directly from the augite to pyroxene ureilites (Fig. 6), as expected for melts related to the low-Ca pyroxene field (Fig. 16). This has the additional such residues (Goodrich and Delaney 2000). attraction of explaining one of the most notable features of the If the augite-bearing ureilites crystallized largely from poikilitic augite-bearing ureilites: the presence of augite → melts (and are therefore cumulates or paracumulates rather low-Ca pyroxene reaction textures. In LEW 88774 (Fig. 17) than residues), then the fact that their mg span the same range and META78008, which provide the most dramatic examples as those of the olivine-pigeonite ureilites need not conflict of this texture, the oikocrysts enclose abundant, rounded and with the smelting model. The crystallization sequence of irregularly shaped augite grains, which (though isolated from these melts can be inferred from textural relationships, and one another) are in optical continuity and have the same from the compositions of melt inclusions (Goodrich et al. crystallographic orientation over areas of up to ∼2 mm 2001; Goodrich and Fioretti 2000), to have been olivine → (Goodrich 1999; Goodrich and Keller 2000; Berkley and augite → low-Ca pyroxene (orthopyroxene). Melts that would Goodrich 2001). By analogy to similar textures seen in originally crystallize olivine + augite could only be generated terrestrial heteradcumulates, these grains appear to be from materials located in the augite primary phase field in remnants of once single cumulus crystals that reacted with the Fig. 16. For our estimated bulk UPB composition, this means pore liquid from which the oikocrysts grew (Hunter 1996; materials having mg in the range of ∼65–70 and representing Jackson 1961; McBirney 1995). Such a reaction is not source regions located at depths greater than (and less reduced expected to occur in a normal igneous fractionation process than) those at which the olivine-pigeonite ureilites formed (unlike the common olivine → low-Ca pyroxene reaction), (Fig. 15). High degree (after removal of plagioclase) melts of but could easily be explained if reduction of an augite- such compositions will lie within the augite field; for saturated melt during ascent on the UPB caused it to pass example, ∼14–25% melts of the bulk UPB composition (mg directly from the augite to the low-Ca pyroxene stability field 65) are shown (∼14% is the minimum amount of melting (Fig. 16). Thus, the orthopyroxene oikocrysts in augite- needed to eliminate plagioclase, and ∼25% melting eliminates bearing ureilites can be interpreted as later products of the augite). If such melts were to crystallize at depths near those melts from which their olivines and augites crystallized. at which they were generated, they would produce olivine- augite assemblages with Fo ∼65. However, if they ascended to Petrogenesis of NWA 1500 on the UPB? shallower depths (lower pressures and therefore lower fO2) they would be reduced, which would have the effect of If NWA 1500 is a ureilite, then it is an augite-bearing moving them directly toward Opx in the phase diagram. Such ureilite and therefore, following our model for the melts, provided they remained in the augite stability field, petrogenesis of augite-bearing ureilites, must be largely a would crystallize to olivine-augite assemblages having mg cumulate from a melt that was generated deep in the UPB and higher than those of their original source regions. A wide ascended to shallower depths before becoming entrapped. variety of crystallization paths that meet this criterion are Many of the petrologic characteristics of NWA 1500 are conceivable (a few examples are illustrated in Fig. 16), consistent with such a model. First, although the coarse- depending on depth of source region (degree of reduction), grained, equilibrated texture and olivine-augite mineralogy of degree of melting, and rate of ascent (rate of reduction versus the majority of NWA 1500 are uninformative as to whether it crystallization), but test calculations with MAGPOX have is a cumulate or a residue (as is the case for many ultramafic demonstrated the basic feasibility of this model in producing rocks), if it is a ureilite then the superchondritic Mn/Mg ratio early olivine-augite assemblages over the range of mg (∼76– of its olivine (Fig. 6) distinguishes it as the former (if it is not 948 C. A. Goodrich et al.

Fig. 18. The oxygen isotopic composition of NWA 1500 compared to ureilites. Data from Clayton and Mayeda (1988, 1996). a ureilite then the Fe-Mn-Mg composition of its olivine does bearing ureilites, can be interpreted as late products of the not provide definitive information as to whether it is a same melt that the large olivine and augite grains formed cumulate or a residue, but the fact that the Mn/Mg ratio is so from. high relative to chondritic values nevertheless strongly However, the petrogenesis of NWA 1500 appears to have points toward formation from a melt). Furthermore, the differed from that of known augite-bearing ureilites in three textures of the plagioclase-rich areas (poikilitic occurrence of significant respects: 1) it formed under more oxidized plagioclase and interstitial/rimming occurrence of conditions, 2) the crystallization sequence of its parent orthopyroxene) are clearly indicative of crystallization from a magma was olivine → augite → plagioclase → melt and, by analogy to the poikilitic areas of bimodal augite- orthopyroxene, rather than olivine → augite → Northwest Africa 1500 949 orthopyroxene, and 3) it equilibrated to significantly lower path differs only slightly from that inferred for known augite- temperatures than did ureilites. Nevertheless, all of these bearing ureilites, principally in that it has a smaller differences could be explained within the context of the component of reduction (it was bent less toward Opx and so model described above. does not miss the plagioclase/augite boundary). This would, A higher oxidation state for NWA 1500, compared to in fact, be expected for a melt that did not ascend as far (as the known ureilites, is indicated by: 1) the more ferroan parent melts of known augite-bearing ureilites) through the composition of its olivine, 2) the presence of chromite and the smelting range and became entrapped at a greater depth low Cr contents of olivine, metal and sulfide, all of which (Fig. 15). The appearance of orthopyroxene only in the final indicate that Cr occurs principally as Cr3+ rather than (as it stages of crystallization, when the peritectic point (Fig. 16) is does in ureilites) as Cr2+ or Cr0, and 3) the presence of apatite reached, is consistent with its occurrence in the rock as small, and the lower P content of metal, which indicate that P occurs interstitial grains. In this case, it appearance might be due principally as P5+ rather than (as it does in ureilites) as P0. more to normal igneous fractionation processes than to Formation under more oxidizing conditions can be explained reduction (consistent with the observation of normal olivine if this cumulate formed at a greater depth than that at which → orthopyroxene reaction textures and the less dramatic the most ferroan ureilite formed (Fig. 15), since (assuming nature of the augite → orthopyroxene reaction textures carbon redox control, which is reasonable, since NWA 1500 compared to those in ureilites). It is likely, in this model, that contains graphite) Fo values of 65–72 imply higher pressures the NWA 1500 parent magma was highly refractory, i.e., it (Fig. 14). It is interesting that the most ferroan Fo value was a high degree fractional melt (melting on the UPB was observed in this rock (65) is almost exactly that predicted to more likely a fractional than a batch process: Kita et al. 2004; be the limiting (most ferroan) value for a UPB with a bulk Cohen et al. 2004). This is suggested by the low abundance of iron content similar to that of CV chondrites, and would imply plagioclase and orthopyroxene (although this could also be that the parent melt originated at a depth above the smelting explained by loss of fractionated liquid from the cumulate limit (Figs. 14 and 15). Assuming that olivine was the pile), and by the evidence for crystallization of a large amount liquidus phase (as indicated by its high abundance and of olivine before the appearance of augite. texture) and that early crystals were entrained in the rising Equilibration temperatures calculated for coexisting magma, the reverse zoning seen in the cores of a few olivine augite and orthopyroxene in NWA 1500 (Table 3, columns 1 grains (Figs. 2 and 5) can be explained by a combination of and 3) using QUILF (Andersen et al. 1993) are 913 °C based reduction (due to pressure drop) and continued growth during on Ca distribution and 835 °C based on Fe/Mg distribution. ascent (the absence of the metal formed by the reduction The olivine composition predicted by QUILF to be in reaction can be explained by the presence of a large amount of equilibrium with these pyroxenes is Fo 69 ± 2 at either 913 or melt, so that metal would easily separate from silicates). The 835 °C, indicating that the dominant olivine composition dominant value of Fo ∼68–69 (Fig. 5) may, then, represent the observed in the rock also reflects these low temperatures (we equilibrium value for the final depth at which the cumulate note that the low CaO content of the olivine may also be formed (Figs. 14 and 15), and the reduction rims (Fo up to indicative of equilibration to low temperatures). Equilibration ∼73) could be interpreted, as for all ureilites (Mittlefehldt temperatures (±40 °C) calculated for coexisting olivine- et al. 1998) as a secondary feature unrelated to primary chromite (the composition with the highest fe#, since those petrogenesis. The fact that a record of reduction during ascent with lower fe# may have experienced secondary reduction) is preserved in NWA 1500 but not in “other” augite-bearing pairs using the olivine-spinel thermometer of Sack and ureilites could be explained by a greater depth of derivation of Ghiorso (1991) are 1030 °C (Fo 65), 975 °C (Fo 68.5), and its parent magma (resulting in longer ascent time). 930 °C (Fo 71.3). Corresponding values calculated with the The silicate mineral crystallization sequence olivine → Fabriès (1979) olivine-chromite thermometer, as modified by augite → plagioclase → orthopyroxene (it is likely that Wlotzka (2005), are 963 °C, 915 °C, and 877 °C. chromite began to crystallize before olivine, and in fact the All of these temperatures are considerably lower than any very low Cr2O3 content of olivine suggests the possibility that equilibration temperatures determined for ureilites (based on early chromite cumulates were formed) is inferred for NWA pyroxene and olivine compositions), which are in the range of 1500 principally from textural relations: augite grains have an 1100–1300 °C (Mittlefehldt et al. 1998; Goodrich et al. 2001; intergranular, semi-poikilitic relationship to olivine (also, the Singletary and Grove 2003). In fact, the thermal history of all lack of reverse zoning in augite suggests that it crystallized ureilites is quite distinctive (and constitutes one of the later than the olivine grains that show such zoning; i.e., after strongest pieces of evidence that they are all derived from the the magma had completed most of its ascent); plagioclase same parent body), showing extremely rapid cooling (on the poikilitically encloses both olivine and augite; and order of 10 °C/h) through the range 1100–650 °C coupled orthopyroxene occurs only as small interstitial grains and as with a sudden drop in pressure (during which the secondary rims on augite and olivine. This sequence can be explained if reduction rims formed on olivine). This P-T history suggests the parent melt followed a fractionation/crystallization path that they were excavated, while still at near-magmatic similar to that shown by the solid arrow in Fig. 16. Such a temperatures, by a single major impact (Takeda 1987; Warren 950 C. A. Goodrich et al. and Kallemeyn 1992), and furthermore were probably not Is NWA 1500 a Ureilite? reaccreted to their parent body (Goodrich et al. 2004). In this context, the significantly lower equilibration In sum, we have argued that the major petrologic temperatures of NWA 1500 would suggest that it formed at a characteristics of NWA 1500 can plausibly be explained greater depth than that from which material was excavated on within the context of a model for the petrogenesis of augite- the UPB (Fig. 15), which would, of course, be consistent with bearing ureilites and the history of the UPB, provided that this our argument from its redox state that it must have formed at rock did not share the final stages of that history with the rest greater depth than any known ureilite. The oxidation state of the ureilites. Is this interpretation likely to be correct? argument needs to be reexamined, however, in light of the low Possibly not. First, we consider the argument of Goodrich equilibration temperatures, since in the discussion above we et al. (2004) that all ureilites are being delivered to Earth from tacitly assumed that NWA 1500 equilibrated in the same T a secondary (daughter) body, to which they accreted after range as ureilites (Fig. 14a). Any estimate of fO2 based on excavation from their original parent. It follows that if NWA olivine compositions in NWA 1500 (no matter which one), 1500 is a ureilite, then it remained on the remnant of the over the T range ∼1000–800 °C, will yield absolute values original parent (UPB), and must have been delivered to us by that are considerably lower than those now recorded in a different mechanism. Although this is certainly not out of ureilites, and which (assuming carbon redox control) imply the question, the likelihood of this happening seems much lower pressures of equilibration (Fig. 14b). If we assume that less than the likelihood of receiving another sample from the the rock cooled isobarically, we arrive at the conclusion that it daughter body (since we already have more than 180 of formed under more reduced conditions (rather than more those), and thus it decreases the plausibility that NWA 1500 is oxidized conditions) than known ureilites, and that its olivine a ureilite after all. composition has reequilibrated from Fo > 95 to Fo 65. This More importantly, however, we now consider the oxygen seems highly unlikely. First, although it is certainly the case isotopic composition (Fig. 18a) of NWA 1500 (Bartoschewitz that olivine compositions can change with decreasing et al. 2003). This composition is significantly outside the temperature, even under fO2-buffered conditions, olivine range of compositions of known ureilites, and cannot be composition is not the only evidence that NWA 1500 formed related to them in any plausible way by fractionation under more oxidized conditions than did known ureilites. The (Bartoschewitz et al. 2003). Furthermore, it is not consistent presence of chromite (clearly an early phase) and phosphate with the observed correlation between Δ17O and mg (which attest to this as well. Second, it is unlikely that carbon redox implies a correlation of Δ17O with depth) amongst ureilites equilibrium could be maintained over such a large range of (Fig. 18b). Of known meteorite groups, it is closest to isobaric cooling, since this would require the continued compositions of winonaites/IAB silicate inclusions, but in presence of both metallic iron (to be oxidized and added to fact does not fall within their range either. olivine) and CO/CO2 gas. The gas, in particular, is unlikely to The oxygen isotopic composition of NWA 1500 could be available (any gas generated by previous reduction would have been taken a priori to mean that this rock was not formed have quickly escaped to the surface), and metal may well on the ureilite parent body. However, we deliberately chose to have been lost in the form of metallic melt. examine this rock from a strictly petrologic point of view We return, then, to the reasonable assumption that NWA before accepting that conclusion, because we were so 1500 formed (at high T) under more oxidized conditions than impressed by its similarities to, and suggestive differences did known ureilites, which would imply that its olivine from, the augite-bearing ureilites. Our conclusion that it could composition did not change dramatically (i.e., on the scale of plausibly be interpreted as an augite-bearing ureilite is indeed things in Fig. 14) during cooling, and that, if cooling was suggestive, but must be tempered by the realization that it can isobaric, carbon redox control was not maintained (isobaric only be so under rather special circumstances. Its strikingly cooling with maintenance of carbon redox control would different thermal history may more plausibly be explained if result in extremely FeO-rich olivine). As we have just argued, it is in fact not a ureilite at all, but rather an ungrouped this would not be surprising. Interestingly, however, our achondrite (Mittlefehldt and Hudon 2004). Since this model for forming NWA 1500 on the UPB requires the interpretation is more satisfactory in terms of its oxygen removal of overburden and hence a decrease in pressure isotopic composition, we are inclined to embrace it. during cooling. This suggests the possibility that carbon Nevertheless, the results of our study suggest that the parent redox control was maintained after all. Finally, we note that in body of NWA 1500 may have been compositionally and this model the narrow reduction rims seen on olivine in NWA petrologically similar to the ureilite parent body, and may 1500 would not have formed under the same conditions as have had a similar differentiation history. The extent to which those in “other” ureilites (i.e., excavation and quenching); that is true is, of course, difficult to assess in the absence of rather, they would have formed when the overlying material other material from the same body, and we caution that many was removed and pressure decreased (a less catastrophic of the characteristics of NWA 1500 could be consistent with process, which might account for their limited nature). interpretations other than those discussed here. Northwest Africa 1500 951

Acknowledgments–We thank J. L. Berkley, H. C. Connolly, Goodrich C. A. 2001b. Origin of chromite in ureilite LEW 88774 J. Mikalopas, A. H. Treiman, P. H. Warren, and (abstract). Meteoritics & Planetary Science 36:A67–68. M. K. Weisberg for helpful discussions and comments. We Goodrich C. A. 2006. Composition of ureilite precursor materials (abstract #1194). 37th Lunar and Planetary Science Conference. are grateful to B. Spettel, M. I. Prud^encio, and N. Lahajnar for CD-ROM. analyses presented in this paper. Helpful reviews by Goodrich C. A. and Delaney J. S. 2000. Fe/Mg-Fe/Mn relations of S. Singletary, A. Yamaguchi, and K. Righter are appreciated. meteorites and primary heterogeneity of primitive achondrite This work was supported by NASA grant NNG05GH72G to parent bodies. Geochimica et Cosmochimica Acta 64:2255– C. A. Goodrich, the Max Planck Society, and NASA grant 2273. Goodrich C. A. and Fioretti A. M. 2000. The parent magmas of NAG5-11591 to K. 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