Eucrite-like basaltic or gabbroic achondrite, ungrouped
31° 20' N., 4° 20' W.
Several small stones were purchased from a man in Rissani, Morocco by a Russian team during a meteorite expedition. Among these stones was a relatively fresh 40 g meteorite, designated NWA 011, which was analyzed and classified at the Vernadsky Institute, Moscow, by S. Afanasiev and M. Ivanova. Initially, the meteorite was classified as a highly metamorphosed, unbrecciated, noncumulate eucrite with Fe/Mn ratios of pyroxene significantly higher (~65) than other eucrites (3050) (Afanasiev et al., 2000). A second, nearly identical stone weighing 137 g, NWA 2400, was also recovered, and it was determined to be most likely paired with NWA 011 (Bunch and Wittke, NAU; Irving and Kuehner, UWS; Rumble CIW). Further searches of the area have resulted in the recovery of more stones that are considered to be paired with NWA 011, including NWA 2976 (219 g) and NWA 4901 (24 g). One stone currently under analysis weighs 95 g, while another stone represents the largest known member of this pairing groupa 530 g stone designated NWA 4587, for which a 360°, 3D image has been skillfully constructed by G. Hupé. Additionally, a 200 g stone has been designated NWA 5644, while a 50 g stone has been designated NWA 7129. The TKW for this unique pairing group is calculated to be over 1 kg.
The cutting and distribution of NWA 011 is well documented (Inoue, Meteorite, August 2002). A 22.236 g end section that was offered for sale at the 2000 Denver Show attracted the attention of S. Inoue of Hori Mineralogy Ltd., who eventually purchased the piece on behalf of A. Yamaguchi of the National Institute of Polar Research and himself. This piece was cut into two sections17.773 g was distibuted to NIPR, and the remaining 4.018 g slice was retained by HML. The remainder of the material, representing ~45 g, is included in the original slice. The other end section, which weighs 11.4 g, is curated at Vernadsky Institute.
Northwest Africa 011 is primarily composed of coarse-grained pyroxene (58.5 vol%, as pigeonite and augite) in fine-grained plagioclase (39.6 vol%, as bytownite), with minor quartz (0.7 vol%), Ca-phosphate (0.5 vol% as merrillite and chlorapatite), Fe-rich olivine (trace), and opaques (0.7 vol%) including ilmenite, troilite, Ti-rich chromite, and ulvöspinel). The presence of igneously-zoned plagioclase laths suggests a complicated petrogenesis, in accord with a history that includes low degrees of fractional crystallization of a partial melt, brecciation and recrystallization, possibly in an impact-melt event (Yamaguchi, 2001), and finally, a period of annealing. Subsequent to this, another heating event occurred, during which olivine grains formed, and this was followed by rapid cooling likely due to impact excavation (Sugiura and Yamaguchi, 2007). During the first thermal event, Ca-phosphates and ilmenite were mobilized from the original mesostasis into the plagioclase assemblages of the recrystallized rock. Similarly, some REE homogenization occurred within pyroxenes. This history of thermal metamorphism presumed for NWA 011 is almost identical to that proposed for the highly metamorphosed eucrites EET 90020 and Y-86763, and the ungrouped eucrite-like Ibitira, and it suggests that they were similar early crustal rocks on their respective parent bodies.
A consortium study was carried out by various researchers to measure the O-isotopes, elemental abundances, and CRE age of NWA 011. On an oxygen 3-isotope diagram, the plot is very distant from that of the eucrites, but rather is more similar to the CR chondrites (Promprated et al., 2003), and plots near the acapulcoite/lodranite group. However, elemental abundances in NWA 011 are likely the result of fractional crystallization rather than partial melting processes that occurred on the acapulcoite/lodranite body (Floss et al., 2005). Oxygen isotope mixing models suggest that a close compositional match can be obtained by blending components of Allende with either H or LL chondrite material (Boesenberg, 2003). This model also calls for the sequestration of a larger amount of metal and olivine to the core than is considered to have occurred on the eucrite parent body.
In its bulk composition, NWA 011 is significantly different from typical eucrites in the following ways: it has a higher P content, a higher siderophile element content, a higher mean Fe/Mn ratio in pyroxene (~65 for NWA 011 vs. ~2840 for typical eucrites), a lower Sc content, and a higher abundance of platinum group elements. In addition, it has an excess of 50Ti and 54Cr (Trinquier et al., 2007). All of these features suggest a more oxidized source than that for typical eucrites (Korotchantseva et al., 2003; Isa et al., 2008). Based on elemental abundance patterns, the PGE enrichment as well as the higher abundances of other siderophile elements has been conjectured to be the result of an impact mixing event on the NWA 011 parent body involving a group-IVB iron projectile (Yamaguchi et al., 2002). In addition, the trace element contents of NWA 011 are distinct from those of other eucritesit has a higher Sr content, a lower REE content in merrillite, and a smaller Eu anomaly in pyroxene and phosphate (Floss et al., 2004). The Th/U ratio of NWA 011 is significantly lower than that of other basaltic meteorites such as eucrites and angrites. In addition, a positive Ce anomaly in merrillite is consistent with formation in an oxidizing environment.
The MnCr isotope systematics were studied by Bogdanovski and Lugmair (2003, 2004), and they found significant differences (much lower abundances of each) between NWA 011 and the HED group, providing further persuasive evidence against a genetic relationship. Interestingly, their Cr data is similar to that of CI and CM carbonaceous chondrites, indicating a possible origin from a differentiated CI- or CM-like (or possibly CR-like) carbonaceous chondrite parent body. In addition to the Cr data, the enriched Fe content of NWA 011 also excludes the planet Mercury as the parental source. Moreover, by utilizing the MnCr data they were able to calculate the time at which differentiation occurred and the source reservoir of NWA 011 was formed, which occurred ~4.563 b.y. ago.
In an isotope systematics study, Sugiura and Yamaguchi (2007) reported the MnCr age for NWA 011 as 4,562.3 (±2.6) m.y., while the AlMg age was calculated to be 4,562.7 (±0.3) m.y. Other AlMg age results include 4563.3 [±0.5] m.y., determined by Schiller et al. (2010), and 4563.1 [±0.38] m.y., determined by Bouvier et al. (2011) for the paired meteorite NWA 2976. The Sugiura and Yamaguchi (2007) ages are concordant with each other, and likely represent the meteorite's crystallization age 35 m.y. after CAI formation; such an early crystallization would be consistent with an asteroidal origin rather than a planetary origin (Scott et al., 2009). In addition, these ages are nearly identical to the ages calculated for the oldest eucrites and the quenched angrites such as Sahara 99555. The SmNd age of NWA 011, calculated to be 4.46 (±0.04) b.y. (Nyquist et al., 2003), is younger than the calculated MnCr and AlMg ages, possibly reflecting late metamorphic resetting. A more precise crystallization age was calculated by Bouvier et al. (2011) utilizing the paired NWA 2976. Based on the internal PbPb isochron, the PbPb age was estimated to be 4,562.89 (±0.59) m.y. This age is concordant with the AlMg age anchored to the D'Orbigny angrite and a Type B CAI from the CV3 chondrite NWA 2364. This likely represents the crystallization age of the basaltic meteorite in the crust of this parent asteroid.
The noble gas data of NWA 011 reveal both a young isochron age of ~800 m.y., which may reflect Ar redistribution and adsorption in this meteorite due to terrestrial weathering effects, and a gas retention age of ~3.23.9 b.y., which is not resolvable from that of the HED meteorites (Bogard and Garrison, 2004; Korochantseva et al, 2005). However, some data is consistent with a gas retention isochron as young as ~3.13.2 b.y. old, which is earlier than the late heavy bombardment period reflected by most eucrites and the Moon.
An initial CRE age of 22.2 (±3.3) m.y. was calculated for NWA 011, an age which falls within one of the five common breakup events determined for the HED PB (Patzer et al., 2003). Subsequent studies by two other research groups calculated 21Ne-derived and 38Ar-derived CRE ages of ~2830 m.y. for NWA 011 (Yamaguchi et al., 2002 and Korochantseva et al, 2005), which also fall near an established HED age cluster.
From these and other comparisons, it may be assumed that NWA 011 is an ungrouped basaltic or possibly gabbroic achondrite that originated from a relatively large parent body other than 4 Vesta, which was located in a different region of the solar nebula, and which experienced a similar petrologic history and had a similar mineralogy. Its CI-like O-isotopic composition and similarity to some metamorphosed eucrites suggests an asteroidal origin in the outer asteroid belt. The possibility of an origin on the ~17 km diameter basaltic asteroid 1459 Magnya, at ~3.15 AU, was raised by Nyquist et al., 2003, but the SmNd data favor a larger parent body. Other differentiated and disrupted parent bodies have been identified in the central main belt (Bottke et al., 2006). These include the S-type, high-Ca pyroxene asteroids 17 Thetis, 847 Agnia, and 808 Merxia, as well as some possible exposed iron cores such as 16 Psyche and 216 Kleopatra. A very small amount of mantle material would be expected to survive the long journey from this distant region of the asteroid belt. Scott et al. (2009) surmise that the parent asteroids of NWA 011 and other ungrouped basaltic achondrites, along with most of the thousands of other Vesta-like bodies that probably occupied the early asteroid belt, were likely removed from the belt in early Solar System history through gravitational perturbations. Previous to the asteroid's removal, crustal portions may have been ejected to form small ~10 km diameter objects from which unbrecciated samples could subsequently be made available for capture by Earth.
A transmitted light view of a petrographic thin section of the paired stone NWA 2976 can be seen on J. Kashuba's page. The top photo shown above is an enlarged image of a 0.001 g (1 mg) portion of NWA 011 acquired from the first known stone. The other two photos show both the fusion-crusted side and an interior view of a 0.56 g fragment from one of the pairings of this meteorite. My thanks to meteorite procurer extraordinaire, Aziz Habibi, who kindly contributed this specimen to my collection.
Thanks to R. A. Langheinrich Meteorites for kindly pursuing the initial sample of this scientifically significant meteorite from S. Afanasiev, Vernadsky Institute of Geochemistry and Analytical Chemistry.