NORTHWEST AFRICA 1500


Achondrite, ungrouped
(brachinite-like)
(ureilite [olivine-augite type] in MetBull 87)

standby for nwa 1500 photo
Purchased 2000
no coordinates recorded

A single, partially fusion-crusted stone weighing 3.3 kg was purchased in Zagora, Morocco. The meteorite was later sold to a dealer in Tucson, and then finally traded to R. Bartoschewitz in April 2002. This meteorite, which was designated NWA 1500, was initially analyzed and classified at the Max Planck Institut für Chemie in Germany as a plagioclase-bearing basaltic ureilite.

The presence of reverse-zoned reduction rims and the resulting grain boundary darkening in NWA 1500 olivines and their absence in other brachinites compelled some investigators to make an initial classification of NWA 1500 as a monomict ureilite. It also exhibited an equigranular texture and abundant triple junctions, and was composed mainly of coarse-grained olivine grains (~95 vol%) along with melt pockets and secondary veins containing a heterogeneous distibution of Ca-rich augite (2–3 vol%), chromite (0.6–1.6 vol%), plagioclase (0.7–1.8 vol%), andesine, and diopside. Other constituents include FeNi-metal, graphite (not found subsequently), lonsdaleite, and diamond. The high olivine fayalite content of Fa28 (as high as Fa35, Goodrich et al., 2006) initially distinguished this meteorite as potentially the most ferroan ureilite known (Bartoschewitz et al., 2003).

One of the most interesting components of this meteorite is the plagioclase grains. They occur as poikilitic grains measuring 0.5–3.0 mm across which enclose olivine and augite. This is consistent with an igneous association with olivine rather than an association through an impact-melting event. This plagioclase potentially represents the basaltic component of the UPB, which has heretofore only been studied in very small clasts from polymict ureilites, it could provide important information relating to the evolution of the UPB (Cohen and Goodrich, 2003).

Continued studies have shown that NWA 1500 is more similar to brachinites than any other group with respect to most of its petrographic, geochemical, and mineralogical features (Kita et al., 2009; Goodrich et al., 2011). Moreover, in contrast to ureilites, which show high equilibrium temperatures in the range of 1200–1300°C, the equilibrium temperature for NWA 1500 calculated by multiple methods is only 880°C (±70°C), inconsistent with having an origin on the ureilite parent body. The measured HSE abundances are consistent with a partially melted parent body in which heating from short-lived radionuclides came to a halt before a core was fully formed.

Initial oxygen-isotopic studies conducted by R. Clayton and T. Mayeda at the University of Chicago in 2003 demonstrated that NWA 1500 had a unique O-isotopic plot among the ureilites, forming an extention of the ureilite trend line. This initial O-isotope plot also fell on the border of the winonaite/IAB field near the lodranite/acapulcoite field, and was very close to the brachinite-like, primitive achondrite Divnoe. It did not fit the results expected for the isotopic fractionation of a basaltic partial melt, which called into question the exact nature of this meteorite. Subsequent highly precise O-isotopic analyses were conducted on NWA 1500 (Kita et al., 2009; Spicuzza et al., 2007; Greenwood et al., 2007). When plotted on an oxygen three-isotope diagram (courtesy of Achim Raphael), the O-isotope values (Δ17O = –0.811‰ [Miller, 2002]) are well removed from the ureilite trend line and plot within the brachinite field (along with Divnoe, NWA 595, and GRA 06128/9). This supports a genetic relationship (i.e., same parent body) with the brachinites (see a linearized plot).

A separate sample of NWA 1500 was studied by Mittlefehldt and Hudon (2004). They found a composition consisting of ~90 vol% coarse-grained olivine exhibiting a weak preferred alignment, with narrow reduction rims of magnesian pyroxene containing abundant Ni-free metal grains, causing silicate darkening. The remaining component, comprising coarse-grained augite and chromite with minor FeNi-metal, is almost totally lacking in plagioclase. They also found different Fe–Mn–Mg compositions than those determined in the initial study, values that plot well outside of ureilite ranges. Furthermore, the CaO and CrO contents in NWA 1500 olivine cores are significantly lower than, and the Wo content significantly higher than, normal ureilite trends. Moreover, in contrast to typical C contents in ureilites (7–66 mg/g), the C content in NWA 1500 was too low to measure, and no CO2 was released upon heating (Murty et al., 2007). Finally, as determined previously and since refined, the O-isotopic composition of NWA 1500 falls outside of the ureilite range and clearly within the brachinite field. For these reasons, they have suggested that NWA 1500 might be a unique, olivine-dominated, ultramafic rock containing trace plagioclase, and not a member of the ureilite group. They suggested that other primitive achondrites with similar textures (but with less ferroan compositions) such as brachinites, winonaites, and acapulcoites should be compared.

Goodrich et al. (2006, 2010) observed fine-grained assemblages of orthopyroxene and opaques lining various olivine grain boundaries nearly identical to those found in other brachinites. These assemblages exhibit several features that indicate the occurrence of a late reduction process, and the Fe–Mn–Mg relations are also consistent with reduction processes on the brachinite parent body. Several methods for the reduction of primary olivine were reviewed by Goodrich et al., 2017), including its reaction with methane to form orthopyroxene + metal (Irving et al., 2013) and through its sulfurization by a S-rich fluid or gas to form orthopyroxene + sulfide (e.g., Singerling et al., 2013).

A noble gas study was conducted by Murty et al. (2007), and it was found that NWA 1500 retains very low abundances of trapped noble gases compared to ureilites. Moreover, the Ar and Xe isotopic ratios were found to differ significantly from ureilites, the cause of which cannot be attributed to terrestrial weathering. A noble gas isotopic plot gives values for NWA 1500 that fall within the brachinite field. In addition, they concluded that the N systematics were unlike that of ureilites. Utilizing the plagioclase grains to determine the 26Al closure age of NWA 1500, it was found to have formed at least 7 m.y. after CAIs. This establishes a younger age for NWA 1500 than for Brachina, the latter meteorite being distinguished in significant ways from the other brachinites. A CRE age of 9.4 m.y. was also determined.

Still, results of an in-depth study of this anomalous meteorite was presented by C. Goodrich et al. (2005, 2006). They found that many of the petrologic features of NWA 1500 were in fact consistent with accumulation on the ureilite parent body from a high degree fractional melt, which occurred at a greater depth than that at which the most ferroan olivine–pigeonite ureilites formed. The Fe–Mn–Mg composition of olivine in NWA 1500 is consistent with the ratios measured for other augite-bearing ureilites, but it would be a ureilite that contains a larger melt component and has undergone a higher degree of smelting/reduction.

Subsequently, Goodrich et al. (2006) provided many examples of textural, chemical, and mineralogical characteristics of NWA 1500 which are consistent with the augite-bearing ureilite group, and they proposed that this is a member of the small group of augite-bearing (lacking pigeonite), monomict ureilites. To account for the many anomalous characteristics shown by NWA 1500, they argued that it experienced higher oxidation given its greater depth of formation, and that it subsequently experienced only slight reduction during ascent as shown by the reverse zoning of olivine; examination of other brachinites for reverse zoning in olivine grains will be beneficial. Low Cr and P in olivine and metal, and the presence of chromite and apatite phases, are also indicative of higher oxidation conditions for this ureilite during formation at greater depth. The presence of plagioclase as poikilitic and intergranular grains is indicative of crystallization from a melt at great depth, and this feature further distinguishes this possible ureilite from all others. Northwest Africa 1500 was equilibrated at lower temperatures than any other ureilite measured, which is consistent with the generally accepted ureilite model involving a breakup with subsequent rapid cooling of its parent body during its magmatic stage.

Further studies of NWA 1500 by Goodrich et al. (2011) determined that texture, modal abundances, mineral compositions, REE abundances, O-isotopic compositions, and siderophile element abundances all follow brachinite trends, and are distinguishable from other olivine-rich, primitive achondrite groups. However, through studies of highly siderophile element (HSE) abundances, and upon examining the metal-sulfide segregation processes, it was determined by Day et al. (2012) that NWA 1500 and similar brachinite-like primitive achondrites were not likely genetically related (i.e. from the same parent body) to brachinites, but rather, originated on similar volatile-rich, oxidized, chondritic precursor asteroids while experiencing similar petrologic processes during their history. Goodrich et al. (2017) determined that brachinites and brachinite-like achondrites have a distinct redox trend and a higher Fe/Mg ratio compared to all other primitive achondrites, consistent with formation in a similar nebula reservoir; therefore, they suggest that brachinites and brachinite-like achondrites be called the brachinite clan.

Notably, an achondrite clast from the Kaidun meteorite has been favorably compared to a brachinite (Higashi et al., 2017, #1874). Verification of this discovery would infer a very old formation age for the Kaidun parent body, since the age of Brachina is 4.564.8 (±0.0005) b.y. The specimen of NWA 1500 shown above is a 3.97 g partial slice.