NORTHWEST AFRICA 2656


Acapulcoite
Acapulcoite–Lodranite Clan
standby for nwa 2656 photo

Purchased 2004
no coordinates recorded

A 386 g meteorite was found in the Sahara Desert in 2003. This was later determined to be part of an ~7.5 kg mass, which, along with other recovered fragments, compose a meteorite with a total weight of ~10 kg. The 386 g fragment was purchased by N. Oakes from a Moroccan dealer, and a sample was submitted for analysis and classification (T. Bunch and J. Wittke, NAU; A. Irving, UWS; D. Rumble III, CIW). Northwest Africa 2656 was determined to be an acapulcoite, exhibiting a recrystallized, polygonal, granular texture. Portions of this meteorite have been analyzed by NAU under several different names, including at least NWA 2699 (1,294 g), NWA 2714 (100 g), NWA 2866 (213 g), and NWA 2871 (3,467 g).

As a member of the lodranite/acapulcoite group, NWA 2656 has been distinguished from the members of the winonaite group, which exhibits similar recrystallized textures, through a plot of the Fa content of olivine vs. the Δ17O-isotopic value. From this diagram, it is apparent that these two groups plot in separate regions, and NWA 2656 is clearly resolved within the lodranite/acapulcoite group (D. Rumble, III et al., 2005). Northwest Africa 2656 consists of orthopyroxene, olivine, and plagioclase, along with minor FeNi-metal, troilite, schreibersite, Cr-diopside, and chromite, and exhibits a grain size of <1 mm. This meteorite has been shocked to stage S2 and has been weathered to grade W3.

A division of the acapulcoite–lodranite meteorite clan based on metamorphic stage was proposed by Floss (2000) and Patzer et al. (2003).

  1. primitive acapulcoites: near-chondritic (Se >12–13 ppm [degree of sulfide extraction])
  2. typical acapulcoites: Fe–Ni–FeS melting and some loss of sulfide (Se ~5–12 ppm)
  3. transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
  4. lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
  5. enriched acapulcoites (addition of feldspar-rich melt component)

Because both acapulcoites and lodranites are derived from the same parent body and have the same O-isotopic ratios, the plagioclase content in this meteorite is an important factor in making the distinction between acapulcoite and lodranite. Lodranites contain no plagioclase (or only trace amounts) since it was depleted from the restite during the partial melt phase. The two groups also have similar mineralogies, thermal histories, and cosmic ray exposure ages. Additionally, lodranites and acapulcoites have identical cosmogenic nuclide abundances and similar shielding conditions. Another factor which distinguishes acapulcoites from lodranites is their grain size. The grain size of NWA 2656 is more consistent with the finer-grained acapulcoites than with the coarser-grained lodranites—the division has been established by some at 500 µm, and the average grain size of NWA 2656 is 400 µm; however, further studies indicate an average grain size for portions of this meteorite of 0.6–0.7 mm, consistent with a lodranite classification. With many more samples to study, it is now evident that a continuum exists for the grainsizes of these two groups, and it has been proposed by Bunch et al. (2011) that an arbitrary group division is no longer justified; the term ‘acapulcoite–lodranite clan’ should therefore be applied to all members of the combined group.

For more complete amd current formation scenarios of the acapulcoite–lodranite parent body, visit the Monument Draw and Lodran pages. The specimen shown above is a partial slice of NWA 2656 weighing 1.21 g.