NORTHWEST AFRICA 1058


Winonaite (primitive)*
('W Chondrite')
standby for northwest africa 1058 photo
Purchased 2001
no coordinates recorded

A 180 g stone was purchased in Erfoud, Morocco and subsequently sold to a meteorite dealer. The meteorite was submitted to the Institut für Planetologie in Münster and given the designation NWA 1058. Although initially considered to be an acapulcoite, NWA 1058 was classified in a broad terminology as a primitive achondrite by Russell et al. (2003) due to its anomalous O-isotopic composition. On an O-isotope plot it falls on the terrestrial fractionation line similar to the winonaites, but with O-isotopes that are more 16O-rich than most winonaites.

*Previously, Floss (2000) and Patzer et al. (2003) proposed that the acapulcoite/lodranite meteorites should be divided based on metamorphic stage:
  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)
A similar distinction could be made among the winonaites in our collections, although there is not yet an analog of the IAB complex irons for the acapulcoite/lodranite PB. Northwest Africa 1463 (and pairing group) ranks as the most primitive member of the winonaites, containing intact chondrules comparable to a petrologic type 5 chondrite (Benedix et al., 2003). However, most winonaites experienced extensive thermal metamorphism involving incipient sulfide melting and exhibit highly recrystallized textures, characteristics analogous to the "typical" acapulcoites. Metamorphic progression in other winonaites led to partial loss of the low-melting phases FeS and plagioclase, and these are designated as a "transitional" stage in the acapulcoite/lodranite metamorphic continuum. Those winonaites which experienced the highest temperatures ultimately crystallized from residual melt material, and they exhibit significant depletions in FeS, FeNi-metal, and plagioclase (including plagiophile trace elements). Samples representing this advanced metamorphic stage are known as lodranites in the acapulcoite/lodranite metamorphic sequence, while the term "evolved" could be used to represent a similar metamorphic stage in the winonaite group (e.g., Tierra Blanca; Hunt et al., 2017).

Further petrographic and compositional analyses have determined that NWA 1058 is very similar to the primitive acapulcoite GRA 98028 (Patzer et al., 2004), and that the anomalous O-isotopic composition might actually be due to terrestrial weathering effects (W2–3). A high abundance of relict chondrules are observed in NWA 1058, which is also a feature of some primitive acapulcoites. However, while most acapulcoites have similar CRE ages of ~6 m.y. (16.8 m.y. for TIL 99002), that calculated for NWA 1058 is significantly higher at 38.2 m.y. (Patzer et al., 2003). Nevertheless, the noble gas abundances of NWA 1058 remain high with 36Ar/132Xe ratios that are similar to the "Q" component, characteristics which are more consistent with acapulcoites.

A study was undertaken by Eugster and Lorenzetti (2005) in which they determined a possible structure for the acapulcoite parent body. They specifically analyzed a number of new acapulcoites, as well as NWA 1058. They found that the data on the whole was more consistent with NWA 1058 being a winonaite rather than an acapulcoite: 1) NWA 1058 is more Mg-poor and Fe-rich than any acapulcoite; 2) its O-isotope composition plots outside the acapulcoite field; 3) its CRE age is much higher than any other acapulcoite (38.9 ±4.0 m.y.), with all except one acapulcoite having almost identical CRE ages of ~6 m.y.

Therefore, this meteorite appears to be neither a typical winonaite nor a typical acapulcoite. Utilizing a diagram which compares the Δ17O to the Fa content of olivine, Rumble, III et al (2005) found that the winonaite and acapulcoite/lodranite groups were readily resolved, and NWA 1058 plots clearly within the winonaite field. In addition, Moggi-Cecchi et al (2011) published a diagram (42nd LPSC, #1398 [2011]) which illustrates the Cr content of diopside vs. Mg# of olivine, and the acapulcoite/lodranite group can be clearly distinguished from the winonaite group.

standby for aca/lod/win diagram photo

These same investigators also published a diagram showing the reduction state of the samples (Fe# in orthopyroxene vs. Mg# of olivine), which distinguishes three separate clusters: the acapulcoites, lodranites, and winonaites. To that end, Irving et al. (2005) have described this meteorite as a metamorphosed chondrite probably representing the regolith of the winonaite parent body. Furthermore, they argued that the occurrence of distinct chondrules precludes the use of the term achondrite to describe this meteorite group, and they suggest that the term metachondrite or 'W chondrite' would be a more appropriate term to describe this texturally evolved group of meteorites (Irving et al., 2005; Irving and Rumble III, 2006 MetSoc #5288).

There is convincing evidence that this meteorite is paired with NWA 725, NWA 1052, NWA 1054, and NWA 1463 (Irving and Rumble III, 2006); a further pairing was found in 2007 and designated NWA 4835 (T. Bunch, NAU). The partial slice of NWA 1058 shown above weighs 0.9 g.