Winonaite (primitive)*
('W Chondrite')
Purchased 2001
no coordinates recorded
A 180 g stone was purchased in Erfoud, Morocco and was 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, a division of the acapulcoite/lodranite meteorites based on metamorphic stage was proposed by Floss (2000) and Patzer et al. (2003). 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. Some winonaites such as NWA 1463 and its likely pairing group contain intact chondrules and are among the most primitive of the winonaites. However, most members have experienced extensive heat metamorphism, and some possibly sustained a low degree of silicate partial melting resulting in a depletion of certain trace elements. Progressive degrees of thermal metamorphism produced samples exhibiting the earliest stages of melting and loss of a low-melting phases, which exhibit highly recrystallized textures analogous to characteristics of the "typical" acapulcoites. Progressing along the metamorphic contiuum led to a loss of some plagioclase and sulfide phases, called the "transitional" stage in the acapulcoite/lodranite metamorphic sequence. Finally, at the highest temperatures, crystallization from residual melt material resulted in a depletion of the low-melting point components including plagioclase (and plagiophile trace elements), FeNi-metal, and FeS. 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.
Further petrographic and compositional analyses determined that NWA 1058 is very similar to the primitive acapulcoite GRA 98028 (Patzer et al., 2004), and that the anomalous O-isotopic composition may 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, more consistent with acapulcoites, the noble gas abundances of NWA 1058 remain high with 36Ar/132Xe ratios that are similar to the "Q" component.
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; NWA 1058 plots clearly within the winonaite field. In addition, Moggi-Cecchi et al (2011) published a diagram (42nd LPSC 2011, #1398) which illustrates the Cr content of diopside vs. Mg# of olivine. This diagram clearly distinguishes the acapulcoite/lodranite group from the winonaite group.
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). The partial slice of NWA 1058 shown above weighs 0.9 g.
