NORTHWEST AFRICA 1463


Winonaite (primitive)*
('W Chondrite')
standby for northwest africa 1463 photo
Purchased November 27, 2000
no coordinates recorded

Three fragments weighing together 1,001 g were found and subsequently sold in Erfoud, Morocco to Canadian collector D. Gregory. This meteorite was analyzed and classified through a collaboration with UCLA and Washington University in St. Louis, and determined to be a primitive winonaite. A 23 g specimen of NWA 1463 is curated by UCLA, while the 975 g type specimen is on deposit with the Royal Ontario Museum.

*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).

Although the mineral composition of NWA 1463 is typical for the primitive achondrite group designated winonaites, it has a more primitive, more chondritic texture than most other members of the group, equilavent to a petrologic type 5 chondrite (Benedix et al., 2003). In contrast to most other winonaites, NWA 1463 does not exhibit features related to igneous fractionation processes, features which supported the original designation of winonaites as primitive achondrites. NWA 1463 contains abundant relict chondrules, found infrequently in only a few anomalous winonaite members (e.g., Pontlyfni, Mount Morris), embedded within a recrystallized ground mass. By contrast, most other winonaites have features consistent with extensive thermal metamorphism, such as uniformly sized grains that form triple junctions. The O-isotope composition of NWA 1463 plots on a line that extends the winonaite trend, while the high abundance of FeNi-metal and troilite as well as the absence of metallic and sulfide veining attests to a lower equilibration temperature than that of other winonaites.

standby for winonaite comparison photo
Textural comparison of four winonaites, L to R: NWA 1463 (with relict chondrule), Winona, Tierra Blanca, HaH 193
Image credit: Floss et al., MAPS, vol. 43, #4, p. 660 (2008)
'Evolution of the winonaite parent body: Clues from silicate mineral trace element distributions'
(http://dx.doi.org/10.1111/j.1945-5100.2008.tb00676.x)

Based on similar silicate textures, reduced mineral chemistry, and O-isotopes, it is presumed that the winonaites and the IAB complex irons originated on a common parent body. Because of its highly primitive nature, NWA 1463 might closely resemble the chondritic precursor material of the typical winonaites and silicate inclusions in IAB complex irons. In a similar way, it was determined by Hidaka et al. (2015) that the FeNi-metal in the winonaite Y-8005 retains a near chondritic composition likely representative of the precursor material of the parent body. Utilizing a Ge/Ni vs. Au/Ni coupled diagram, they found that metal in this winonaite plots in the field of the sLL subgroup of the IAB complex irons. In view of these findings, they suggest that the sLL subgroup rather than the main group of the IAB complex represents the primitive metal of the IAB–winonaite parent body, with the main group possibly representing a partial melt of the sLL subgroup.

This winonaite does not fit into the scheme that has commonly been used to define the primitive achondrite group, and it could be instrumental in the future in redefining the metamorphic progression of chondrites to primitive achondrites. To that end, Irving et al. (2005) have described this winonaite as possibly representing the regolith of the winonaite parent body. Furthermore, the occurrence of distinct chondrules precludes the use of the term achondrite to describe this meteorite, and they suggest that the term metachondrite or 'W chondrite' would be more appropriate to describe this texturally evolved group of meteorites (Irving et al., 2005; Irving and Rumble III, 2006 MetSoc #5288).

Northwest Africa 1463 contains certain features that are unique compared to most other winonaites, and while it is plausible that this meteorite may represent the winonaite precursor material, it has been conjectured that it may instead have originated on a separate parent body (Floss et al., 2008). These unusual features include the lowest degree of metamorphism of all winonaites, an anomalous chromite composition, an anomalous O-isotopic composition, a lack of graphite, the presence of merrillite rather than apatite, and an abundance of incompatible trace elements intermediate to other winonaites. The variability in Ar–Ar ages obtained for some winonaites indicates they may have been excavated from different depths over an extended period of time (Scott et al., 2014). Furthermore, cooling rates standardized at ~500°C were determined for a number of winonaites and IAB irons, and the results demonstrate that a wide continuum exists:

Based on the oxygen isotope data obtained by Hunt et al. (2012) for silicate inclusions in IAB irons, along with the observed volatile element depletions, it can be inferred that the winonaite precursor likely had a volatile-depleted carbonaceous chondrite-like composition. From results of their trace element analyses of a broad sampling of winonaites, Hunt et al. (2017) recognized that CM chondrites represent the closest match; however, the important differences that exist indicate that the precursor to winonaites is unlike any meteorite class currently known. There is convincing evidence that NWA 1463 is paired with NWA 725, NWA 1052, NWA 1054, and NWA 1058 (Irving and Rumble III, 2006); a further pairing was found in 2007 and designated NWA 4835 (T. Bunch, NAU).

Further details about the petrogenetic history of the winonaites can be found on the Tierra Blanca page. Shown above are two views of a 0.12 g cut fragment of NWA 1463 that show its chondritic texture and a relatively fresh fusion crust. This specimen was kindly provided by Dr. David Gregory.