NORTHWEST AFRICA 725


Winonaite (primitive)*
('W Chondrite')**
standby for northwest africa 725 photo
Found July 4, 2000
30° 36' N., 5° 3' E.

Eleven pieces of this Moroccan primitive winonaite, weighing a total of 3,824 g (total weight of the entire pairing group is ~5.1 kg), were recovered by a French team under the organization of Bruno Fectay and Carine Bidaut. Although the coordinates of the recovery location in Tissemoumine, Morocco, were recorded by GPS equipment, a NWA-series designation was accepted as the name for this meteorite by the Meteoritical Society NomCom. This primitive achondrite is of great interest due to its abundance of relict chondrules.

*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 NWA 725 was initially classified as an acapulcoite, a recent O-isotopic analysis of a portion of the main mass (from the collection of S. Turecki) by the Open University, UK, resolves the material clearly within the winonaite field: "In particular, the Δ17O value of -0.431 is in reasonable agreement with the mean value of -0.48 for the winonaite–IAB complex group determined by Clayton and Mayeda (1996)." In addition, when plotted on a diagram comparing Δ17O vs. Fa mol% in olivine (Rumble, III et al, 2005), NWA 725 (Fa6.1) and the winonaites NWA 1463 (Fa7.4; Δ17O = -0.45‰), NWA 1457 (Fa5; Δ17O = -0.40 [±0.03]‰), and NWA 1058 (Fa6.5; Δ17O = -0.53‰) all plot very close together within the winonaite field. The inference can thus be made that NWA 725 is actually a winonaite, likely paired with the primitive winonaites NWA 1463 and 1058 (NWA 1052 and NWA 1054 are also possible members of this pairing group; see Irving and Rumble III, 2006 MetSoc #5288).

Northwest Africa 725 (and pairings) contains relict intact chondrules, and accordingly it has been grouped among the most primitive winonaites with near chondritic composition. However, it has still experienced extensive heating and possibly a low degree of partial melting resulting in a depletion of certain trace elements. Features suggest that the winonaites are actually metamorphosed chondrites, or metachondrites (or 'W-chondrites' for those containing relict chondrules) based on terminology associated with several newly recognized groups of chondrule-free, texturally evolved chondrites with elemental ratios and O-isotopic compositions which show affinities to existing chondrite groups (Irving et al., 2005).

Oxygen isotope data for IAB silicate inclusions along with observed volatile element depletions infer that the winonaite precursor most likely had a volatile-depleted carbonaceous chondrite-like composition that is not sampled in current meteorite collections (Hunt et al., 2012). Preliminary data based on Al–Mg chronometry shows that NWA 725 was formed ~1.4 m.y. after CAIs. This age is consistent with Hf–W ages in the range of 1.5–5 m.y. which were calculated for more highly metamorphosed winonaites (Hidaka et al., 2014).

Further information about the classification of this meteorite and its probable pairings can be found on the NWA 1058 page. The NWA 725 specimen pictured above is a 1.3 g partial slice showing many distinct, intact chondrules. The photo below is an excellent petrographic thin section micrograph of NWA 725, shown courtesy of Peter Marmet.

standby for lodran photo
click on photo for a magnified view
Photo courtesy of Peter Marmet