NORTHWEST AFRICA 2993


Winonaite (evolved)*
(ungrouped achondrite in MetBull 91)
standby for nwa 2993 photo
Found June 2006
no coordinates recorded

A fresh, fusion-crusted, 625 g stone was found in the Algerian desert in June of 2006; shortly thereafter it was purchased by a collector (A. Aaronson) in Erfoud, Morocco, and a portion of the meteorite was submitted for analysis and classification to Northern Arizona University (T. Bunch and J. Wittke) and the University of Washington in Seattle (A. Irving). The meteorite was subsequently sold to another collector (S. Turecki). The section of NWA 2993 under analysis was found to consist of 37 vol% orthopyroxene, 32 vol% olivine, and 30 vol% FeNi-metal (kamacite-rimmed taenite with occasional plessite cores), along with minor sulfide.

While a preliminary petrographic examination of a cut face revealed the texture and composition of a coarse-grained (0.3–3.4 mm), plagioclase-depleted, metal-rich lodranite, subsequent O-isotopic analyses conducted at Carnegie Institute (D. Rumble, III) showed that it actually plots on an extension of the trend line for the winonaite/IAB-iron complex meteorites. It is also notable that NWA 2993 plots close to the brachinite group and also very close to the pyroxene pallasite Vermillion (see Figure 3 in the 38th LPSC abstract #2211 [2007]). To help discriminate between the winonaite, brachinite, and acapulcoite–lodranite groups, a plot of Δ17O (‰) with olivine Fa content (mol%) was found to be a useful technique (Rumble III et al., 2005). Northwest Africa 2993 plots nearest to the winonaite field, but because it has the largest values for both parameters, it plots in a unique location along an extension of the winonaite trend line slightly distant from the main winonaite concentration (see diagrams below).

standby for o vs. fa diagram
Diagram credit: Bunch et al., 38th LPSC, #2211 (2007)

standby for o-isotopic diagram
Diagram credit: Bunch et al., 41st LPSC, #1281 (2010)

standby for o-isotopic diagram
click on image for a magnified view

Diagram credit: Greenwood et al., Chemie der Erde—Geochemistry, vol. 77, p. 23 (2017)
'Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies'
(open access: http://dx.doi.org/10.1016/j.chemer.2016.09.005)

*Previously, Floss (2000) and Patzer et al. (2003 #1352, 2004) 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).

Layered Structure of the Winonaite Parent Asteroid
standby for winonaite parent body diagram
click on diagram for a magnified view

Diagram credit: Zeng et al., Earth, Planets and Space, vol. 71, #38, p. 12 (2019 open access link)
'The layered structure model for winonaite parent asteroid implicated by textural and mineralogical diversity'
(https://doi.org/10.1186/s40623-019-1015-9)

Dey et al. (2019) made use of 17O and ε54Cr values for several irons and their associated silicates/oxides to investigate i) if each iron and its associated phases originated on a common parent body (i.e., an endogenous mixture of core and mantle vs. an exogenous mixture through impact), and ii) if any genetic connection exists between the irons and other meteorite groups (e.g., IAB with winonaites, IIE with H chondrites, and Eagle Station pallasites with CK chondrites). Caddo County is one of three IAB irons employed in the study, and it was demonstrated on an O–Cr coupled diagram that although the ε54Cr values for the iron component plot in the winonaite field, the silicate component plots in a distinct region at higher values (see diagram below). From these results they ascertained that the IAB silicated irons formed through an impact-generated mixture comprising iron from a winonaite-like parent body and silicate from an unrelated and otherwise unsampled parent body. It may also be reasonably inferred that winonaites derive from a separate parent body (Goldstein et al., 2021). Incorporation of the silicates into the FeNi-metal host took place at a depth greater than 2 km, allowing time for a Thomson (Widmanstätten) structure to develop during a long duration cooling phase. Fractional crystallization occurred in some large molten metal pools, followed by very slow cooling, which produced the broad range of features found in certain IAB meteorites (e.g., silicate-poor, graphite–troilite-rich inclusions and extremely high Ni contents). Other results from their study can be found on the Miles and Eagle Station pages.

ε54Cr vs. Δ17O for Irons and Pallasites
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click on diagram for a magnified view

Diagram credit: Dey et al., 50th LPSC, #2977 (2019)

Northwest Africa 2993 represents the first known coarse-grained, plutonic sampling from the winonaite/IAB-iron complex parent body, and is chemically and texturally analogous to the lodranites of the ACA–LOD parent body. Determination of the exact petrogenetic history which led to the formation of this unique meteorite is ongoing, and further details about the winonaite group can be found on the Tierra Blanca page. The photo of NWA 2993 shown above is a 8.93 g slice, while that below is a view of the main mass in the possession of the owner.

standby for nwa 2993 photo
Photo courtesy of S. Turecki