NWA 516

Winonaite (primitive)*
(‘W Chondrite’)
standby for northwest africa 516 photo
Found November 2000
no coordinates recorded A small meteorite weighing only 68 g was purchased in the Moroccan market by a group of American collectors. The meteorite was classified at UCLA (A. Rubin) as a winonaite similar to the anomalous winonaite, Pontlyfni, the only winonaite witnessed fall. Pontlyfni is a fine-grained, reduced, brecciated rock containing silicate clasts (48.3 vol%) in a heterogeneous FeNi-metal- and troilite-rich (51.7 vol%) matrix in a similar manner to some IAB complex irons (Hunt et al., 2011). A third winonaite from Antarctica, Y-74025, has also been shown to be very similar to Pontlyfni (Kallemeyn, 1997). Northwest Africa 516 has a shock stage of S2 and a weathering grade of W3.

*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). Northwest Africa 516, Pontlyfni, and Y-74025 differ from most other winonaites in having a finer-grained, less equilibrated texture. Notably, not only has Pontlyfni been found to contain relict porphyritic and radiating-pyroxene chondrules, but it has also been interpreted as a petrologic type 6 chondrite by several investigators. By contrast, most other winonaites have coarse-grained, highly recrystallized achondritic textures. The Antarctic winonaite QUE 94535 is transitional in texture to Pontlyfni and the recrystallized Winona.

Other significant differences exist between the anomalous members and the majority of the winonaite group. The anomalous members have values for fayalite content in olivine that are lower than those of other group members, with NWA 516 having a value of Fa1.1 (±0.1). Moreover, they have a ferrosilite content in low-Ca pyroxene lower than that of other winonaites. Only some chondritic silicate inclusions identified in a few IAB complex iron meteorites, such as Pine River and Kendall County, have comparable FeO-poor compositions (Benedix et al., 2000). In addition, the anomalous members have a different HREE/LREE pattern with a smaller positive Eu anomaly, and contain esentially unfractionated refractory lithophiles compared to the depleted refractories of other winonaites. While the low Ca/Al ratios found in most winonaites are consistent with fractionation during igneous differentiation processes, the near-chondritic Ca/Al ratio present in the anomalous winonaites is indicative of a much lower degree of thermal processing.

Yet, on an oxygen 3-isotope diagram all winonaites plot in a similar region with silicates from IAB complex irons, suggesting that their formation at least occurred within a common O-isotope reservoir. The IAB-silicates also show close similarities to all winonaites in chemistry and mineralogy and in REE fractionation patterns. The abundant graphite found in Pontlyfni has similar C-isotopic compositions to that of IAB silicates. Furthermore, a comparison of the trace element abundances in the metal fractions of the IAB silicates and the anomalous winonaites supports the conclusion that a close relationship exists between them. Evidence of a magmatic event is shown to have occurred during a similar timeframe for both winonaites and Caddo County, while a major metamorphic event has been shown to have occurred in both Winona and Campo del Cielo at about the same time (Schulz et al., 2010). The wide range of Ar–Ar ages determined for IAB silicates spans the range of ages determined for the winonaites—from the old age of Pontlyfni to the young age of Winona and Mount Morris—delineating the range of burial depths and cooling rates for these meteorites. All of these findings attest to a common IAB-winonaite parent body which experienced incomplete differentiation followed by catastrophic impact disruption and reassembly, consistent with late-stage thermal events ensuing ~10–14 m.y. after CAI formation (Hunt et al., 2017).

However, in some studies data are shown to be inconsistent with a common origin for winonaites and IAB irons. For example, Pontlyfni crystallized earlier (~4.538 b.y. ago) than IAB silicates (~4.49 b.y. ago). This could be explained by a more rapid cooling (~35K/m.y. in the temperature range 1150–550K) and an earlier isotopic closure for the winonaites in a location nearer to the surface of the parent body than the IAB silicates. In addition to these crystallization ages, Schulz et al. (2007, 2010) determined a Hf–W isochron for selected winonaites, reflecting the end of Hf–W redistribution between metal and silicate during progressive cooling. They revealed an age of <4.45 b.y. for Winona, which is somewhat younger than that of Pontlyfni. This suggests either that some winonaites cooled very slowly (~4K/m.y. in the temperature range 1150–550K) while at a significant depth, or that the winonaite Hf–W age reflects a late impact-related re-equilibration event on the parent body. The presence of relict chondrules in Pontlyfni but not in Winona is consistent with the former scenario. One other inconsistency for a common origin is that winonaites and IAB irons have very different CRE ages of 20–80 m.y. and 400–1,000 m.y., respectively (Benedix et al., 2000); however, this could be explained by the much longer space longevity of iron meteoroids than for stone meteoroids.

There are significant textural, mineralogical, and chemical differences that exist among Pontlyfni, NWA 516, and Y-74025 compared to other winonaites, with the possible exception of NWA 725 (and pairing group). Although the mineral composition of NWA 725 is typical of the winonaite group, it has a more primitive, chondritic texture than other members of the group, equivalent to a petrologic type 5 chondrite (Benedix et al., 2003). In contrast to most other winonaites, NWA 725 does not exhibit features related to igneous fractionation processes, those features which initially led to the designation of winonaites as primitive achondrites. Northwest Africa 725 contains abundant relict chondrules, which are also found in lower abundances in Pontlyfni and Mt. Morris. Its O-isotopic composition plots on a line that extends the winonaite trend, while the absence of metallic veining attests to a lower equilibration temperature than that of most winonaites. Because of its highly primitive nature, NWA 725 might closely resemble the chondritic precursor material of the winonaites and silicate inclusions of IAB complex irons.

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.

Approximately two dozen winonaites have been identified thus far, including Winona, Pontlyfni, Mt. Morris, Tierra Blanca, HaH 193, NWA 516, NWA 725 (and pairing group, likely including NWA 1463, 1058, 1054, and 1052), NWA 1457, and others from the deserts of the Sahara and Antarctica. Further details about the petrogenetic history of the winonaites can be found on the Tierra Blanca page. The specimen of NWA 516 shown above is a 0.5 g partial slice, while the photo below is the reverse side exhibiting abundant FeNi-metal grains. standby for northwest africa 516 photo

Leave a Reply