Winonaite (primitive)*
(W Chondrite)
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). Pontlyfni Northwest Africa 516 has a shock stage of S2 and a weathering grade of W3.
*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.
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 at least 10 m.y. after CAI formation.
In some studies however, 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). However, this could be explained by a more rapid cooling (~35°K/m.y. in the temperature range 1150–550°K) 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 (~4°K/m.y. in the temperature range 1150–550°K) 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—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 (and pairing group) 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 (and pairing group) 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 (and pairing group) 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 (and pairing group) may closely resemble the chondritic precursor material of the winonaites and silicate inclusions of IAB complex irons.
Over a dozen winonaites have been identified to date, among them being Winona, Tierra Blanca, Mt. Morris, Hammadah al Hamra 193, NWA 516, NWA 725 (and pairing group, likely including NWA 1463, 1058, 1054, and 1052), Pontlyfni, NWA 1457, and others from the Sahara and Antarctica. The specimen of NWA 516 shown above is a 0.5 g partial slice. Shown below is the reverse side exhibiting abundant FeNi-metal grains.
