NWA 516

Winonaitea partially differentiated asteroid that was disrupted just as it began to form an Fe core and a silicate-rich crust. This disrupting impact mixed silicates into molten Ni-Fe metal forming the silicated IAB irons, and mixed olivine-rich residues of partial melts into unmelted silicates, forming the winonaites. A few winonaites Click on Term to Read More (primitive)*
(‘W Chondrite’)

Found November 2000
no coordinates recorded A small meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More 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 fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More. Pontlyfni is a fine-grained, reducedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More, brecciated rock containing silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the clasts (48.3 vol%) in a heterogeneous FeNi-metal- and troilite-rich (51.7 vol%) matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More 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 stageA petrographic assessment, using features observed in minerals grains, of the degree to which a meteorite has undergone shock metamorphism. The highest stage observed in 25% of the indicator grains is used to determine the stage. Also called "shock level".               Click on Term to Read More of S2 and a weathering grade of W3.

*Previously, Floss (2000) and Patzer et al. (2003) proposed that the acapulcoitePrimitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated Click on Term to Read More/lodranite meteorites should be divided based on metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More stage:

1. primitive acapulcoitesPrimitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated Click on Term to Read More: near-chondritic (Se >12–13 ppmParts per million (10⁶). Click on Term to Read More [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 plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More (Se <5 ppm)
4. lodranites: sulfide, metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More, 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 chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine Click on Term to Read More comparable to a petrologic typeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes. 5 chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More (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 fayalitePure* iron end-member (Fe2SiO4) of the olivine solid solution series and an important mineral in meteorites. When iron (Fe) is completely substituted by magnesium, it yields the the pure Mg-olivine end-member, forsterite (Mg2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) Click on Term to Read More content in olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More that are lower than those of other group members, with NWA 516 having a value of Fa1.1 (±0.1). Moreover, they have a ferrosiliteA mineral that is composed of Fe-rich pyroxene, FeSiO3. It is the iron endmember of the pyroxene silicate mineral series – enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More content in low-Ca pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. Click on Term to Read More 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 fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. Click on Term to Read More during igneous differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities.   When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including Click on Term to Read More 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 oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read More 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 REEOften abbreviated as “REE”, these 16 elements include (preceded by their atomic numbers): 21 scandium (Sc), 39 Yttrium (Y) and the 14 elements that comprise the lanthanides excluding 61 Promethium, an extremely rare and radioactive element. These elements show closely related geochemical behaviors associated with their filled 4f atomic orbital. Click on Term to Read More fractionation patterns. The abundant graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More found in Pontlyfni has similar C-isotopic compositions to that of IAB silicates. Furthermore, a comparison of the trace elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More 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 bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More which experienced incomplete differentiation followed by catastrophic impact disruption and reassembly, consistent with late-stage thermal events ensuing ~10–14 m.y. after CAISub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they Click on Term to Read More 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 crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More 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 mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More 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 isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: ¹H, ²H (deuterium), and ³H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More data obtained by Hunt et al. (2012) for silicate inclusions in IAB irons, along with the observed volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). 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 chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More 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.

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