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Eagles Nest

Brachinite
standby for eagles nest photo
Found Summer of 1960
no coordinates recorded A well-oriented, fusion-crusted meteorite weighing 154 g was found in Central Australia lying next to an eagle’s nest. Although the type specimen Brachina was found in central South Australia, differences between it and Eagles Nest in mineralogy, chemistry, and average grain size indicate that they are not fall-related. Analysis and classification of Eagles Nest was conducted at the University of Arizona, Lunar and Planetary Laboratory (W. V. Boynton), and a classification of brachinite was proposed. Eagles Nest shows some differences to most brachinites including its lack of fine-grained assemblages of orthopyroxene and opaques lining various olivine grain boundaries. Goodrich (2010) described reduction features that exist on such assemblages in some brachinites, but which are absent in Eagles Nest. It was conjectured that Eagles Nest might have originated from a different brachinite-like parent body.

To further resolve the petrogenesis of the brachinite group, a consortium of institutions was established to make an in-depth study of the unique Antarctic meteorite GRA 06128/9. This is a high-phosphate, high-Na, coarse-grained troctolitic anorthosite (Zeigler et al., 2008), perhaps also properly called an alkalic leucodiorite (Treiman et al., 2008) or a basaltic trachyandesite/trachyandesite (mugearite/benmoreite). The meteorite is composed predominantly (~75 vol%) of albitic plagioclase (oligoclase: Ab85An15 mol%); this is an extraordinarily high abundance of plagioclase compared to other meteorite magma types. It has an O-isotopic composition that plots close to the TFL and which is indistinguishable from the plot of brachinites. In a similar manner, the Fe/Mn ratio measured for both olivine and pyroxene are indistinguishable from that of other brachinites (this ratio remains constant regardless of differentiation processes and is diagnostic for the origin of each planetary body). Moreover, the major, minor, and trace element chemistry of GRA 06128/9 is very similar to that of brachinites. Studies of highly siderophile element (HSE) abundances and examination of the metal–sulfide segregation processes led to the determination by Day et al. (2012) that GRA 06128/9 was likely genetically related (i.e., from the same parent body) to the brachinites.

Consistent with the ancient crystallization age of brachinites, the GRA 06128/9 samples have a Sm–Nd crystallization age of ~4.55 b.y. (Nyquist et al., 2008, 2009) and an Al–Mg age relative to D’Orbigny angrite of ~4.566 b.y. (Shearer et al., 2009), attesting to commencement of magmatism on the parent body within a couple of m.y. after CAI formation. As with brachinites and the other inner Solar System objects, the Sm–Nd age of the GRA meteorite was reset ~3.4 b.y. ago, close to the Late Heavy Bombardment period. The CRE age calculated for GRA 06128/9 is 2.9–3.0 m.y., which is within the range of CRE ages (2.0–3.5 m.y.) calculated for Brachina (Mittlefehldt et al., 1998 and references therein; Matsuda et al., 2008), but is significantly different from that of Eagles Nest (44–49 m.y.).

Many of the known brachinites have disparate cosmic-ray exposure ages, indicating that they represent numerous separate ejection events. According to a study by Patzer et al. (2003), the CRE ages of EET 99402/407, Hughes 026, and Eagles Nest form a cluster at ~48 m.y., and those of Reid 013 and ALH 84025 coincide at ~10 m.y. In a separate study by Ma et al. (2003), the cosmogenic nuclide calculations establish a range of CRE ages from 4 m.y. for Brachina to ~25.5 m.y. for Eagles Nest. From their noble gas analyses of 15 brachinite and brachinite-like meteorites, together with the literature values for seven others, Beard et al. (2018) identified three potential CRE age clusters. The oldest cluster reflects a possible ejection event that occurred ~49.9 (±4.1) m.y. ago, comprising the six brachinites Eagles Nest, EET 99402, Hughes 026, NWA 4969, NWA 7605, and NWA 10637. Importantly, two of these CRE age clusters include both brachinite and brachinite-like meteorites, which attests to a common parent body for all of these meteorites (see diagram below). standby for o-cr diagram
click on image for a magnified view

Diagram credit: Beard et al., 81st MetSoc, #6170 (2018) It has been argued that GRA 06128/9 possibly represents a lower crustal cumulate developed after 10–30% partial melting of a chondritic source on the brachinite parent body. An estimated cooling rate of 10–20°C/year was derived (University of Tokyo; Miyamoto), consistent with a near-surface burial at a depth of 15–20 m. This is contrasted with the deep, ultramafic mantle region from which typical brachinites are thought to have formed as residues of partial melting (Ash et al., 2008). It can be inferred from petrological, geochemical, and mineralogical data that GRA 06128/9, and thus all brachinites, originated on a large partially differentiated planetary body distinct from the Earth, Moon, or Mars; interestingly, speculation has surfaced about a possible origin on Venus (Shearer et al., 2008). Nevertheless, in light of its isotopic, chemical, and mineralogical similarities to IAB complex irons such as Caddo County, which similarly contains inclusions of albitic plagioclase (Ab84 mol%), it was considered plausible that this iron asteroid could be the parental source of the brachinites (Nyquist et al., 2009). However, a comparison between brachinites and IAB irons utilizing nucleosynthetic isotope anomalies (Mo systematics) shows them to be unrelated. standby for carbonaceous vs. non-carbonaceous irons mo diagram standby for aca-lod mo diagram
Diagrams credit:
(left) Kruijer et al., PNAS, vol. 114, #26, p. 6713 (2017), ‘Age of Jupiter inferred from the distinct genetics and formation times of meteorites’ (http://dx.doi.org/10.1073/pnas.1704461114)
(right) Budde et al., 49th LPSC, #2353 (2018) Laboratory melting experiments conducted by Gardner-Vandy et al. (2013) have demonstrated that an FeO-rich (oxidized) R chondrite-like precursor asteroid can undergo significant partial melting (14–31% at ~1250°C) and melt removal to produce a brachinite-like residue, and possibly also a low degree partial melting (<10% at <1250°C) and melt removal to produce a complementary evolved melt having a composition like that of the brachinite-related GRA 06128/9. In their continued effort to attain the composition of the GRA 06128/9 meteorite, considered to be a likely representative of the brachinite parent body feldspathic crust, Sosa et al. (2017) employed multiple modeling techniques and conducted melting experiments utilizing R4 chondrite LAP 03639. Their results demonstrate that an R chondrite-like precursor asteroid can undergo low-degree partial melting (~16–20%) at 1140°C with an oxygen fugacity near the iron–wï ¿ ½stite buffer (~IW) to produce a brachinite-like residue and a complementary evolved melt with a composition like that of GRA 06128/9.

Additional experimental data and modeling results attained by Lunning et al. (2017) has further constrained the conditions of formation for GRA 06128/9. Their investigation indicates that both equilibrium and non-equilibrium partial melting (the latter condition corresponding to lower temperatures and degrees of melting) on an oxidized parent body similar to R chondrites, in which 14–22% melt is generated at a temperature of 1120–1140°C and a redox state of IW–IW+1, reproduces most closely the whole rock composition of the GRA 06128/9 meteorite. The authors also posit that unsampled lithologies containing higher silica abundances may have been produced on the GRA 06128/9 (or the brachinite) parent body, in association with very low degrees of non-equilibrium partial melting. These potential lithologies might be akin to the Almahata Sitta trachyandesite samples MS-MU-011/035, which are thought to represent the primary crust of the ureilite parent body. Additional information concerning the origin and petrogenesis of brachinites and GRA 06128/9 can be found on the NWA 3151 and Reid 013 pages.

The specimen of Eagles Nest shown above is a 1.2 g fusion crusted partial slice (photo courtesy of K. Regelman). The top photo below shows a complete slice of Eagles Nest, while the bottom photo shows a small preserved portion of the oriented face of this meteorite exhibiting radial flowlines.

eagles nest
click on image for a magnified view

Photos courtesy of Ken Regelman


For additional information on GRA 06128/9, read the PSRD article by G. Jeffrey Taylor: ‘More Evidence for Multiple Meteorite Magmas‘, Feb 2009.


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Zakłodzie (Zaklodzie)

Primitive Enstatite Achondrite or
EL melt rock or E chondrite IMB
(E achondrite-ung in MetBull 84)
standby for zakłodzie photo
Found September 1998
50° 45′ 46′ N., 22° 51′ 58′ E. Zakłodzie is an 8.68 kg enstatite-rich achondrite that was found beside a dirt road by Mr. Stanislaw Jachymek, a mineral and fossil collector. This extremely weathered meteorite with remnant fusion crust was found about 40 km west of Zamosc, Poland, near the small village of Zakłodzie. Its meteoritic nature was first verified by Dr. Lukasz Karwowski of the Silesian University Geology Department. The mineralogical analysis and classification were performed at the Max-Planck-Institut für Chemie (Dr. Frank Wlotzka), the Polish Geological Institute (Dr. Marian Stepniewski), and the Bartoschewitz Meteorite Laboratory (Rainer Bartoschewitz).

 

Zakłodzie has a fine-grained, granular texture similar to that of several other recrystallized EL chondrites, including Happy Canyon, MIL 090807, and Ilafegh 009 (Boesenberg et al. (2014). It is composed of ~60 vol% orthoenstatite with ~20 vol% Si-bearing FeNi-metal, along with ~10 vol% of both troilite and feldspar. It also contains accessory schreibersite, tridymite, cristobalite, oldhamite, albandite, amphibole, and ubiquitous graphite. Keilite [(Fe,Mg)S], a mineral phase associated with strongly reducing conditions, has been observed in Zakłodzie (Karwowski et al., 2007; Uribe et al., 2017). Sinoite, a silicon nitride associated with crystallization of an impact melt, has been identified as inclusions within the keilite. While there are no visible chondrules, some elongated enstatite grains arranged in an oval configuration might represent extremely metamorphosed relict chondrules. There are abundant metal flakes, interspersed gas vesicles, and various light and dark inclusions present as well. While 0.1–4 mm (rarely as large as 9 mm; Przylibski et al., 2011) spherical graphite nodules containing metal inclusions (and vice versa), are present in the innermost portion of the meteorite, finely dispersed graphite producing a dark coloration occurs in the outer portion. Raman analysis shows that the graphite has a semi-ordered to ordered structure.

 

Zakłodzie shows similarities to the QUE 94204 pairing group. The bulk composition and petrography of both meteorites, especially the zoned and skeletal feldspar, unequilibrated anorthite contents in the feldspar, and polysynthetically twinned enstatite grains, suggests a possible rapid cooling at some stage. Moreover, the retention of abundant opaque phases, FeNi-metal surrounding enstatite laths, and relict chondrules, are all features consistent with impact melting and rapid cooling. In addition, the mineral keilite is only stable in conditions of rapid cooling of impact melts. All of these features taken together are considered by some investigators (Burbine et al. [2000], Keil [2007], and Keil and Bischoff [2008]) to best represent a classification for Zakłodzie as an E chondrite impact-melt breccia.

 

This classification is contrary to that proposed by Przylibski et al. (2005), who argue that rapid cooling from an impact-melt is inconsistent with the characteristics of Zakłodzie (e.g., cumulate texture, relict chondrules, triple junctions, slow cooling rate, silica content, and plagioclase enrichment). They believe instead that the features best fit a scenario in which Zakłodzie experienced initial slow cooling during cumulate processes, all the while undergoing fractional crystallization of forsterite sequentially forming protoenstatite ⇒ orthoenstatite ⇒ clinoenstatite, and also silica, feldspar, FeNi-metal, and sulfides. This rock is the residual material resulting from rapid partial melting of enstatite chondrite material. They propose that Zakłodzie subsequently experienced a rapid cooling phase as it was entrained in a rising magma column, assimilating chondrule-bearing rock as it rose. Following this stage, the low-melting-point components such as feldspar, metal, and sulfides were partially remelted in a subsequent impact event. Given this scenario, Przylibski et al. (2005) favor a classification for Zakłodzie as a primitive enstatite achondrite. In addition, they favor this classification for the enstatite meteorites Happy Canyon, Ilafegh 009, QUE 94204, and Y-8404. A study of Zakłodzie by Krzesińska et al. (2015) revealed micro-textural features and a mineralogy which suggests that the source rock experienced a severe shock event. This was followed by burial under an ejecta blanket where recrystallization and annealing occurred.

 

Zakłodzie has a K–Ar closure age of ~4.50 (+0.02, –0.01) b.y. (Bogard et al., 2010). Other radiometric chronometers reveal that a melting event occurred ~2.1 b.y. ago, possibly reflecting the remelting impact event. The Al–Mg age of Zakłodzie was determined to be 5.4 (±0.4) m.y. after CAIs (4.5671 b.y.), or an absolute age of ~4.5617 b.y. (Sugiura and Fujiya, 2008). This age is consistent with other basaltic meteorites that experienced early melting and differentiation due to radioactive decay of short-lived radionuclides such as 26Al.

 

The cosmogenic nuclides in Zakłodzie, including 4He, 40Ar, 129Xe, and Q-type noble gases, indicate a CRE age of 55.3 (±5.5) m.y. This high exposure age is also characteristic of the aubrites, which cluster around 50 m.y. Noble gas studies indicate that Zakłodzie is distinct from equilibrated E chondrites. During its terrestrial residence, considered to be >100 years, Zakłodzie experienced very high oxidation of Fe throughout much of its mass. The Fe in the outer crust has progressed to hematite, with an admixture of the loess minerals in which the mass was found; nevertheless, 14C results indicate that Zakłodzie is a relatively recent fall.

 

Despite the very close similarities in O-isotope compositions between Zakłodzie and the ungrouped enstatite achondrite Itqiy, their chemical and mineral compositions, noble gas contents, and terrestrial ages exclude an origin from a common parent body. Likewise, differences between Zakłodzie and enstatite achondrites in some elemental ratios such as Mn/Cr exclude a common origin. The parent body was most likely a member of the enstatite chondrite group of asteroids, and the chemistry is most similar to the EL group (Stepniewski et al., 2000)—the Si concentration in kamacite of 1.5 wt% is more consistent with the EL group (EH: 1.9–3.8 wt%; EL: 0.3–2.1 wt%).

 

In 2006, the 685 g achondrite NWA 4301 was found in Algeria. Because this meteorite is almost identical to Zakłodzie in petrology and mineral composition, and since the two stones have similar young terrestrial ages of ~300 years, scientists have deduced that these two ungrouped enstatite meteorites could be source-crater paired (Irving and Kuehner, UWS). Micro-computed tomography (µCT) scans performed on samples of both of these meteorites indicate differences in their degree of weathering and in their metal abundances, with NWA 4301 exhibiting more weathering and more abundant and interconnected metal veining (Uribe et al., 2015). Continued comparative studies of both meteorites by Uribe et al. (2015) revealed some textural differences in these two meteorites, and they suggest that NWA 4301 might have experienced slower cooling than Zakłodzie. The specimen of Zakłodzie shown above is a 0.74 g partial slice showing a portion of a metal vein. The top photos below show a large mass of Zakłodzie shown courtesy of Marcin Cimala. The middle photo is a thumbnail image to a high-resolution photo shown courtesy of Tomek Jakubowski. The bottom photo is an excellent petrographic thin section micrograph of Zakłodzie shown courtesy of Peter Marmet.
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standby for zaklodzie photo
Photos courtesy of Marcin Cimala—PolandMET.com
 
standby for zaklodzie photo
click on image for a magnified view
Photo courtesy of Tomek Jakubowski
 
standby for lodran photo
click on image for a magnified view
Photo courtesy of Peter Marmet


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Zag (B)

Achondrite, ungrouped
(brachinite-like)
standby for zag (b) photo
Found 1999
no coordinates recorded An oriented, fusion-crusted stone weighing 300 g was found near the site of the Zag meteorite fall in Morocco (see photo of main mass with flow lines at the RSPD website). Zag (b) is an unbrecciated achondrite rich in forsteritic olivine (68 vol%) that incorporates mm-sized orthopyroxene channels containing metal or sulfide or weathered metal inclusions. These orthopyroxene and opaque assemblages exhibit several features that indicate the occurrence of a late reduction process, and the Fe–Mn–Mg relations are also consistent with reduction processes on the brachinite parent body. Several methods for the reduction of primary olivine were reviewed by Goodrich et al., 2017), including its reaction with methane to form orthopyroxene + metal (Irving et al., 2013) and through its sulfurization by a S-rich fluid or gas to form orthopyroxene + sulfide (e.g., Singerling et al., 2013). Olivine and orthopyroxene exhibit variability in their distribution (Day et al., 2012). The oxygen isotope ratios plot close to those for Divnoe, NWA 4042, and the brachinites, although Zag (b) has undergone a more thorough reduction process.

Zag (b) shares very close similarities with Divnoe and the brachinites in Fe ratios and oxygen isotopic systematics. Moreover, the acapulcoites and lodranites fall on the same oxygen mixing line, and they have Fe–Mn–Mg features consistent with a close relationship to Zag (b), Divnoe, and the brachinites. All of these different meteorites likely formed in the same nebular region from common precursor material, with Zag (b) having an intermediate position between Divnoe and the ACA–LOD cluster. Because of its close relationship to Divnoe, and by extension, the brachinites, Zag (b) is sometimes conveniently included with the brachinites in classification studies. Further information about Zag (b) can be found in the abstract ‘Zag (b): A Ferroan Achondrite Intermediate Between Brachinites And Lodranites’ by Delaney et al., 31st LPSC, #1745 (2000).

On a newly compiled O-isotope diagram for brachinites and other planetary achondrites, published by Rumble III et al. (2008), Zag (b) has a Δ17O value that plots within a select grouping of brachinites including NWA 3151, NWA 595, and the ungrouped brachinite-like achondrite NWA 4042, and these investigators believe that Zag (b) should probably be lumped with the brachinites. However, through studies of highly siderophile element (HSE) abundances, and upon examining the metal-sulfide segregation processes, it was determined by Day et al. (2012) that Zag (b) and similar brachinite-like achondrites were not likely genetically related (i.e. from the same parent body) to brachinites, but rather, originated on similar volatile-rich, oxidized, chondritic precursor asteroids while experiencing similar petrologic processes during their formation history. Goodrich et al. (2017) determined that brachinites and brachinite-like achondrites have a distinct redox trend and a higher Fe/Mg ratio compared to all other primitive achondrites, consistent with formation in a similar nebula reservoir; therefore, they suggest that brachinites and brachinite-like achondrites be called the brachinite clan.

The measured HSE abundances are consistent with a partially melted parent body in which heating from short-lived radionuclides came to a halt before a core was fully formed. Studies of the fractionation trends for Zag (b) led Day and Warren (2015) to conclude that this meteorite might not have been a residue after partial melting, but instead represents a cumulate that incorporated a residual metallic melt with a high Pt/Os ratio; the ungrouped cumulate achondrite NWA 6704 and the brachinite-like cumulate achondrite MIL 090206 (and pairings) exhibit similar elevated Pt/Os ratios.

The specimen of Zag (b) shown above is a 2.2 g partial slice. A microscopic examination reveals the yellow-green olivine enclaves scattered throughout this meteorite.


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Tierra Blanca

standby for tierra blanca photoWinonaite (evolved)*
Found Before 1965, 34° 56′ N., 102° 01′ W.

An 860 g stone covered 80% by weathered fusion crust was found by a local rancher near Tierra Blanca Creek (translated: white earth creek), about 10 km SW of Canyon, Texas. It was brought to the Department of Geology, West Texas State University, where it was identified by F. Daugherty as a meteorite. To date only a small number of winonaites have been identified; some of those found outside Antarctica include Pontlyfni, which is the only fall of the group, Winona, Tierra Blanca, Mount Morris, HaH 193, NWA 516, NWA 1457, NWA 1463 and pairing group, and NWA 1617. Pontlyfni, Mount Morris, and the NWA 725 pairing group contain relict chondrules (porphyritic pyroxene and radial pyroxene in Pontlyfni).

*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).

Winonaites define a group of meteorites that have mineral compositions intermediate between groups E and H chondrites, with O-isotope compositions that are unique from all other groups except IAB complex irons. They have a metamorphically heterogeneous chondritic composition and a reduced state (Tierra Blanca is among the most oxidized of the winonaites). Winonaites are considered by some to derive from the breakup and reassembly of a hot, partially differentiated body ~60–200 km in diameter on which sulfur-rich molten metal had begun forming a core, and silicates had undergone varying degrees of partial melting forming basaltic melts and olivine-rich residues (Benedix et al., 1995, 1996; Hunt et al., 2017). About 10–14 m.y. after CAIs, near the stage of peak temperatures, a catastrophic impact disrupted the winonaite–IAB parent body excavating molten core material and injecting it into cooler silicates, which quickly solidified to form the IAB irons with silicate inclusions. Deep burial of these silicated irons resulted in slow cooling rates and permitted the formation of a Thomson (Widmanstätten) structure.

 

The reassembly that followed this catastrophic collision also mixed olivine-rich residues into unmelted silicates nearer the surface to form the winonaites, while subsequent impact gardening contributed to the mixing of various lithologies. Varying degrees of thermal metamorphism produced the wide variation of trace element concentrations observed within the winonaite group. 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.

 

Evidence was presented by Yugami et al. (1998) indicating that local textural and mineralogical variations on a cm-scale are the result of petrological processes rather than the reassembly of heterogeneous clastic material. In a similar argument, Benedix et al. (2005) proposed that this small scale heterogeneity is the result of localized heating and cooling rates of fragments following the reassembly after a catastrophic breakup. Utilizing helium pycnometry, Consolmagno et al. (2007) determined a porosity for Tierra Blanca of 14% (±4%).

 

Tierra Blanca is an Fe-rich winonaite and is among the coarsest-grained members of the group (0.1–0.2 mm). It has an equigranular texture with abundant triple junctions, and shows no evidence of mixing with a molten metallic phase. Benedix et al. (1998) concluded that the growth of large, poikilitic, Ca-rich pyroxene grains enclosing olivine in Tierra Blanca occurred during later metamorphic processes. Similar large poikilitic orthopyroxene grains present in HaH 193 have been attributed by Floss et al. (2007) to an extended period of thermal metamorphism and slow cooling at depth. Tierra Blanca contains a lower abundance of Ca-rich materials and a higher abundance of olivine and chromite than other winonaites. It exhibits Fe/Mg reverse zoning in olivine which is attributed to solid state reduction. However, another study involving oxygen fugacities of winonaites (related to the partial pressure of available oxygen) suggests that most of the reduction observed is an intrinsic property of the chondritic precursor (Benedix et al., 2005).
standby for winonaite comparison photo
Textural comparison of four winonaites, L to R: NWA 1463 (with relict chondrule), Winona, Tierra Blanca, HaH 193
Image credit: Floss et al., MAPS, vol. 43, #4, p. 660 (2008)
‘Evolution of the winonaite parent body: Clues from silicate mineral trace element distributions’
(http://dx.doi.org/10.1111/j.1945-5100.2008.tb00676.x)

Some regions of coarse-grained olivine grains may represent partial melt residues produced by the extraction of a basaltic melt and FeNi–FeS through veins. These features attest to a moderate degree of silicate partial melting on the precursor body at a temperature of 1200°C, which has been confirmed through two-pyroxene geothermometry analysis (900–1100°C estimated by Lindsley, 1983). However, Floss et al. (2008) analyzed the suspected silicate partial melt and melt residue lithologies in Tierra Blanca, Winina, and HaH 193 for expected incompatible element enrichments and depletions, respectively. Despite variable incompatible trace element abundances, they did not find differences in plagioclase among winonaites and were unable to unequivocally demonstrate that a silicate partial melt exists. Instead, they propose that the rare fine-grained plagioclse-rich, and coarse-grained olivine lithologies present in some winonaites, as well as the ubiquitous FeNi-metal veining, were produced through impact-induced shock melting; they infer that any occurrence of silicate partial melting was not widespread. In a subsequent study of eight winonaites, Hunt et al. (2017) utilized major element and REE data as well as two-pyroxene thermometry to ascertain that only Tierra Blanca experienced temperatures high enough (1473 [±100] K) to produce silicate melting and extraction. However, previous studies (Benedix et al., 1998, 2005) have revealed that Winona is heterogeneous in both its texture and in its range of peak temperatures, concluding that some portions of this meteorite were likely to have experienced some degree of silicate partial melting. It is interesting that in their trace element study of Winona, Hunt et al. (2017) found that it has a similar positive Ce anomaly to achondrites recovered from Antarctica. They reason that since this Ce anomaly is produced through terrestrial weathering in a cold desert environment, it is likely that Winona was transported south to Arizona from a similar cold desert location.

 

Based on similar silicate textures, reduced mineral chemistry, and O-isotopes, it is presumed that the winonaites and the IAB complex irons originated on a common parent body. Utilizing a Ge/Ni vs. Au/Ni coupled diagram, Hidaka et al. (2015) determined that FeNi-metal in the winonaite Y-8005 plots in the field of the sLL subgroup of the IAB complex irons. In addition, the metal in this winonaite retains a near chondritic composition likely representative of the precursor material of the parent body. In view of these findings, they suggest that the sLL subgroup rather than the main group of the IAB complex represents the primitive metal of the IAB–winonaite parent body, with the main group possibly representing a partial melt of the sLL subgroup.

 

Oxygen isotope data obtained by Hunt et al. (2012) for silicate inclusions in IAB irons, along with the observed volatile element depletions, led to the inferrence 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. Yugami et al. (1999) speculate that these and other primitive achondrites may have been heated early in the Solar System by both radiogenic 26Al decay and by slow-speed collisions of planetesimals. The Tierra Blanca main mass of 465 g was traded from the Dr. Elbert A. King Collection to the Natural History Museum, London. The specimen shown above is a 1.1 g cut fragment.


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SAHARA 03500

Primitive Achondrite, ungrouped
(impact melt rock)
standby for nwa s03500 photo
Found February 2003
no coordinates disclosed A 221.33 g, fusion-crusted, partly broken stone was found in the Sahara Desert by the French team of Caillou Noir under the organization of Michel Franco. A 21.36 g section of Sahara 03500 was provided to the Université Pierre & Marie Curie in Paris (A. Jambon and O. Boudoma; #5272) for analysis, and a smaller 2.0 g sample to the Université Blaise Pascal in Clermont–Ferrand (B. Devouard).

Centimeter-size raised metallic clasts are present over the regmaglypted surface, the result of selective ablation processes on the rounded metal–sulfide globules that occur throughout (~16 vol%). Inside these pyrrhotite globules there is a visible texture resulting from dendritic exsolution of kamacite or taenite. Smaller globules and sub-mm-sized FeNi-metal grains have associated phosphides. The meteorite exhibits an unusual light gray-green color.

The silicate matrix is composed of olivine and orthopyroxene and has an igneous texture consistent with impact-melting and rapid cooling at the surface (Jambon et al., 2005). Small feldspathic glassy shock veins are scattered throughout the meteorite. The bulk composition of Sahara 03500 is similar to that of LL chondrites, but with a higher K/Na ratio, an enrichment in LREE, and a depletion in siderophile elements. An oxygen isotope analysis is currently underway which could help resolve the nature of its parent body.

Sahara 03500 exhibits minor terrestrial weathering (W1) in the form of carbonate veins. The specimen of Sahara 03500 shown above is a 0.80 g ultrathin part slice. Two views of the complete slice from which this specimen was removed can be seen below; the magnified view shows the coarse exsolution (graphic) texture of the large metal–sulfide globules. A photo of the main mass as found is shown courtesy of Michel Franco. standby for nwa s03500 photo
standby for nwa s03500 photo
Photos courtesy of S. Turecki
standby for nwa s03500 photo
Photo courtesy of Michel Franco