HAMMADAH AL HAMRA 193


Winonaite (typical)*
standby for hah 193 photo
Found October 1996
28° 39.28' N., 13° 27.52' E.

A single stone weighing 259 g was found in the Libyan Sahara. Hammadah al Hamra 193 has a weathering grade of W3 and a shock stage of S1. Unlike most other winonaites having recrystallized equigranular textures, HaH 193 contains large poikilitic orthopyroxene grains up to 5 mm in size which enclose smaller grains of olivine, plagioclase, and clinopyroxene (Floss et al., 2003, 2007). These large poikilitic orthopyroxene grains have been attributed to an extended period of thermal metamorphism and slow cooling at depth. Interstitial regions contain clumps of coarse-grained (200–300 µm), equigranular olivine grains possibly representing partial melt residues, as well as amphibole (fluoro-edenite) grains up to 2 mm in size. The fluoro-edenite, which poikilitically encloses a variety of other mineral grains, has not been identified previously in any other meteorite. Based on trace element evidence, it is argued that this fluoro-edenite is the product of a chemical reaction between clinopyroxene and plagioclase with the addition of F, the latter possibly being derived from a F-bearing metasomatic fluid; the fluor-apatite present in this meteorite might also be a product of this fluid. Veins of FeNi-metal and troilite and associated schreibersite occur throughout the meteorite.

*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).

Although winonaites exhibit significant mineralogical and trace element heterogeneity on a large scale, they likely originated on a common parent body generally considered to be the same as that of IAB complex irons. Evidence for this includes nearly identical O-isotopic compositions and silicate textures in certain inclusions, and a similar compositional range in silicate mineralogies. The heterogeneous nature of winonaites is thought to be the result of incomplete differentiation followed by the catastrophic impact disruption and reassembly of the parent body (Benedix et al., 2000). This event was followed by a sustained period of impact brecciation.

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)

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. Further details about the petrogenetic history of the winonaites can be found on the Tierra Blanca page. The specimen of HaH 193 shown above is a 2.2 g partial slice.