TIERRA BLANCA


Winonaite (evolved)*
standby for tierra blanca photo
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, Mt. Morris, HaH 193, NWA 516, NWA 1457, NWA 1463 and pairing group, and NWA 1617. Pontlyfni, Mt. 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)

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.