TIERRA BLANCA

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 Winona, Tierra Blanca, Mt. Morris, Hammadah al Hamra 193, NWA 516, NWA 1457, NWA 1463 and its likely pairing group, and NWA 1617, along with the only fall of the group, Pontlyfni. Pontlyfni, Mt. Morris, and the NWA 725 pairing group contain relict chondrules (porphyritic pyroxene and radial pyroxene in Pontlyfni).

*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.

Winonaites define a group that has mineral compositions intermediate between group E and H chondrites, and has O isotope compositions that are unique from all other groups except IAB complex irons. They have a reduced (Tierra Blanca is among the most oxidized of the winonaites) and metamorphically heterogeneous chondritic composition, and are considered by some to be derived from the breakup and reassembly of a hot, partially differentiated, 100–300 km diameter body 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). Near the stage of peak temperatures, a catastrophic impact disrupted the 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 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 samples. 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.

Evidence was presented by Yugami et al. (1998) 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 and abundant triple junctions, with 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 present in Tierra Blanca occurred during later metamorphic processes. 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, 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). 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°C–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, and that any occurrence of silicate partial melting was not widespread.

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. 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 has recently been 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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