HAMMADAH AL HAMRA 193

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 other winonaites having recrystallized equigranular textures, HaH 193 contains large (up to 5 mm), continuous, poikilitic orthopyroxene grains which enclose grains of olivine, plagioclase, and clinopyroxene (Floss et al., 2003). Interstitial regions may contain large clumps of coarse-grained, equigranular olivine grains, or they may contain the amphibole fluoro-edenite (up to 2 mm) in combination with clinopyroxene and other mineral grains. 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, this amphibole is thought to be the product of a chemical reaction between clinopyroxene, plagioclase, and F-bearing apatite.

*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.

Although winonaites exhibit significant mineralogical and trace element heterogeneity on a large scale, they likely originated on a common parent body, which is considered by most to be the same as that of the 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 may be the result of incomplete differentiation followed by the catastrophic impact disruption and reassembly of the parent body (Benedix et al., 2000). This was followed by a sustained period of impact brecciation. More in-depth information on winonaite petrogenesis can be found on the NWA 516 page of this website. The pictured specimen is a 2.2 g partial slice of HaH 193.

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