NORTHWEST AFRICA 1463

Three fragments weighing together 1,001 g were found and subsequently sold in Erfoud, Morocco to Canadian collector D. Gregory. This meteorite was analyzed and classified through a collaboration with UCLA and Washington University in St. Louis, and determined to be a primitive winonaite. A 23 g specimen of NWA 1463 is curated by UCLA, while the 975 g type specimen is on deposit with the Royal Ontario Museum.

*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 the mineral composition of NWA 1463 is typical for the primitive achondrite group designated winonaites, it has a more primitive, more chondritic texture than most other members of the group, equilavent to a petrologic type 5 chondrite (Benedix et al., 2003). In contrast to most other winonaites, NWA 1463 does not exhibit features related to igneous fractionation processes, features which supported the original designation of winonaites as primitive achondrites. NWA 1463 contains abundant relict chondrules, found infrequently in only a few anomalous winonaite members (e.g., Pontlyfni, Mt. Morris), embedded within a recrystallized ground mass. By contrast, most other winonaites have features consistent with extensive metamorphism, such as uniformly sized grains that form triple junctions. The O-isotope composition of NWA 1463 plots on a line that extends the winonaite trend, while the high abundance of FeNi-metal and the absence of metallic and sulfide veining attests to a lower equilibration temperature than that of other winonaites.

Because of its highly primitive nature, NWA 1463 may closely resemble the chondritic precursor material of the typical winonaites and silicate inclusions in IAB complex irons. This winonaite does not fit into the scheme that has commonly been used to define the primitive achondrite group, and it could be instrumental in the future in redefining the metamorphic progression of chondrites to primitive achondrites. To that end, Irving et al. (2005) have described this winonaite as possibly representing the regolith of the winonaite parent body. Furthermore, the occurrence of distinct chondrules precludes the use of the term achondrite to describe this meteorite, and they suggest that the term metachondrite or ‘W chondrite’ would be more appropriate to describe this texturally evolved group of meteorites (Irving et al., 2005; Irving and Rumble, III, 2006).

Northwest Africa 1463 contains certain features which are unique compared to most other winonaites, and while it is plausible that this meteorite may represent the winonaite precursor material, it has been conjectured that it may instead have originated on a separate parent body (Floss et al., 2008). These unusual features include the lowest degree of metamorphism of all winonaites, an anomalous chromite composition, an anomalous O-isotopic composition, a lack of graphite, the presence of merrillite rather than apatite, and an abundance of incompatible trace elements intermediate to other winonaites. There is convincing evidence that NWA 1463 is paired with NWA 725, NWA 1052, NWA 1054, and NWA 1058. Shown above are two views of a 0.12 g cut fragment of NWA 1463 that show its chondritic texture and a relatively fresh fusion crust. This specimen was kindly provided by Dr. David Gregory.

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