NORTHWEST AFRICA 2993

A fresh, fusion-crusted, 625 g stone was found in the Algerian desert in June of 2006; shortly thereafter it was purchased by a collector (A. Aaronson) in Erfoud, Morocco, and a portion of the meteorite was submitted for analysis and classification to Northern Arizona University (T. Bunch and J. Wittke) and the University of Washington in Seattle (A. Irving). The meteorite was subsequently sold to another collector (S. Turecki). The section of NWA 2993 under analysis was found to consist of 37 vol% orthopyroxene, 32 vol% olivine, and 30 vol% FeNi-metal (kamacite-rimmed taenite with occasional plessite cores), along with minor sulfide.

While a preliminary petrographic examination of a cut face revealed the texture and composition of a coarse-grained (0.3–3.4 mm), plagioclase-depleted, metal-rich lodranite, subsequent O-isotopic analyses conducted at Carnegie Institution (D. Rumble, III) showed that it actually plots on an extension of the trend line for the winonaite/IAB-iron complex meteorites, very close to the IAB-related Vermillion (see Figure 3 plot in LPSC XXXVIII #2211). To help discriminate between the winonaite and acapulcoite–lodranite groups, a plot of Δ17O (‰) with olivine Fa content (mole%) was found to be a useful technique (D. Rumble, III et al., 2005). Northwest Africa 2993 plots nearest to the WIN field, but since it has the largest values for both parameters, it plots in a unique location slightly distant from the main WIN concentration (see Figure 4 in abstract #2211 above).

*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 continuum 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.

Northwest Africa 2993 represents the first known coarse-grained, plutonic sampling from the winonaite/IAB-iron complex PB, and is chemically and texturally analogous to the lodranites of the ACA–LOD PB. The exact petrogenetic history which led to the formation of this unique meteorite is an ongoing study. The photo of NWA 2993 shown above is a 8.93 g slice. Shown below is a view of the main mass in the possession of the owner.