SAHARA 00182


C3-ungrouped w/ CV/CR affinities
(related to HaH 073 and possibly to the Coolidge–Loongana grouplet)
standby for sah00182 photo
Found January 2000
z° 06' 13" N., w° 09' 35" W.

A single stone weighing 70 g was found by the Labenne Family while searching for meteorites in the Sahara. This carbonaceous chondrite contains metal-rich, multi-layered chondrules up to 3 mm (ave. 790 µm) in diameter that are texturally similar to those in the CR group. These concentrically layered chondrules reflect early nebular activity in which multiple phases of aggregation and heating occurred.

The cores of these chondrules are composed of Mg-rich silicate and/or metal that was once in a partially to completely molten state producing barred or cryptocrystalline textures, respectively. Surrounding the core is a layer composed of metal blebs, which either condensed directly from the nebula or was the result of reduction processes in the chondrule. The next layer to be accreted, composed of olivine and/or pyroxene, exhibits enrichments in FeO and depletions in CrO compared to other CR chondrites studied, suggesting that this material experienced higher-temperature metamorphism or melting in a transient heating event (Kakazu et al., 2007).

Surrounding some type-I chondrules is a silica-rich outer rim composed of tridymite and cristobalite, along with low- and high-Ca pyroxene, as well as a REE-enriched, Ca-rich glass. These rims were formed as a result of condensation of 16O-poor silica-rich material, which was followed by a transient heating event that resulted in the crystallization of the rim phases observed (Kakazu et al., 2008). These rims are homogeneously enriched in S compared to similar rims in CR chondrites, indicative of a lower-temperature pre-accretion history than that which was experienced by other CR chondrites. All of these successive phases constituting the chondrules, from core to outer rim, are thought to have condensed from a single, fractionated, cooling nebular reservoir.

While previously classified as C3-ungrouped, Sah 00182 shares many similarities with the CR chondrite group (Weisberg, 2001). In addition to having similar chondrule textures, most chondrules in both Sah 00182 and other CR members are Mg-rich type-I. Metal within chondrules has a similar Ni content (4.7–6.6 wt%) and Co/Ni ratio. The abundance of refractory inclusions such as AOAs is similar to that of CR chondrites. Although Sah 00182 lacks hydrous phyllosilicates as is expected for a petrologic grade of 3, it is nevertheless petrographically similar to the CR2 chondrites in light of their having experienced aqueous alteration. All CR chondrites have retained a pristine nebular signature within their components (chondrules, FeNi-metal, CAIs, AOAs) and have experienced very little thermal metamorphism.

Despite the many similarities of Sah 00182 to the CR chondrites, its plot on an oxygen 3-isotope diagram in comparison to carbonaceous chondrite groups, it plots close to the CCAM line in the CV field—between Coolidge and HaH 073 (Smith et al., 2004). According to another analysis, the O-isotopic plot is near the CO field and above the CV field (Huber et al., 2006). On the other hand, some characteristics are quite different from those of CV chondrites: 1) the abundance of refractory inclusions in Sah 00182 is 1.1 vol% compared to 6.1–11.5 vol% in CV chondrites; 2) the chondrule abundance in Sah 00182 is 64.9 vol% compared to 33.0–52.5 vol% in CV chondrites; 3) the matrix in Sah 00182 constitutes only 27.6 vol% compared to 34.5–51.3 vol% in CV chondrites. In addition, many bulk rock elemental ratios are significantly different when compared to the CV chondrites, and some ratios are more similar to the CM or CO chondrites. Moreover, volatile lithophiles in Sah 00182 are significantly lower than in CR chondrites, while other elemental abundances are either lower or higher, inconsistent with a CR classification (Choe et al., 2010). Another distinction between Sah 00182 and both the CR and CV chondrites is the high abundance of sulfides in the former.

It is possible that Sah 00182 represents primary, anhydrous, unaltered CR material. However, based on the total data for Sah 00182 it best remains assigned as an ungrouped type 3 carbonaceous chondrite with affinities to both CV and CR groups. Furthermore, in light of similar volatile siderophile and chalcophile patterns between Sah 00182 and HaH 073, it has been conjectured by Choe et al. (2010) that these two meteorites are related. They also argue, given a heterogeneous carbonaceous chondrite asteroid, that this relationship could be extended to include the Coolidge–Loongana grouplet; therefore, one or both could conceivably be recognized as additional samples constituting the membership of five which is necessary to establish a new carbonaceous chondrite group. The Sah 00182 specimen pictured above is a 0.85 g partial slice showing an abundance of metal-rich chondrules. It has been shocked to stage S2 and weathered to grade W2. The magnified image below speaks for itself.

standby for sah 00182 photo
Photo courtesy of Labenne Meteorites