CBb, bencubbinite
standby for hammadah al hamra 237 photo
Found October 18, 1997
28° 36.56' N., 13° 02.95' E.

A single mass of 3,173 g was found in the Libyan Sahara Desert in the fall of 1997. This breccia consists of a mechanical mixture of silicate and metal fragments that are similar to those in Bencubbin, but smaller in size. While the FeNi-metal abundance in HaH 237 is exceptionally high (>70 vol%), the opposite is true for the abundance of fine-grained matrix. Compositionally and isotopically the bencubbinites are most similar to carbonaceous chondrites, thought to have formed in a vapor plume resulting from a collision between two CR-like parent bodies.

The bencubbinites have been divided into two petrologic subgroups, CBa and CBb, representing those with cm-sized metal and silicate chondrules, and those with mm-sized chondrules, respectively. Hammadah al Hamra 237 is a member of the CBb subgroup of the bencubbinites, with an especially close relationship to QUE 94411 (paired with QUE 94627). HaH 237 is a metal-rich chondritic breccia formed from a combination of two separate nebular condensates; i.e., the collisional debris from two planetary embryos. These highly primitive components underwent a size-sorting process within a nebular region enriched in siderophile elements (9.6 x that of lithophiles, relative to solar composition), leading to equilibrium condensation at variable pressures, temperatures, and cooling rates, and within distinct local environments containing variable dust enrichments, particularly Si/gas and Ni/gas ratios. The resulting zoned and unzoned metal grains, silicate chondrules, and other condensation components accreted to form the bencubbinites and CH chondrites. (Fedkin et al., 2015).

Silicates are present in the form of mm-sized cryptocrystalline (CC) and barred olivine (BO) chondrules and chondrule fragments, similar to those found in members of the CH group such as Acfer 214. In light of their non-igneous textures, absence of relict grains, depletion in volatiles, unfractionated REE patterns, and absence of FeNi-metal, the chondrules in HaH 237 are thought to represent first generation chondrules that condensed directly from an impact vapor plume. Large polycrystalline, chondrule-like metal spheres (up to 5 mm) and their fragments are also present. The nearly solar Ni/Co ratio and the strong compositional zoning in some metal grains (60–70%) is indicative of a volatility-based condensation origin in an impact vapor plume. This was followed by diffusion outward from the refractory siderophile-rich core at a total pressure of only 10 Pa (one ten-thousandth of a bar) (Campbell et al., 2005). It was initially ascertained that formation of zoned metal grains in CBb chondrites occurred at high temperatures during a temperature interval of 1092°C to 987°C, and then experienced rapid cooling. The unzoned metal grains were considered to have formed at lower temperatures, cooled more slowly, and then underwent minimal low-temperature metamorphism with little if any reduction. The observation of a sub-grain microstructure exhibiting deformation in areas remote from the core indicates a limited heating event occurred following the condensation/diffusion phase (Duffy et al., 2008).

Based on kinetic condensation modeling, Fedkin et al. (2015) ascertained a more detailed scenario for the formation of the bencubbinites. They determined that the chemical and isotopic compositions of all of the components, including zoned and unzoned metal grains, and cryptocrystalline (CC) and barred olivine (BO) chondrules, can be explained by equilibrium condensation in a vapor plume caused by the collision of two differentiated CR-type chondritic planetesimals, each composed of a core (~29 wt%), a CaO-, Al2O3-poor mantle (~57 wt%), and a CaO-, Al2O3-rich crust (~14 wt%), in addition to the presence of significant hydrous materials. Different fractions of each of these lithologies were sampled by the various components in the bencubbinites, each forming within distinct local regions of the impact vapor plume. They ascertained that formation of the unzoned metal grains occurred by equilibrium condensation as a liquid under one of two probable conditions: 1) 0.01 bar pressure and an enrichment in the Ni:gas ratio of 3,000 relative to solar composition, or 2) 0.001 bar pressure and an enrichment in the Ni:gas ratio of 30,000 relative to solar composition. By similar means, they showed that the BO chondrules could have formed under these same equilibrium condensation conditions, but with additional constraints including enrichment in the Si:gas ratio of 500 relative to solar composition, a water abundance of 20 wt% of the total vaporized silicate, and with formation occurring in a region sampling 40–70 wt% of the vaporized mantle lithology (or ≤40 wt% if sequestration of refractory condensates had occurred). Likewise, the CC chondrules could have formed under the same conditions as the unzoned metal grains, but with unique additional constraints including enrichment in the Si:gas ratio of 300 relative to solar composition, a water abundance of 15 wt% of the total vaporized silicate, and with formation occurring in a region sampling ≤40 wt% of the vaporized mantle lithology after sequestration of refractory condensates. In contrast, the calculations show that the zoned metal grains had to have formed in a separate region of the vapor plume, where condensation occurred in the solid state under significantly lower pressure involving a lower enrichment in the Ni:gas ratio (2,500 relative to solar composition), and cooling proceeded at a high rate under conditions of rapidly decreasing pressure.

A clear, isotropic glass component is found within some chondrules, reflecting the unequilibrated type-3 nature of the meteorite. Other shock-melted silicate glass (5–20 GPa) containing miniscule Fe–Ni–S metallic blebs occurs between metal and silicate fragments, similar to that found in Bencubbin and Weatherford. This shock melt glass is considered by some to be the transformed matrix material, now preserved as sparce hydrated lithic clasts (see following paragraph). Refractory inclusions are a minor constituent in HaH 237, QUE 94411, and Gujba, but none have yet been found in Bencubbin or Weatherford. The CAIs present in the CH-group segment of the CR clan contain the most refractory minerals, providing evidence that they condensed from a hotter nebular region than those in the CR and CB groups, and that they experienced only very low degrees of alteration. The 16O-depleted, pyroxene-rich CAIs present in metal-rich chondrites are unique, and they have textural and mineralogical characteristics that exclude them from an origin on the CR parent asteroid.

Similar to the CAIs, hydrated lithic clasts (or ‘matrix lumps’) are present in low abundance in the CBb group as well as in the CH and CR groups, but none have been identified in the CBa group. These clasts consist of magnetite, sulfides, and carbonates embedded within a hydrous phyllosilicate matrix composed of serpentine and minor smectite. These hydrated lithic clasts are very similar in composition to carbonaceous chondrite matrix material of types 1 and 2, and they were formed independently of the anhydrous CB components. Following aqueous alteration, the lithic clasts were accreted together with the high-temperature components in a cooler region of the Solar System, or through regolith gardening on the CB parent body.

Subsequent shock-lithification fused the porous, fine-grained matrix material that initially constituted the CBa chondrites (Meibom et al., 2004). The shock wave resulted in higher temperatures in this hydrated, porous material than in the denser metal and silicate components, which served to weld the latter components together (Meibom et al., 2005). Heating was localized and cooling was rapid, consistent with the low degree of chondrule melting and shock effects observed. Both the metallic and silicate chondrules in HaH 237 and several other CB members (QUE 94411, Bencubbin, Weatherford, and Gujba) exhibit preferential orientation, presumably resulting from this deformational event.

As with all bencubbinites, HaH 237 contains an abundance of isotopically heavy N. The main N carrier phase in this meteorite is molten metal, possibly residing in sub-microscopic carbide and nitride within kamacite. Another N carrier is taenite, or less often, carbide present around Cr-rich sulfide. More rarely, silicate glass and gas within vesicles are also found to contain heavy N. The hydrated lithic clasts are also being investigated as a carrier of heavy N (see the Bencubbin page for details).

Extraterrestrial amino acids (0.2–2 ppm) were found to be present in a sampling of CB chondrites studied by Burton et al. (2013), abundances of which are slightly lower than those found among aqueously altered type-1 carbonaceous chondrites. The types of amino acids are different from those identified in other carbonaceous chondrite groups, and were likely synthesized through different chemical pathways under different environmental conditions (e.g., degree of aqueous alteration).

Initial studies based on Pb-isotope systematics revealed that the silicates in Gujba (CBa) and HaH 237 (CBb) formed simultaneously ~4.5627 b.y. ago. Subsequently, high precision isotopic studies of HaH 237 conducted by Pravdivtseva et al. (2015, 2016) led them to suggest a refinement in the absolute I–Xe age for the Shallowater standard of 4.5624 (±0.0002) b.y. Based on this new refinement, the age of HaH 237 relative to Shallowater was ascertained to be 4.5621 (±0.0003) m.y., which is consistent with the U-corrected Pb–Pb age determined for Gujba chondrules by Bollard et al. (2015) of 4.56249 (±0.00021) b.y., as well as that determined for HaH 237 silicates by Krot et al. (2005) of 4.5619 (±0.0009) b.y. Agreement in the ages for these various components reflects the simultaneous closure of these chronometers following chondrule formation within a late-stage protoplanetary impact-generated plume.

The CB, CH, and CR chondrites constitute the CR clan, comprising groups which likely formed in the same isotopic reservoir under similar conditions in the solar nebula; current evidence argues for an origin of the metal-rich carbonaceous chondrites in a common collision between planetary embryos (Krot et al., 2009). The CRE age of HaH 237 is calculated to be greater than 3 m.y. The specimen of HaH 237 shown above is a 1.1 g thin partial slice, while that pictured below is a grand 76.33 g complete slice, courtesy of the J. Piatek Collection.

standby for hammadah al hamra 237 photo
Specimen size ~ 123mm by 63mm
Photo courtesy of the J. Piatek Collection

For additional information on the petrogenesis of HaH 237 and the CB chondrites, read the PSRD article by G. Jeffrey Taylor: "Little Chondrules and Giant Impacts", Oct 2005.