HAMMADAH AL HAMRA 237

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.

HaH 237 is a member of the CB group, 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—one being a metal-rich chondritic component and the other a volatile-depleted metallic component. These highly primitive components underwent a size-sorting process within a nebular region favorable to metal-enrichment, leading to the accretion of the bencubbinites. The bencubbinites have been divided into two petrologic subgroups, CB_a and CB_b, representing those with cm-sized metal and silicate chondrules (Bencubbin, Weatherford, NWA 1814, Fountain Hills, Gujba, and NWA 4025), and those with mm-sized chondrules (HaH 237, QUE 94411, and Isheyevo), respectively. Based on Pb-isotope systematics, the silicates in Gujba (CB_a) and HaH 237 (CB_b) formed simultaneously ~4.5627 b.y. ago.

Large polycrystalline, chondrule-like metal spheres (up to 5 mm) and their fragments are 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 and/or evaporation origin in a nebular or vapor plume environment. 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). Formation of zoned metal grains occurred at high temperatures during a temperature interval of 1092°C to 987°C, which was then rapidly cooled. On the other hand, unzoned metal grains in CB_b chondrites formed at lower temperatures, were cooled more slowly, and 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). The CB_b chondrites have endured as some of the most pristine representatives of the nascent Solar System.

Silicates are present in the form of mm-sized cryptocrystalline and barred olivine 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 a vaporized region of the solar nebula (or alternatively, from a vapor plume created by the collision of two planetesimals) at pressures low enough to allow silicate to condense before metal. As an illustration of this scenario, some FeNi-metal grains contain inclusions of cryptocrystalline chondrules. After the condensation of chondrules and metal grains, they were radially transported by the solar wind to a colder, isolated region of the nebula prior to the condensation of volatile elements.

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 the 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 CAIs from the CB group 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 CB_b group as well as in the CH and CR groups, but none have been identified in the CB_a 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 CB_a 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). The CRE age of HaH 237 is calculated to be greater than 3 m.y.

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 specimen of HaH 237 shown above is a 1.1 g thin partial slice, while that shown below is a grand 76.33 g complete slice.

Specimen size ~ 123mm by 63mm
Photo courtesy of the Jay 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, from this link: