HaH 237

CBb, bencubbinite

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 fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If of 1997. This brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material consists of a mechanical mixture of silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the and metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn 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 matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents.. Compositionally and isotopically the bencubbinites are most similar to carbonaceous chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such, 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 chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine, 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 (9.6x that of lithophiles, relative to solar composition), leading to equilibriumTerm used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A system in static 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 cryptocrystallineCrypto meaning "hidden" refers to a rock texture in which individual crystals are too small to be distinguished even using a standard petrographic microscope. Crystals are typically less than a few μm in size - any smaller and the texture would be considered amorphous. Among sedimentary terrestrial rocks, chert and (CC) and barred olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion (BO) chondrules and chondruleRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine 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 REEOften abbreviated as “REE”, these 16 elements include (preceded by their atomic numbers): 21 scandium (Sc), 39 Yttrium (Y) and the 14 elements that comprise the lanthanides excluding 61 Promethium, an extremely rare and radioactive element. These elements show closely related geochemical behaviors associated with their filled 4f atomic orbital. 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 diffusionMovement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Because diffusion is thermally activated, coefficients for diffusion outward from the refractory siderophile-rich coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. at a total pressure of only 10 Pa (one ten-thousandth of a barUnit of pressure equal to 100 kPa.) (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 reductionOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants. 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 planetesimalsHypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many –, each composed of a core (~29 wt%), a CaO-, Al2O3-poor mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in (~57 wt%), and a CaO-, Al2O3-rich crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the (~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, isotropicSame in all directions. glass component is found within some chondrules, reflecting the unequilibrated type-3 nature of the meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and. Other shock-melted silicate glass (5–20 GPa) containing miniscule Fe–Ni–S metallic blebs occurs between metal and silicate fragments, similar to that present 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 inclusionsInclusions found predominantly in carbonaceous chondrites and are rich in refractory elements particularly calcium, aluminum and titanium that in various combinations form minerals such as spinel, melilite, perovskite and hibonite. There are two types of refractory inclusion: • Ca Al-rich inclusions (CAIs) • Amoeboid olivine aggregates (AOAs) Refractory inclusions were are a minor constituent in HaH 237, QUE 94411, and Gujba, but none have yet been found in Bencubbin or Weatherford. The CAIsSub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they 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 magnetiteFe oxide, Fe²⁺Fe³⁺2O4, containing oxidized iron (Fe³⁺) found in the matrix of carbonaceous chondrites and as diagnostic component in CK chondrites. In CK chondrites, magnetite is typically chromian, containing several wt. % Cr2O3., sulfides, and carbonates embedded within a hydrous phyllosilicate matrix composed of serpentineName used for a large group of phyllosilicate minerals with the generalized formula X2-3 Y2 O5 (OH)4. Due to their various structures (meteoritics focuses primarily on (Fe, Mg)3Si2O5(OH)4), serpentine can be used to understand the chemistry and progress of aqueous alteration (hydration) of olivine, amphibole, or pyroxene dating back to and minor smectite. These hydrated lithic clasts are very similar in composition to carbonaceous chondriteCarbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than 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 SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids., or through regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles).   gardening on the CB parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today..

Subsequent shock-lithification fused the porous, fine-grained matrix material that initially constituted the CBa chondrites (Meibom et al., 2004). The shock waveAbrupt perturbation in the temperature, pressure and density of a solid, liquid or gas, that propagates faster than the speed of sound. 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 kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word. Another N carrier is taeniteLess common than kamacite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Taenite, γ-(Fe,Ni), has 27-65 wt% Ni, and forms small crystals that appear as highly reflecting thin ribbons on the etched surface of a meteorite; the name derives from the Greek word for "ribbon.", 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 ppmParts per million (10⁶).) 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 chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such 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.

Specimen size ~ 123mm by 63mm
Photo courtesy of the J. Piatek Collection

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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.

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