Bencubbin

CBa, bencubbinite

Found July 30,1930
30° 45′ S., 117° 47′ E. A mass of 54.2 kg was found NW of Bencubbin, Western Australia by Fred Hardwick while plowing his farm. A second paired mass weighing 64.6 kg was found in 1959 in nearby Mandinga, and in 1974 an additional mass weighing 15.76 kg was recovered. Bencubbin is a primitive, polymict chondritic 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 Click on Term to Read More containing 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 Click on Term to Read More clasts (~60 vol%; 63 vol% for a sample as determined by heliumHelium (He) Second lightest and second most abundant element (after Hydrogen) in the universe. The most abundant isotope is 4He (99.9998%), 3He is very rare. Helium comprises ~8% of the atoms (25% of the mass) of all directly observed matter in the universe. Helium is produced by hydrogen burning inside Click on Term to Read More pycnometry, Consolmagno et al., 2007), achondritic 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 clasts, and chondritic xenoliths, all of which are fused together by a glass–metal-melt. The often rounded metal clasts, which occur in sizes up to ~10 mm, are aggregates of sub-mm-sized 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 Click on Term to Read More and sulfide grains that have been sintered together. These clasts show evidence of fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. Click on Term to Read More based on volatility-controlled processes. Other sub-mm- to mm-sized metal grains also occur. The silicate clasts in Bencubbin have skeletal 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 Click on Term to Read More or 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 Click on Term to Read More textures, and lack FeNi-metal inclusions, possibly attesting to their formation prior to FeNi-metal condensation at the source region.

The xenolithic chondritic inclusions include an LL-type, L-type, CR-type, and R-type, E-type, all likely constituents of a 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).   Click on Term to Read More. In addition, a unique dark inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. Click on Term to Read More (DI) has been studied in Bencubbin which shows some similarities to CM and CO 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 Click on Term to Read More, with an O-isotopic composition similar to that of CM and CR chondrites. However, many unusual features of this DI (e.g., ‘flame-like’ structures) indicate that it is a new type of primitive material possibly reflecting impact-generated sedimentary processes (Nehru et al., 2014).

Bencubbinites contain the heaviest N found in any 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 Click on Term to Read More (up to δ15N ~+1500‰, where δ15N is the deviation in parts per thousand relative to the atmospheric 15N/14N ratio), an enrichment likely having an interstellar origin. Other studies attribute the source of the heavy N to N2 self-shielding or low-temperature ion-molecule reactions which occurred either in the protosolar molecular cloudAn interstellar gas cloud that is dense enough to allow the formation of molecules and comprised of a cold dense complex mixture of interstellar gas and dust roughly 75% hydrogen and 21-24% helium. Clouds contain trace amounts of other molecules, of which well over 100 different types have now been Click on Term to Read More or in the protoplanetary diskFlattened and rotating disk of dense gas and dust/solids orbiting a young star from which planets can eventually form.   Click on Term to Read More. This 15N component is now considered to have been present on the Bencubbin 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. Click on Term to Read More prior to the major shock event that produced the silicate melt, probably residing in the now destroyed hydrated matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More lumps (Perron et al., 2008). As with the water component, this heavy N is now present within vesicles that are located in silicate clasts, in the mesostasisLast material to crystallize/solidify from a melt. Mesostasis can be found in both chondrules, in the matrix around chondrules, and in achondrites as interstitial fine-grained material such as plagioclase, and/or as glass between crystalline minerals.   Click on Term to Read More of chondritic inclusions, at the edges of metal, and in grains of mesostasis thought to be derived from 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 Click on Term to Read More of the chondritic inclusions (the latter grains are referred to as ‘bubble grains’ by Perron et al., 2008). The evidence that these bubble grains originated in the chondritic inclusions is provided by their similar elemental and isotopic compositions, and by their close proximity to the chondritic inclusions; however, other features still require an adequate explanation. Further information on the hydrated lithic clasts and the heavy N enrichment can be found on the Isheyevo page.

Based on the Perron et al. (2008) model, the formation of the vesicles occurred when water and 15N-bearing organics, derived from the hydrated clasts, were degassed during the impact of a chondritic object onto the Bencubbin parent body, which probably arrived from the outer Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids.. The 15N and water became dissolved in the low-temperature melt phases local to the impact, and further degassing left bubbles as the melt phase solidified. Evidence shows that vesicleVesicles appear in nature when they are produced within lava (extrusive aphanitic igneous rock) whose dissolved gases come out of solution (are released) due to the drop in pressure during an eruption. The resulting lava solidifies around the gas bubbles capturing their shape inside and outside the rock. Vesicles do Click on Term to Read More formation had to occur prior to the major impact event which produced the silicate glass–metal-melt that fused all of the components together. Bencubbin has a low average porosityThe volume percentage of a rock that consists of void space. Vesicular porosity is a type of porosity resulting from the presence of vesicles, or gas bubbles, in igneous rock such as the pumice presented here. Vesicular porosity is very rare in meteorites and is often associated with slag, one Click on Term to Read More of 3.9% (Macke et al., 2011).

The Bencubbin metal component records the effects of at least two late shock events during which recrystallization and minor differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities.   When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including Click on Term to Read More occurred. It was during one of these events that shock-melted silicate glass containing miniscule Fe–Ni–S metallic blebs was produced from the existing porous aggregate of clastic material. Perron et al. (2008) calculated that the precursor material of this silicate glass was composed of approximately equal proportions of silicate clasts and hydrated clasts, of which the latter were composed of water-bearing phyllosilicatesClass of hydroxyl-bearing silicate minerals with a sheet-like structure. They result from aqueous alteration are dominantly serpentine and smectite in meteorites; found in the matrixes of carbonaceous chondrites. Phyllosilicates consist of repeating sequences of sheets of linked tetrahedra (T) and sheets of linked octahedra (O). The T sheet consists of Click on Term to Read More; these phyllosilicates were likely the source of the oxidationOxidation 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 Click on Term to Read More that is observed in the melt phase (high FeO). This silicate glass has welded together the various components of this 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 Click on Term to Read More during an impact-heating event, calculated by K–Ar chronometry to have occurred 4.2 (±0.05) b.y. ago (Marty et al., 2010). Another evaluation by Trinquier et al. (2008) based on Mn–Cr systematics revealed that impact-related metamorphism on the CB parent body occurred much earlier in Solar SystemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with history, at 4.5649 (±0.0040) b.y. ago. Sub-µm- to µm-sized diamonds are present within both metal and silicate portions and along their boundaries, mostly associated with shocked graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More. They provide evidence of high-shock events with corresponding pressures reaching at least 15–20 GPa.

Previously published studies (31st LPSC [2000]) designed to locate the source of the heavy 15N in Bencubbin are in agreement with the model described above. Some of the heavy nitrogenPrincipal constituent of the Earth’s atmosphere (78.08 vol. % at ground level). Nitrogen is the fifth most abundant element in the universe by atom abundance. Nitrogen comprises only 3.5 vol. % of the atmosphere of Venus and 2.7 vol. % of Mars’s atmosphere. Nitrogen has two isotopes: 14N (99.632 %) and 15N Click on Term to Read More, along with rare gases such as radiogenic 40Ar, were found to reside in µm-sized vesicles associated with the silicate melt phase. However, rather than implicating the hydrated clasts as the source of the heavy N and the oxidation as proposed above, it was hypothesized that the high oxide content within the vesicle-containing silicate melt phase was most consistent with fractionation processes occurring as a consequence of a high-temperature shock event. This chemically reactive environment could have led to the release of N, creating the N- and Ar-rich vesicles. In suceeding studies utilizing micro-infrared spectroscopyTechnique of splitting electromagnetic radiation (light) into its constituent wavelengths (a spectrum), in much the same way as a prism splits light into a rainbow of colors. Spectra are not smooth but punctuated by 'lines' of absorption or emission caused by interaction with matter. The energy levels of electrons in, Guilhaumou et al. (2006) first discovered the water present in the vesicles and melt phase, and this team suggested that the vesicles were formed when water from the hydrated matrix was degassed during a shock event.

It was proposed in earlier studies that the heavy nitrogen was likely incorporated in the Bencubbin parent body directly from an isotopically heterogeneous region of the solar nebulaThe primitive gas and dust cloud around the Sun from which planetary materials formed.. The occurrence of N-rich material in 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." Click on Term to Read More located at the sulfide-metal boundary and in the molten metal phase, shows that N was mobilized during shock heating and then redistributed during the later cooling stage. For this to be true, the N carrier would not be a pristine presolar component. The discovery of hydrated matrix lumps in other meteorites of the CR clan containing organics and phyllosilicates is consistent with these previous conclusions. The hydrated clasts which were once a component in Bencubbin were likely destroyed in a major impact-heating event.

The siderophile elemental trends reflected in the metal clasts of Bencubbin are consistent with higher partial pressures than those associated with typical nebular formation models (Fedkin et al., 2014). Instead, a model consistent with the known properties of Bencubbin supports a formation within a highly siderophile-enriched impact vapor plume produced in a collision between a metal-rich chondritic body and a reducedOxidation 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 Click on Term to Read More silicate (low-FeO) body (Campbell et al., 2001). This catastrophic collision between two molten 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 – Click on Term to Read More (or a hypervelocity impact between two solidified objects) occurred within the first few m.y. of solar system history. The impact produced a high-temperature metal-enriched gas, from which accreted the CB-chondrite daughter object. Results of computations and modeling by Fedkin et al. (2014, 2015) indicate that both of these colliding planetesimals could have been differentiated CR-type chondritic planetesimals composed of a 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. Click on Term to Read More, 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 Click on Term to Read More, 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 Click on Term to Read More, along with significant hydrous materials. A more detailed scenario of the formation environment for the bencubbinites was ascertained through kinetic condensation modeling by Fedkin et al. (2015), a synopsis of which can be found on the HaH 237 page.

It has been posited that after Jupiter had grown to a massive size (>50 M_⊕) by ~4 m.y. at an initial heliocentricCentered around a sun. Our own Solar System is centered around the Sun so that all planets such as Earth orbit around the Sun. Note that 25% of Americans incorrectly believe the Sun revolves around the Earth. Click on Term to Read More distance of ~3 AUThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The Click on Term to Read More, it underwent a chaotic migration in a 3:2 (or 2:1) resonance with Saturn—first inward for ~100,000 years to ~1.5–2.0 AU while clearing the inner disk of planetesimals, and then outward for ~4–5 m.y. years to its current location near 5.2 AU (‘Grand Tack’ scenario of Walsh et al., 2011; Johnson et al., 2016; Brasser et al., 2016). Planetary modeling employed by Johnson et al. (2016) demonstrates that only during a relatively short timeframe within this migration period will dynamical excitement produce impact velocities that reach levels high enough (>18 [±5] km/s) to vaporize Fe in a planetesimal core. It is notable that the timing of the inward migration of Jupiter and Saturn is consistent with the timing of the accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the Click on Term to Read More of CB chondrites from an impact-generated vapor plume, occurring ~4.8. m.y. after 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 Click on Term to Read More (Scott et al., 2018). It has been determined that the zoned FeNi-metal grains present in CB chondrites were derived from core material of a CR chondriteClass named for the Renazzo meteorite that fell in Italy in 1824, are similar to CMs in that they contain hydrous silicates, traces of water, and magnetite. The main difference is that CRs contain Ni-Fe metal and Fe sulfide that occurs in the black matrix and in the large chondrules Click on Term to Read More parent body. Kruijer et al. (2017) have demonstrated through coupled Mo- and W-isotopic diagrams (see below) that the CR parent body accreted in a reservoir beyond Jupiter, in the outer protoplanetary disk, and that group IIC irons are also associated with this carbonaceous reservoir. Compared to all other meteorite groups, only CR chondrites and IIC irons share certain characteristics such as i) significant δ183W excesses, ii) elevated δ15N, and iii) Mo isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: ¹H, ²H (deuterium), and ³H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More systematics; therefore, a genetic link is inferred (Kruijer et al., 2017; Budde et al., 2018). Further details about the ‘Grand Tack’ scenario and the carbonaceous and non-carbonaceous reservoirs can be found in the Appendix Part III.
Diagrams credit: Kruijer et al., PNAS, vol. 114, #26, p. 6713 (2017)
‘Age of Jupiter inferred from the distinct genetics and formation times of meteorites’
http://dx.doi.org/10.1073/pnas.1704461114 The identification of CAIs in HaH 237, QUE 94411, Gujba, and Isheyevo is more consistent with a primitive origin, i.e., condensation from a nebular gas. These CAIs are isotopically (26Al-poor) and mineralogically distinct (grossite- and hibonite-rich) from those of other chondrites, which supports the proposition that the CB chondrites, CH chondrites, and Isheyevo were derived from a common nebular reservoir. Still, investigations into the I–Xe systematics of the CB group indicate that the chondrules were formed in a high temperature environment ~100 m.y. after the solar system began, more consistent with an origin through impact rather than within the solar nebulaAn immense interstellar, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense and accrete. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive Click on Term to Read More (Whitby et al., 2003).

Raman spectra results for Gujba have led to the identification of the first occurrence in a 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 Click on Term to Read More of several high pressure phases located within barred olivine fragments and matrix components; these include majorite garnetMineral generally found in terrestrial metamorphic rocks, although igneous examples are not uncommon. Garnet is a significant reservoir of Al in the Earth's upper mantle. The garnet structure consists of isolated SiO4 tetrahedra bound to two cation sites. The A site holds relatively large divalent cations (Ca2+, Mg2+, Fe2+, Mn2+); the Click on Term to Read More, majorite-pyrope solid solutionCompositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with, and wadsleyiteHigh pressure polymorph of olivine, β-Mg2SiO4, found on Earth and in some meteorites. It is thought to make up 50% or more of Earth's mantle between depths of 400 and 525 km. Wadsleyite transforms into ringwoodite at high pressure, but the exact pressure depends strongly on composition. At lower pressures,, along with minor grossular-pyrope solid solution and coesiteHigh-pressure polymorph of silicon dioxide (SiO2). Has the same chemical composition as cristobalite, stishovite, seifertite and tridymite but possesses a different crystal structure. Coesite forms at intense pressures of above about 2.5 GPa (25 kbar) and temperature above about 700 °C, and was first found naturally on Earth in impact Click on Term to Read More (Weisberg and Kimura, 2010). These high pressure phases formed either through solid-state transformation of pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. Click on Term to Read More, or through crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More from an impact melt during a heterogeneous planetesimal-wide impact shock event; this impact involved minimum pressures of ~19 GPa and temperatures of ~2000°C. The investigators argue that these high pressure phases are inconsistent with the subsequent formation of chondrules within an impact-generated, gas-melt plume since at such high temperatures these phases would be rapidly back-transformed to their low temperature polymorphs. Moreover, the measured cooling rates of chondrules (ave. 100K/hr) are much too slow than that at which shock veins with high pressure polymorphs would survive (~1000K/hr). Therefore, they determined that the barred chondrules and metal in CB chondrites were formed prior to the impact event which produced the high pressure polymorphs in Gujba.

The designation of a new primitive, metal-rich chondrite grouplet, the CB chondrites, was first proposed in the paper A new metal-rich chondrite grouplet, by Weisberg et al. (2001). The bencubbinites are represented by a relatively small number of samples, with Gujba as the only observed 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 Click on Term to Read More. With the exception of Fountain Hills, which is anomalous in several of its characteristics, the bencubbinites have similar oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read More and nitrogen isotopic compositions and petrologic characteristics, including shock histories. They have highly reduced silicates, metal abundances of 60–70 vol%, Cr-bearing troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More, metal with near solar Ni/Co ratios, and similar elemental abundances.

A study of the CBa Fountain Hills (image: Fountain Hills) by La Blue et al. (2004), has led to the consideration of this bencubbinite as a transitional type between the CB chondriteClass also known as bencubbinites, are named after the Bencubbin meteorite found in 1930 in Australia. Only a handful of these strange meteorites are known, all composed of >50 vol. % Ni-Fe metal, together with highly reduced silicates, and chondrules similar to those found in the CR group. Click on Term to Read More group and the genetically related CR chondrite group. Fountain Hills has an identical O-isotopic composition to other bencubbinites with a similar metal and silicate composition, but it has experienced the least amount of metal-silicate fractionation. Despite its similarities to the CBa subgroup, it exhibits several important features that distinguish it from both of the bencubbinite subgroups. Fountain Hills contains a large abundance of relatively small, sometimes armored porphyritic chondrules, a feature it shares with CR chondrites. In addition, it contains large barred-olivine chondrules and smaller pyroxene-rich chondrules of radial and granular textures (Lauretta et al., 2009). This diversity of 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 Click on Term to Read More types has been attributed to variations in peak temperatures of the chondrule precursor material; e.g., porphyritic chondrules experienced incomplete melting of precursor material, whereas barred chondrules crystallized from a completely molten precursor. Calculations of peak temperatures and heating duration during formation of Fountain Hills was presented by Lauretta et al. (2009). They determined a peak temperature range of between 878°C and 535°C, commensurate with a heating duration ranging from ~2,000 yr to ~10 m.y., respectively.

Fountain Hills has a significantly lower content of metal than other bencubbinites—~25 vol% compared to the typical 60–70 vol%—which might be the result of gravitational draining following impact heating/melting. It contains large 2–3 mm-sized olivine phenocrysts that likely crystallized from such a melt. Unique to the bencubbinites, Fountain Hills has a partially recrystallized texture, comparable to a petrologic type-4 ordinary chondriteWork in Progress Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% of all falls (Meteoritical Society database 2018)1. Meteorites & the Early Solar System: page 581 section 6.1 OC of type 5 or 6 with an apparent shock stage of S1, Click on Term to Read More. It exhibits general shock features consistent with S2–S3, but some as high as S4, suggesting a history of shock, burial, and long duration annealing. Notably, only in Fountain Hills does metal occur interstitialTerm applied to ions or atoms occupying sites between lattice points. Click on Term to Read More to the silicates rather than as separate metal clasts, and metal is present within the silicate chondrules (as sub-micron-sized inclusions) as well. The occurrence of spinelMg-Al oxide, MgAl2O4, found in CAIs. is also unique. Furthermore, although it has an O-isotopic composition indistinguishable from CBa members, it has N-isotopic systematics that are significantly different from the other bencubbinites. The δ15N values in Fountain Hills (48‰) are much lower than in both the CBa (1000‰) and CBb (200‰) subgroups; the value is actually much closer to that of the CR chondrites (Weisberg and Ebel, 2009 and references therein). Based on all of these findings, both similarities and differences with CB chondrites, it was proposed that the porphyritic chondrules in Fountain Hills may have been formed in a high-temperature, high-pressure region of the nebula from an impact-induced partial melt phase of an earlier generation of CB chondrite material. A portion of the metallic melt was removed along with the sulfides, and these depletions may be the cause of the anomalous 15N values (Weisberg and Ebel, 2005).

In a study of the shock-modified bencubbinite Fountain Hills in particular, and all bencubbinites in general, Weisberg and Ebel (2009) discussed the pronounced impact-related characteristics of this meteorite and the other CB members as they attest to a major planetary-scale collision early in the formation history of the bencubbinite object. They presented an abundance of evidence showing that Fountain Hills experienced impact shock forces greater than those observed in any other chondritic body, and they contrasted this severe impact with the hypothesized collision on Mercury—an impact which is considered to have stripped away much of its original mantle. They argue that the general characteristics of bencubbinites, i.e., metal-rich, refractory-rich, and volatile-depleted, are consistent with its formation in the innermost Solar System, possibly near the orbitThe elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around Click on Term to Read More of Mercury. Furthermore, they contend that the bulk composition of bencubbinites shows some similarities to Mercury as well. It may be speculated that bencubbinites formed from a vapor plume that was produced by a massive collisional impact on Mercury. A possible link between this carbonaceous group and Mercury will be the subject of future investigations through data gathered by the MESSENGER spacecraft.

In a study by Weisberg et al. (2001), the bencubbinites were divided into two petrologic subgroups, CBa and CBb, representing those with cm-sized metal and silicate clasts (e.g., Bencubbin, Fountain Hills, Gujba, NWA 1814, NWA 4025, Weatherford), and those with mm-sized clasts (e.g., HaH 237, QUE 94411). Based on precise I–Xe and U–Pb systematics, the chondrules in Gujba (CBa) and HaH 237 (CBb) were found to have formed simultaneously ~4.5621 b.y. ago (Pravdivtseva et al., 2016). See the HaH 237 page for details on the accurate determination of formation ages for these meteorites.

Gujba has a 21Ne-based CRE age of 27 m.y., similar to the CRE age of Bencubbin (~39 m.y.) and Isheyevo (~34 m.y.), attesting to a common ejection event. However, while the metal and silicate clasts in Gujba are mostly complete spheres, those in Bencubbin and Weatherford are fragmented and distorted; both clastA mineral or rock fragment embedded in another rock. Click on Term to Read More types in both meteorites exhibit a preferred orientation as a result of a deformation event.

This newly designated CB carbonaceous chondrite group, along with the CH and CR groups, has been considered to constitute the CR clan. Other meteorites presently classified as metachondrites and achondrites have O-isotopic compositions that plot within or near the CR field, and may eventually be shown to belong to this clan. Further information about the genetic relationship between the CB and CH groups and the transitional member Isheyevo can be found on the Isheyevo page. The specimen of Bencubbin pictured above is a 6.3 g polished partial slice. Pictured below is a large slice of Bencubbin on display at the United States National Museum, Smithsonian Institution.

click on photo for a magnified view