BENCUBBIN


CBa, bencubbinite
standby for bencubbin photo
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 breccia containing metal clasts (~60 vol%; 63 vol% for a sample as determined by helium pycnometry, Consolmagno et al., 2007), achondritic silicate 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 kamacite and sulfide grains that have been sintered together. These clasts show evidence of fractionation based on volatility-controlled processes. Other sub-mm- to mm-sized metal grains also occur. The silicate clasts in Bencubbin have skeletal olivine or cryptocrystalline 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 regolith. In addition, a unique dark inclusion (DI) has been studied in Bencubbin which shows some similarities to CM and CO chondrites, 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 chondrite (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 cloud or in the protoplanetary disk. This 15N component is now considered to have been present on the Bencubbin parent body prior to the major shock event that produced the silicate melt, probably residing in the now destroyed hydrated matrix 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 mesostasis of chondritic inclusions, at the edges of metal, and in grains of mesostasis thought to be derived from chondrules 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 system. 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 vesicle 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 porosity 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 differentiation 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 phyllosilicates; these phyllosilicates were likely the source of the oxidation that is observed in the melt phase (high FeO). This silicate glass has welded together the various components of this meteorite 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 System history, at 4,564.9 (±4.0) m.y. ago. Sub-µm- to µm-sized diamonds are present within both metal and silicate portions and along their boundaries, mostly associated with shocked graphite. They provide evidence of high-shock events with corresponding pressures reaching at least 15–20 GPa.

Previously published studies (LPSC XXXI [2000]) designed to locate the source of the heavy 15N in Bencubbin are in agreement with the model described above. Some of the heavy nitrogen, 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 spectroscopy, 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 posited in earlier studies that the heavy nitrogen was likely incorporated in the Bencubbin parent body directly from an isotopically heterogeneous region of the solar nebula. The occurrence of N-rich material in taenite 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 reduced silicate (low-FeO) body (Campbell et al., 2001). This catastrophic collision between two molten planetesimals (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 core, a CaO-, Al2O3-poor mantle, and a CaO-, Al2O3-rich crust, 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.

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 nebula (Whitby et al., 2003).

Raman spectra results for Gujba have led to the identification of the first occurrence in a carbonaceous chondrite of several high pressure phases located within barred olivine fragments and matrix components; these include majorite garnet, majorite-pyrope solid solution, and wadsleyite, along with minor grossular-pyrope solid solution and coesite (Weisberg and Kimura, 2010). These high pressure phases formed either through solid-state transformation of pyroxene, or through crystallization 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. 100°K/hr) are much too slow than that at which shock veins with high pressure polymorphs would survive (~1000°K/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 fall. With the exception of Fountain Hills, which is anomalous in several of its characteristics, the bencubbinites have similar oxygen and nitrogen isotopic compositions and petrologic characteristics, including shock histories. They have highly reduced silicates, metal abundances of 60–70 vol%, Cr-bearing troilite, 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 chondrite 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 chondrule 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 y. 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 chondrite. 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 interstitial 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 spinel 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 orbit 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, Weatherford, NWA 1814, Fountain Hills, Gujba, NWA 4025), 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 clast 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.

smithsonian bencubbin
click on photo for a magnified view