BENCUBBIN

A mass of 54 kg was found NW of Bencubbin, Western Australia, by Fred Hardwick while plowing his farm. A second paired mass weighing about 64.5 kg found in 1959 in nearby Mandinga. 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, 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 chondritic inclusions consist of an LL-type, an L-type, a CR-type, an R-type, and a black metamorphosed-type, along with rare enstatite clasts. Silicate clasts in Bencubbin have skeletal olivine or cryptocrystalline textures, but they lack FeNi-metal inclusions, possibly attesting to their formation prior to FeNi-metal condensation at the source region.

Bencubbinites contain the heaviest N found in any chondrite (δ15N ~1000%), an enrichment likely having an interstellar origin. This 15N component is now considered to have been present on the Bencubbin parent body prior to the major shock event which 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, 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.

Based on the Perron et al. (2008) model, the formation of the vesicles occurred when water and 15N-bearing organics from the hydrated clasts were degassed during the impact of a chondritic object onto the Bencubbin parent body. The 15N and water were then dissolved in the low-temperature melt phases local to the impact, and finally degassed again, leaving 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.

The metal 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, the latter of which were composed of water-bearing phyllosilicates that were likely the source of the oxidation (high FeO) observed for the melt phase. This silicate glass has welded together the various components of this meteorite, and it has been calculated that this process occurred 3.7–4.0 b.y. ago (Kelly and Turner, 1987). Another evaluation by Trinquier et al. (2008) based on Mn–Cr systematics found that impact-related metamorphism on the CB parent body occurred much earlier in Solar System history, 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 shock events with pressures of at least 15–20 GPa.

Previously published studies (LPSC XXXI) 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 revealed 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 at the sulfide-metal boundary, as well as in the molten metal phase, shows that N was mobilized during shock heating and redistributed during the later cooling stages. For this to be true, the N carrier would not be a pristine presolar component. The discovery of hydrated matrix lumps in other meteorites in the CR clan containing organics and phyllosilicates is consistent with these previous findings. The hydrated clasts that were once a part of Bencubbin must have been destroyed in a major impact-heating event.

The siderophile elemental trends in the metal clasts demand higher pressures than those associated with nebular models. 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), one of which may have been composed of hydrous materials formed at low temperatures. This impact likely occurred between two molten planetesimals within the first few m.y. of solar system history. Alternatively, such a high-temperature metal-enriched gas may have been created by the hypervelocity impact of two solidified bodies. Yet, 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. 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).

The designation of a new, primitive, metal-rich chondrite grouplet, the CB chondrites, was proposed in the paper A new metal-rich chondrite grouplet, by Weisberg et al. (2001). The bencubbinites are represented by only a small number of samples, among them are Bencubbin, Weatherford, HaH 237, QUE 94411, Isheyevo, NWA1814, NWA 4021, Fountain Hills, the provisional and anomalous NWA 5492, the only observed fall Gujba, and several Antarctic specimens. 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 CB_a 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 CB_a subgroup, it exhibits several important features which distinguish it from both 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 range of ~2,000 y. and ~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 may be the result of gravitational draining after being melted by impact heating. It contains large 2–3 mm-sized olivine phenocrysts that may have 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.

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 submicron-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 they are in both the CB_a (1000‰) and CB_b (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 severe impact-related characteristics of this meteorite and the other CB members as they attest to a major collision early in the 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 contrast this severe impact with the hypothesized collision on Mercury that 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, 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 cloud 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 the study by Weisberg et al. (2001), the bencubbinites were divided into two petrologic subgroups, CB_a and CB_b, representing those with cm-sized metal and silicate clasts (e.g., Bencubbin, Weatherford, NWA 1814, Fountain Hills, Gujba, and NWA 4025), and those with mm-sized clasts (e.g., HaH 237 and QUE 94411), respectively. The Gujba meteorite has an identical CRE age (27 m.y.) to that of Bencubbin, 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 which 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 40.6 g slice in the J. Piatek Collection, acquired in trade from the United States National Museum, Smithsonian Institution.

Photo courtesy of the J. Piatek Collection