CBa, bencubbinite
standby for weatherford photo
Found 1926, recognized 1940
35° 30′ N., 98° 42′ W. A mass of 2 kg was plowed up in Custer County, Oklahoma and later recognized as a meteorite. Weatherford is a primitive, polymict, chondritic breccia containing cm-sized clasts of a metal host (~60 vol%) and a mafic silicate host (~40 vol%), along with ordinary chondrite, R-chondrite, carbonaceous, and other xenolithic clasts.

One ordinary chondrite xenolith classified petrologically as >3.5 is composed of 85% chondrules of a size closest to that of H chondrites, as well as an FeNi and FeS content very similar to that of H chondrites. However, the O-isotopic composition is unlike that of any other ordinary chondrite group or a chondritic clast that was found in CBa Bencubbin. The unique type 3 carbonaceous xenoliths have been characterized as dehydrated CM-like chondritic material. Very small olivine-rich clasts resembling Rumuruti-like material have also been found. These xenoliths are highly equilibrated to type 6 and devoid of chondrules.

While refractory inclusions have only been found in the HaH 237, QUE 94411, and Gujba bencubbinites, and the transitional member Isheyevo, cm-size chondrule fragments of barred olivine composition occur in all group members. The metal in Weatherford records the effects of a late shock event ~0.5 b.y. after accretion (~4.2 b.y. ago) in which recrystallization and minor impact-melting occurred. It is proposed that this silicate melt phase, which itself contains tiny immiscible blebs of FeNi-metal, thereafter infiltrated the space between the metal and silicate fragments leading to reheating of the metal particles to temperatures of ~400°C (Chappell et al., 2011; Srinivasan et al., 2013). A significant component of these kamacite grains (~40%) have both homogenous and exsolved (the latter from late-stage impact reheating to <600°C) Cr-rich sulfide inclusions occurring in arcuate textures along metal grain boundaries, rarely containing metal blebs (Srinivasan et al., 2014). Host metal elemental abundance ratios are correlated with those of the silicate host phase, providing evidence for a common nebular reservoir origin for the two host phases; the FeNi-metal blebs likely formed through nebular condensation or silicate reduction processes (possibly initiated through impact shock heating).

All of the bencubbinites are characterized by a significant enrichment in planetary-type rare gases and isotopically heavy nitrogen (15N). Contrary to other bencubbinites, the main N-carrier phase in Weatherford is located in the shock-melted veinlets. A lesser source of heavy nitrogen, along with rare gases such as radiogenic 40Ar, has been found inside µm-sized vesicles within the silicate melt phase. The high oxide content within this vesicle-containing melt phase is consistent with fractionation processes resulting from a high-temperature shock event. This chemically reactive environment could lead to the release of N, creating the N- and Ar-rich vesicles.

Bencubbinites have been divided into two petrologic subgroups, CBa and CBb, representing those with cm-sized metal and silicate chondrules, and those with mm-sized chondrules. 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. The best current evidence supports an origin for these late-formed metal-rich carbonaceous chondrites in an impact plume generated by a hypervelocity collision between planetary embryos (Krot et al., 2009). The ‘Grand Tack’ model of Walsh et al. (2011) could have generated such high velocities; further details about the ‘Grand Tack’ scenario can be found in the Appendix Part III. Planetary modeling employed by Johnson et al. (2016) demonstrates that only during a relatively short timeframe within this migration period will impact velocities 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 accretion of CB chondrites from an impact-generated vapor plume, occurring ~4.8. m.y. after CAIs (Scott et al., 2018).

Following condensation of the various components, they were aerodynamically sorted according to their velocity, size, and density as they spread out into the local nebular gas in a typical fan-shaped pattern. It was calculated by Morris et al. (2012) that in ~1% of the impacts the host planetesimal would be propelled in the direction of the impact plume, sweeping up some of the aerodynamically sorted ejecta within a short time period measured in weeks. They reason that this re-accreted material would have been mixed with existing crustal components to form a layer many meters thick. See the HaH 237 page for a more detailed scenario of the CB group formation process ascertained by Fedkin et al. (2015) through kinetic condensation modeling.

The specimen shown above is a 2.54 g polished partial slice of Weatherford. This specimen was acquired from Steve Arnold (International Meteorite Brokerage) in 2000, who had obtained it through a trade with the Smithsonian Institution, and was subsequently traded to another collector in 2018. The photo below shows a larger 22 g slice of this rare bencubbinite in the J. Piatek Collection, acquired from the National Museum of Natural History, Smithsonian Institution. standby for weatherford photo
Photo courtesy of the J. Piatek Collection

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