WEATHERFORD

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, carbonaceous, and R chondrite 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 of the clast found in Bencubbin. The unique type-3 carbonaceous xenoliths have been characterized as dehydrated CM-type 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 formation, in which recrystallization and minor impact-melting occurred. Silicate glass containing tiny immiscible blebs of FeNi-metal was produced between the metal and silicate fragments. Host metal elemental abundance ratios are correlated with those of the silicate host phase, providing evidence for a common nebular origin for the two host phases; the FeNi-metal formed through nebular condensation or silicate reduction processes.

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.

Recent studies provide evidence in support of both a fractionation origin in a high-temperature, high-pressure shock event, and a condensation origin in a primitive nebula or impact vapor plume. A detailed discussion of these possible formation models can be found on the Bencubbin and Isheyevo pages.

The bencubbinite group currently consists of only a small number of samples, among these are Bencubbin, Weatherford, HaH 237, QUE 94411, Isheyevo, NWA 1814, NWA 4021, Fountain Hills, the only observed fall in 1984, Gujba, and several Antarctic meteorites. The bencubbinites have been divided into two petrologic subgroups, CB_a and CB_b, representing those with cm-sized metal and silicate chondrules (Bencubbin, Weatherford, NWA 1814, NWA 4021, Gujba, and Fountain Hills [the latter perhaps best described as transitional between CB and CR]), and those with mm-sized chondrules (HaH 237, QUE 94411, and Isheyevo), respectively.

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 specimen shown above is a 2.54 g polished partial slice of Weatherford made available by International Meteorite Brokerage through a trade with the Smithsonian Institution. The photo below shows a more representative 22 g slice of this rare bencubbinite acquired by its owner from the National Museum of Natural History, Smithsonian Institution.

Photo courtesy of the J. Piatek Collection