BISON


LL6 Impact-Melt Breccia
(LL6 in MetBull DB)
standby for bison photo
Found 1958
38° 18' 24" N., 99° 42' 36" W.

Initially, a 3 kg fragment was reported to have been found in Rush County, Kansas, about 6.8 km N. of Bison (G. Huss, 1982). Subsequently, meteorite recovery field work conducted by Terry Schmidt led to the recovery of another 8 kg fragment, a mass which had been found by a farmer in 1938 a few miles from the later 3 kg mass find. This larger mass was stored by the farmer under a work bench in his garage during the intervening years.

Bison is an LL6 chondrite with a monomict composition of dark-colored, unmelted clasts containing highly shocked, relict chondrules, along with light-colored impact-melt breccia clasts devoid of chondrules, all embedded within an opaque, blackened host rock. This blackening is primarily the result of shock-induced microscopic troilite inclusions and veinlets interspersed throughout the matrix. The poikilitic texture, depletion of metal and sulfides, and high CaO content in orthopyroxene present in the light-colored clasts of Bison are all consistent with a rapid crystallization from a melt (Dominik and Bussy, 1994). A two-stage shock history has been suggested: 1) the first impact caused localized impact melting and brecciation; and 2) a subsequent impact produced features of shock stage S4. In an Ar–Ar dating study of two LL chondrites, Weirich et al. (2009) determined an age of ~1 b.y., possibly reflecting the last major impact on the LL chondrite parent body.

Bison has experienced a low degree of weathering. Shown above is a 13.79 g partial end section of Bison showing its brecciated structure.