Pallasite, PMG-am (main-group, anomalous metal composition)
Found 1882, known before
37° 36' N., 99° 12' W. approx.
Soon after Frank and Mary Kimberly made their home on the Kansas prairie in Kiowa County, Mary began collecting the black iron rocks she found around the farm believing that they resembled the meteorite she had seen as a child. Then for years, she tried to have her large pile of iron rocks authenticated by a professional, without success. Finally, a geologist from Washburn College, Dr. Cragen, recognized them as meteorites and purchased a large portion of her collection. This began a profitable business for the Kimberly family selling meteorites from their "Kansas Meteorite Farm".
In 1929, Harvey Nininger explored an old wallow on the Kimberly farm and discovered that it was a 36 x 55-foot rimmed impact crater containing numerous pallasite masses. A total weight of over 1.5 tons of this weathered pallasite has been recovered in and around the Kimberly farm during the past years, many individual masses weighing hundreds of pounds. A half-ton mass was unearthed in 1948 near the Kimberly farm. Due to an extended period buried in moist soil, much of the material has become oxidized, and the completely oxidized fragments were named meteorodes by Nininger.
Brenham iron artifacts, including hand tools and personal ornaments, have been found associated with the Hopewell Mounds in Ohio, which are located about 1,400 km to the east of the Brenham crater. The Hopewell Indians, whose culture ended about 500 B.C., considered this material sacred, and many meteoritic objects were used ceremonially. Objects were found on the alters of several of the mounds, and some were associated with human skeletons.
In a 3-dimensional study of Brenham utilizing Electron Tomography Segmentation, it was revealed that all of the olivine is interconnected, as is most of the FeNi-metal (Spinsby et al., 2008). The remaining constituents, troilite and schreibersite, occur as a large but discontinuous network with some isolated pockets. The volume of olivine in their sampling (67%) approximates well the volume expected given a scenario of accumulation through settling from an established silicate layer. It was presumed these olivine crystals then recrystallized to form a continuous network. Thereafter, troilite and schreibersite were infused throughout the solid olivine matrix by a late-stage sulfidephosphide-bearing melt prior to complete solidification of the pallasite.
Since cooling rates are the same for both rounded and angular olivines among main-group pallasites, and Brenham metal is also low in Ir, Yang et al. (2010) proposed that the olivines were annealed near the core-mantle boundary prior to the glancing impact that disrupted the body, and thereafter the residual molten metal from the outer core solidified around the rounded olivines. Alternatively, Boesenberg et al. (2012) argue that the rounding of the olivine crystals occurs primarily from resorption at high temperatures (above ~1250°C) in the presence of silicate melt. Formation scenarios and classification schemes for the main-group pallasites can be found on the Imilac page.
Studies of Brenham involving cosmogenic nuclide production rates have resolved a CRE age of 156 (±8) m.y., a pre-atmospheric radius of ~46 m, and a minimum pre-atmospheric mass of ~500 (±100) tons, of which 6,800 kg have been recovered (Honda et al., 2002; Kollar et al., 2003). Homma and Iizuka (2018) obtained a HfW model age for Brenham of 2.0 (±2.9) m.y. after CAIs, which is consistent with the ages of other main-group pallasites derived through other isotopic chronometers. In addition, Homma and Iizuka (2018) recognized that the age accords well with other magmatic iron groups, and the ε183W value indicates an accretion location in the non-carbonaceous (inner solar system) reservoir.
Based on recent excavations from Pleistocene strata, the terrestrial age of Brenham has been estimated to be 10 t.y. (R. Hegeman, Associated Press, 2006). The photo above is a partial slice of Brenham weighing 42.4 g. The olivines near the outer edge have become darkened, possibly as a result of weathering or by the heat of atmospheric entry, while those farther away from this zone maintain their orange color. The photo below shows a dinner plate-sized, 2,100 g slice containing virtually no olivines that was acquired by Jay Piatek.
Photo courtesy of the Dr. J. Piatek Collection
On Sunday, October 16, 2005, after securing lease agreements to search the properties encompassing the Brenham strewnfield, meteorite hunter Steve Arnold and geologist/attorney Phil Mani instituted a search by metal detector on the farm of Allen and Mary Binford. Prompted by a strong signal, they dug down to find an oriented 1,430 pound (650 kg) Brenham mass buried 88 inches underground. This is the largest Brenham mass recovered to date, dubbed King of the Pallasites. Many additional masses continue to be unearthed by the team. The photos below show the final stage of its excavation, and below that, a close-up of Steve Arnold with his prize (Allen Binford behind).