A 36.5 kg mass was found about 0.75 miles from Eagle Station, Carroll County, Kentucky. Eagle Station has the highest fayalite and Ni contents of all other pallasites, while Cold Bay and Itzawisis have nearly the same levels. In consideration of these and other anomalous elemental ratios (e.g., high Ge/Ga, high Ni, and high Ir), along with unique O-isotopic ratios, these three pallasites define a grouplet distinct from the pallasites of the main-group, the pyroxene grouplet, and the ungrouped Milton. Two very closely related silicated irons, Bocaiuva and Northwest Africa 176, share many compositional and O-isotopic similarities with the Eagle Station pallasites, and probably originated from similar chondritic material in the same region of the solar nebula.
Dispersed throughout the metal matrix of Eagle Station are angular, highly fragmented, cm-sized olivine crystals, intermixed with sharp, irregular, sub-mm-sized olivine splinters. A multi-stage formation history is proposed in which an initial impact generated enough heat to form a melt, followed by gravitational differentiation into metal and cumulus silicates. A subsequent impact shattered the olivine and mobilized the metal, which flowed into existing cracks. Later deformational events produced shock forces that incorporated angular shards of olivine, schreibersite, and chromite into melted troilite. Rapid cooling at shallow depth resulted in only a slight rounding of edges on olivine crystals, a thermodynamic process that minimizes the capillary forces along the olivine–metal interface (Saiki et al., 2003). A more technical theory of main-group pallasite origins can be found on the Imilac page.
The three Eagle Station pallasites are confidently resolved from main-group pallasites in having higher Ni, Ge, Ir, Co, Re, Pt, and Cu contents, and lower As, Au, and Ga in the metal. They also have higher Fe and Sc, and lower Mg and Mn contents in the silicates compared to the main-group. These elemental compositions, along with the O-isotopic ratios, are similar to those in group IIF irons and the CO–CV carbonaceous chondrites, particularly Felix (CO3) and Tibooburra (CV3). Of particular interest, the O- and Cr- isotopic signatures of Eagle Station have been used to establish a formation age of 4.557 (±0.6) b.y., and to support a genetic relationship with the CV chondrite parent body (see the Allende page for further details).
Application of the Hf–W isotopic chronometer to Eagle Station reveals that core formation occurred relatively late, ~10 m.y. after differentiation of the HED parent body 4 Vesta (Dauphas et al., 2005). It is calculated that radiogenic heat-generated melting and core–mantle differentiation should cease by ~7–8 m.y. (Sahijpal et al., 2007). This implies that heating of the Eagle Station asteroid continued until after all radiogenic 26Al and 60Fe was extinct, and that this late heating may have been generated through large impact events. Alternatively, the estimated initial Solar System ratio of 60Fe/56Fe may have been higher than previously considered leading to conditions conducive to a more prolonged core–mantle differentiation. Utilizing three methods, Yang et al. (2010) determined the cooling rate of the Eagle Station grouplet at ~15 °K/m.y., near the rate of the fastest cooled main-group pallasites. The CRE age of Eagle Station was determined by some to be 32 (±6) m.y., while others arrived at a value of 388 (±74) m.y. (Cook et al., 2010). Remarkably, multiple approaches conducted by Huber et al. (2010) resulted in a much longer CRE age of ~1 b.y. An estimate of the pre-atmospheric mass was calculated to be ~83.3 kg. The photo above shows a 0.69 g thin partial slice of Eagle Station (photo courtesy of Sergey Vasiliev—SV Meteorites). The photo below is a larger partial slice showing the typical distribution of silicate and metal in Eagle Station.
Photo courtesy of The American Museum of Natural History
