Pallasite, Eagle Station group
(possibly CK- or CO-related)
38° 37' N., 84° 58' W.
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 2010 the Karavannoe pallasite was recognized as the the fourth member of this pallasite grouplet (Korochantsev et al., 2013), and in 2012 the fifth member, Oued Bourdim 001, was foundthe grouplet has become a group. 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 pallasites define a group distinct from the pallasites of the main-group, the pyroxene-bearing pallasites, and the other ungrouped pallasites Milton and Choteau. It is noteworthy however that Milton plots proximate to the O-isotopic trend line (CCAM slope) of the CVCK-related meteorites and the Eagle Station pallasites (Korochantsev et al., 2013) (see diagram below). Also of interest is that the 'Vermillion pallasite grouplet' of pyroxene pallasites, comprising Vermillion, Choteau, and Y-8451, plots with the acapulcoite-lodranite clan on an oxygen three-isotope diagram.
Diagram credit: Gregory et al., 47th LPSC, #2393 (2016)
Two very closely related silicated irons, Bocaiuva and NWA 176, also share many compositional similarities with the Eagle Station pallasites, and probably originated from similar chondritic material in the same region of the solar nebula. Calculations indicate that the oxidized CV chondrite parent body and the Eagle Station pallasites shared a similar composition of their parental metallic melts (Humayun and Weiss, 2011). Precise O-isotopic compositions for Eagle Station and Itzawisis (Δ17O = 5.22 [±0.05] ), along with bulk metal Ni-isotopic compositions and siderophile element abundances, were used support a genetic relationship among the Eagle Station pallasites, the Northwest Africa 176 silicated iron, and the CV/CK chondrites (Ali et al., 2013, 2014). 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.
It was determined by Papanastassiou and Chen (2011) that the 54Cr isotopic compositions among the chromite and olivine phases in Eagle Station are different from those of CV chondrites, and consequently these differences in ε54Cr must also exist between the Eagle Station pallasite and carbonaceous chondrite precursor material. Despite the established O-isotopic similarities, this ε54Cr heterogeneity across the two groups calls into question the existence of a genetic relationship between them. In an effort to better resolve potential genetic relationships that might exist, a Cr-isotopic analysis of olivine from the Milton pallasite was conducted by Sanborn et al. (2018). Sanborn and Yin (2019) demonstrated on a coupled Δ17O vs. ε54Cr diagram that Milton plots among the CV clan with the achondrites NWA 7822 and NWA 10503, and it is plausible that they share a genetic relationship. Furthermore, they also demonstrated that Eagle Station plots closer to the CK (or CO) chondrite group along with the achondrite NWA 8186. It could be inferred that both the CV and CK planetesimals experienced a similar petrogenetic history in a similar isotopic reservoir of the nascent solar system (see diagram below).
17O vs. ε54Cr for Carbonaceous Achondrites
click on photo for a magnified view
Diagrams credit: Sanborn and Yin, 50th LPSC, #1498 (2019)
In a more recent isotopic analysis of Eagle Station, Dey et al. (2019) obtained a Δ17O value of 4.93 , which is more consistent with that of the CK chondrites. They employed newly obtained 17O and ε54Cr values for several irons, a pallasite, and their associated silicates/oxides to investigate i) if each iron/pallasite and the associated phases originated on a common parent body (i.e., an endogenous mixture of core and mantle vs. an exogenous mixture through impact), and ii) if any genetic connection exists between the irons/pallasite and other meteorite groups (e.g., IAB with winonaites, IIE with H chondrites, and Eagle Station pallasites with CK chondrites). It was demonstrated on an OCr coupled diagram (UCD in diagram below) that the ε54Cr values for both the silicates and the oxide phase (chromite) in Eagle Station are identical and indicate an origin from a common reservoirand thus, less consistent with an impact origin for the pallasite. Other results from their study can be found on the Caddo County and Miles pages.
17O vs. ε54Cr for Irons and Pallasites
click on photo for a magnified view
Diagrams credit: Dey et al., 50th LPSC, #2977 (2019)
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 has been proposed in which an initial impact generated enough heat to form a melt. After 20% fractional crystallization of this melt, both silicates and solid metal precipitated from the parental melt and accumulated, representing the material that would become the Eagle Station pallasites. A subsequent impact shattered the olivine and mobilized the metal, which flowed into existing cracks. Thereafter, deformational events produced shock forces which incorporated angular shards of olivine, schreibersite, and chromite into melted troilite. Rounding of olivine crystals, once considered to be due to thermodynamic processes that minimize the capillary forces along the olivinemetal interface (Saiki et al., 2003), is now thought to occur primarily from resorption at high temperatures (above ~1250°C) in the presence of silicate melt (Boesenberg et al., 2012).
The 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. Karavannoe exhibits some anomalous elemental abundance ratios, possibly the result of very extensive terrestrial weathering. The Eagle Station pallasites also have higher Fe contents in the silicates compared to the main-group (Fa1920 vs. Fa1113); moreover, they have higher Sc and lower Mg and Mn. These elemental compositions, along with the O- and Cr-isotopic ratios, are similar to those in group IIF irons and the CO, CK, and oxidized CV carbonaceous chondrites, particularly Felix (CO3) and Tibooburra (CV3). In addition, Humayun et al. (2014) observed that many of the siderophile element abundances measured in Karavannoe and Eagle Station are a good match to the CV chondrites, and are indicative of formation in an oxidizing environment. Their studies suggest a sequencial formation for Karavannoe involving fractional crystallization of a CV-like metallic melt that was more evolved than the Eagle Station metal.
A scenario has been considered in which the Eagle Station pallasites were once a part of a large differentiated parent body that was collisionally disrupted. This is consistent with the finding of natural remanent magnetization in the CV chondrites, attesting to the existence of a core dynamo at the time these meteorites were formed (Weiss et al., 2010). Likewise, paleomagnetic studies conducted by Tarduno et al. (2014) revealed a strong natural remanent magnetization in tiny magnetic inclusions in Eagle Station olivines. Importantly, this remanent magnetization attests to the fact that the Eagle Station pallasite was not formed near the core-mantle boundary, because a rotating core dynamo would necessarily cease prior to any significant cooling of adjacent material; therefore, no remanent magnetization would exist.
Application of the HfW isotopic chronometer to Eagle Station reveals that core formation occurred relatively late, ~10 m.y. after differentiation of the likely HED parent body 4 Vesta (Dauphas et al., 2005). It has been calculated that melting and coremantle differentiation due to radiogenic heating should cease by ~78 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 such 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 coremantle differentiation. The chemical composition of Karavannoe is consistent with formation after 60% fractional crystallization of metallic melt on the (CK?) parent body. Other features observed in this member of the Eagle Station group, such as troilite globules, and rounded olivines containing inclusion chains, are thought to record multiple severe and/or disruptive impact events.
Employing three methods, Yang et al. (2010) determined the cooling rate of the Eagle Station pallasites 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. The photo below shows a large slice showing the typical distribution of silicate and metal in Eagle Station, courtesy of Sergey Vasiliev.