UDEI STATION

A 103 kg mass was seen and heard to fall in the daytime near the Benue River in Nigeria. The Geological Survey of Nigeria learned of the event in 1935 when the mass was recovered from a shallow hole six miles west of the railway station at Udei and 23 miles north of Makurdi. Some reports state that a second mass exists.

The IAB iron-meteorite complex, recently proposed by Wasson and Kallemeyn (2002), comprises iron meteorites from the former IAB-IIICD group, as well as numerous related irons. Many of the members contain silicate inclusions with roughly chondritic compositions. Udei Station is a low-Au member of the IAB complex that is closely related to the main group. On a Ni–Au diagram, a grouplet of six members has been resolved in an area close to the sLL subgroup, but with lower Ni contents. These meteorites were designated the Udei Station grouplet. Although theories of formation of these irons commonly attribute their origin to impact melt pools, correlations between Ni abundance and several chemical and physical properties alternatively suggest an interaction with a crystallizing metallic core. The absolute I–Xe retention age, relative to the Shallowater standard, was calculated to be at least 4.537 b.y., while the metamorphic resetting of the K–Ar chronometer occurred ~4.31 b.y. ago (Bogard et al, 2005).

Udei Station is a medium octahedrite that contains a high proportion of silicates and troilite. Silicate inclusions range in size from single grains to as large as 8 cm and are composed of enstatite, olivine, oligoclase, and diopside. They exist as subchondritic, basaltic/gabbroic, feldspathic orthopyroxenitic, and harzburgitic mineralogies, which represent crystallization from both a low degree partial melt—~3–10% at a peak temperature of <1180°C—and a silicate residue (Ruzicka and Hutson, 2009). Identification of an FeO-poor, Na-rich gabbroic inclusion similar to one found in Caddo County is only the second discovery of such material. Ruzicka and Hutson (2009) argue that interstitial silicate melt migration and removal occurred on the endogenously heated parent body, assisted by CO gas (now manifest as graphite), while low abundances of metallic melt (~37%) became trapped in veins between solid silicate assemblages.

A formation model which accounts for such a wide variety of inclusions and disparate petrographic evidence was proposed by Ruzicka and Hutson (2009). They conceive of a major collisional disruption/jumbling of a partially molten, incompletely differentiated planetesimal. They consider winonaites to be derived from the outer layers of this body. At the same time, they envision the IAB complex irons that were more rapidly cooled, that exhibit older I–Xe ages, and that are Ni-rich to be derived from the middle layers, while those that were more slowly cooled, that exhibit younger I–Xe ages, and that are Ni-poor to be derived from the lowest layers. This severe impact mixed material from different depths—solid subchondritic material derived from cooler regions of the body and fractionated material derived from the hotter regions—and induced metallic melt injection into silicate.

Udei Station has a Cl–Ar-based CRE age of 123 m.y. (Albrecht et al., 2000). It plots as an anomalous member of group I in several regards. It contains a clast best described as a peridotite, composed of the most Fe-rich mafic minerals found among IAB irons. The specimen shown above is a 26.3 g partial slice exhibiting a high content of silicates.

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