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Udei Station

Iron, IAB complex, Udei Station grouplet
standby for udei station photo
Fell spring 1927
7° 57′ N., 8° 5′ E. 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 anomalous Ni contents; these meteorites were designated the Udei Station grouplet. Although theories of formation for 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 ~4.558 b.y., while the metamorphic resetting of the K–Ar chronometer occurred ~4.31 b.y. ago (Bogard et al, 2005; Pravdivtseva et al, 2013).

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 they are composed of enstatite, olivine, oligoclase, and diopside representing various subchondritic rock types including basalt–gabbro, feldspathic orthopyroxenite, lherzolite, and harzburgite. Some of the inclusions crystallized from a low degree partial melt (~3–10%) reaching a peak temperature of <1180°C, while others represent a partial melt 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. It was concluded that interstitial silicate melt migration and removal occurred, assisted by CO gas (now manifest as graphite), while low abundances of metallic melt (~37%) became trapped in veins between solid silicate assemblages (Ruzicka and Hutson, 2009). One plausible model involves impact processing within small melt pools that led to brecciation and mixing of rock clasts from different locations having different thermal histories (ranging from igneous to chondritic) with metallic melt. These melt pools were then slowly cooled, consistent with burial within a deep regolith.

A formation model that accounts for such a wide variety of inclusions and disparate petrographic evidence was proposed by Ruzicka and Hutson (2009) and Ruzicka (2014). 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, exhibit older I–Xe ages, and which are Ni-rich, to have derived from the middle layers, while those that were more slowly cooled, exhibit younger I–Xe ages, and which are Ni-poor, to have derived from the lowest layers. This was a severe impact that mixed material from different depths on the parent body—solid subchondritic material from the cooler regions, and fractionated material from very hot regions—causing metallic melt to be injected into the silicates.

Utilizing the short-lived 182Hf–182W chronometer, corrected for neutron capture by 182W due to galactic cosmic rays, Hunt et al. (2018) derived the timing of metal–silicate separation of all genetically-related IAB irons (at least the MG and sLL subgroup [possibly also the sLM subgroup] and the ungrouped Caddo County [Udei Station grouplet] and Livingstone [Algarrabo duo]) to 6.0 (±0.8) m.y. after CAIs. Based on the constraints provided by the timing of metal segregation, they modeled the early history of the 120(+)-km-diameter IAB parent body as outlined in the following diagram: standby for IAB formation history diagram
Diagram credit: Hunt et al., EPSL, vol. 482, pp. 497 (2018, open access link)
‘Late metal–silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling’
(https://doi.org/10.1016/j.epsl.2017.11.034)’
Dey et al. (2019) employed 17O and ε54Cr values for several irons and their associated silicates/oxides to investigate i) if each iron and its 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 and other meteorite groups (e.g., IAB with winonaites, IIE with H chondrites, and Eagle Station pallasites with CK chondrites). Three IAB irons were employed in the study, and it was demonstrated on a coupled diagram that although the ε54Cr values for the iron component plot in the winonaite field, values for the silicate component plot in a distinct region on an O–Cr coupled diagram (see diagram below). From these results they ascertained that the the IAB silicated irons formed through an impact-generated mixture comprising iron from a winonaite-related parent body and silicate from an unrelated and otherwise unsampled parent body. Incorporation of the silicates into the FeNi-metal host took place at a depth greater than 2 km, allowing time for a Thomson (Widmanstätten) structure to develop during a long cooling phase. Fractional crystallization occurred in some large molten metal pools, followed by very slow cooling, to produce the broad range of features found in certain IAB meteorites (e.g., silicate-poor, graphite–troilite-rich inclusions and extremely high Ni contents). Other results from their study can be found on the Miles and Eagle Station pages. 17O vs. ε54Cr for Irons and Pallasites
standby for o-cr isotope diagram
click on photo for a magnified view

Diagrams credit: Dey et al., 50th LPSC, #2977 (2019)
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 IAB in several regards. It contains a clast best described as a peridotite, composed of the most Fe-rich mafic minerals found among IAB irons. Some variability in appearance is apparent in some members of this grouplet, as can be seen in the Four Corners meteorite. The specimen shown above is a 26.3 g partial slice exhibiting a high content of silicates.


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Caddo County

Iron, IAB complex, Udei Station grouplet
standby for caddo county photo
Found 1987
35° 0′ N., 98° 20′ W. approx. A single mass weighing ~35 pounds along with fragments having a combined weight of ~5 pounds were found by a farmer while plowing. This is an unusual meteorite in which chondritic and nonchondritic silicates are poorly mixed with the FeNi-metal host.

 

Silicate inclusions typically contain olivine, pyroxene, plagioclase, chromian diopside, graphite, troilite, and phosphate. The mm-sized chromian diopside crystals within the silicate clasts have a pronounced green color. Na-rich plagioclase-diopside gabbros have been found in Caddo County, the first such basaltic material found associated with iron meteorites. Apart from this basaltic material, eucrites, angrites, and the ungrouped achondrite NWA 011 represent the only other asteroidal basalts known, while some basaltic plagioclase-enriched regions occur in two meteorites from the acapulcoite–lodranite parent body. This coarse-grained, augite–albite-rich gabbroic material in Caddo County formed as an early-stage, localized partial melt from a chondritic parent body. Because of the high silica content (59 wt%) of this material, along with its low olivine and orthopyroxene content, it represents the first asteroidal andesitic material positively identified.

 

Formation of IAB irons began with the partial melting of a unique chondritic parent body, probably through a combination of both endogenous radiogenic heating (26Al decay) and impact events. Temperatures varied from as low as 950°C to as high as 1400°C, producing a range of metal–silicate lithologies. Migration of the partial melt into a S-rich core, or into numerous smaller pools distributed throughout the parent body, resulted in the segregation of silicates from metal–sulfide partial melts, probably resulting in the partial differentiation of the asteroid. Based on the Hf–W system, this metal–silicate segregation began very early, within ~2.5 m.y. of the formation of CAIs, and therefore silicate inclusions in IAB irons represent some of the oldest silicates available for study (Schulz et al., 2010).

 

It has been demonstrated through HSE data that the IAB complex subgroups were likely formed in distinct parental melt pools, possibly including a core component, with the observed fractionation resulting primarily from crystal segregation rather than fractional crystallization processes (Wasson and Kallemeyn, 2002; Worsham et al., 2013). However, studies of the Mo isotopic compositions of representative meteorites from the IAB iron complex have demonstrated that both the sHL and sHH subgroups might derive from distinct parent bodies in separate nebular regions compared to the IAB complex irons (Dauphas et al., 2002; Ruzicka et al., 2006; Ruzicka, 2014; Worsham et al., 2014; Worsham and Walker, 2015; Worsham et al., 2017). Furthermore, in their studies of Mo isotopic compositions of IAB complex irons, Worsham and Walker (2015) reported anomalous negative µ95Mo values (where µ denotes parts in 106 deviation from terrestrial standards) for Caddo County, which if verified would suggest a formation on a unique parent body. However, HSE data for the Udei Station grouplet reported by Wasson and Kallemeyn (2002) indicates that these irons are closely related to the sLL subgroup, and a new study conducted by Worsham et al. (2016) coupling Pd vs. other HSEs supports this conclusion.

 

Utilizing the short-lived 182Hf–182W chronometer, corrected for neutron capture by 182W due to galactic cosmic rays, Hunt et al. (2018) derived the timing of metal–silicate separation of all genetically-related IAB irons (at least the MG and sLL subgroup [possibly also the sLM subgroup] along with Caddo County and Livingstone [Algarrabo duo]) to 6.0 (±0.8) m.y. after CAIs. Based on the constraints provided by the timing of metal segregation, they modeled the early history of the 120(+)-km-diameter IAB parent body as outlined in the following diagram:

standby for IAB formation history diagram
Diagram credit: Hunt et al., EPSL, vol. 482, pp. 497 (2018, open access link)
‘Late metal–silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling’
(https://doi.org/10.1016/j.epsl.2017.11.034)’

Dey et al. (2019) employed 17O and ε54Cr values for several irons and their associated silicates/oxides to investigate i) if each iron and its 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 and other meteorite groups (e.g., IAB with winonaites, IIE with H chondrites, and Eagle Station pallasites with CK chondrites). Caddo County is one of three IAB irons employed in the study, and it was demonstrated on a coupled diagram that although the ε54Cr values for the iron component plot in the winonaite field, values for the silicate component plot in a distinct region on an O–Cr coupled diagram (see diagram below). From these results they ascertained that the the IAB silicated irons formed through an impact-generated mixture comprising iron from a winonaite-related parent body and silicate from an unrelated and otherwise unsampled parent body. Incorporation of the silicates into the FeNi-metal host took place at a depth greater than 2 km, allowing time for a Thomson (Widmanstätten) structure to develop during a long cooling phase. Fractional crystallization occurred in some large molten metal pools, followed by very slow cooling, to produce the broad range of features found in certain IAB meteorites (e.g., silicate-poor, graphite–troilite-rich inclusions and extremely high Ni contents). Other results from their study can be found on the Miles and Eagle Station pages.

17O vs. ε54Cr for Irons and Pallasites

 

Diagrams credit: Dey et al., 50th LPSC, #2977 (2019)

Since the highest Ar–Ar age estimate for Landes would still make it younger than Caddo County, and since the cooling rate of metal is lower for Landes than that for Caddo County, it was inferred that Landes was the more deeply buried of the two source lithologies, both pre-disruption and post-reassembly of the IAB planetesimal (Vogel and Renne, 2008). Bogard et al, (2005) calculated the absolute I–Xe retention age relative to the Shallowater standard (4.5623 ±0.0004 b.y.) to be 4.5579 ±0.0001 b.y. (given that cooling was initiated 4.53 b.y. ago with an I–Xe closure temperature of 1100°C). In addition, they calculated the K–Ar closure age of Caddo County to be ~4.507 b.y.; a lower limit of 4.536 (±0.032) b.y. was calculated in a separate study (Vogel and Renne, 2006). Caddo County had a minimum pre-atmospheric diameter of ~40 cm, and a cosmic-ray exposure age of only ~2 m.y., based on 3He, 21Ne, and 38Ar in metal; this CRE age is significantly lower than that of other IAB irons and the winonaites (Vogel and Leya, 2008).

 

Futher research on the petrogenetic history of the IAB silicated irons is presented by A. Ruzicka in Chemie der Erde, vol. 74, #1, pp 3–48 (2014); see also the Landes page. The specimen of Caddo County pictured above is a 19.6 g etched partial slice.