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 acapulcoitelodranite parent body. This coarse-grained, augitealbite-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 metalsilicate 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 metalsulfide partial melts, probably resulting in the partial differentiation of the asteroid. Based on the HfW system, this metalsilicate 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 deviation from terrestrial standards in parts per million) 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 182Hf182W chronometer, corrected for neutron capture by 182W due to galactic cosmic rays, Hunt et al. (2018) derived the timing of metalsilicate 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:
Diagram credit: Hunt et al., EPSL, vol. 482, pp. 497 (2018, open accesslink)
'Late metalsilicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling'
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 OCr coupled diagram (see diagram below). From these results they ascertained that 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, which produced the broad range of features found in certain IAB meteorites (e.g., silicate-poor, graphitetroilite-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
click on diagram for a magnified view
Diagrams credit: Dey et al., 50th LPSC, #2977 (2019)
Since the highest ArAr 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 IXe 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 IXe closure temperature of 1100°C). In addition, they calculated the KAr 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). A PbPb age of 4.563 (±0.029) b.y. (isochron for all samples, see diagram below) was calculated by Litasov et al. (2019) for apatite grains in some other Udei Station grouplet irons; a somewhat younger PbPb age was obtained when excluding several incongruent data points.
Diagram credit: Litasov et al., 82nd MetSoc, #6125 (2019)
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 ErdeGeochemistry, vol. 74, #1, pp 348 (2014); see also the Landes page. The specimen of Caddo County pictured above is a 19.6 g etched partial slice. The image below is an excellent petrographic thin section micrograph of Caddo County shown courtesy of Peter Marmet.
click on image for a magnified view
Photo courtesy of Peter Marmet