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 all 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). 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 1,000,000 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.

Following a period of thermal metamorphism, a catastrophic impact likely caused the breakup and rapid gravitational reassembly of the body, resulting in the mixing of solid silicates with still molten metal and sulfide. 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).

Since the highest Ar–Ar-based age estimate for Landes is younger than the highest measured for Caddo County, and since the cooling rate of metal is determined to be lower for Landes than that for Caddo County, it was inferred that Landes was the more deeply buried of the two, 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, based on 3He, 21Ne, and 38Ar in metal, of only ~2 m.y., which is significantly less than that of other IAB irons and the winonaites (Vogel and Leya, 2008). The specimen of Caddo County pictured above is a 19.6 g etched partial slice.