CADDO COUNTY

A mass weighing about 35 pounds, along with fragments weighing about 5 pounds, were found by a farmer while plowing in Oklahoma. This is an unusual meteorite in which chondritic and nonchondritic silicates are poorly mixed with the metal host.

Silicate inclusions typically contain olivine, pyroxene, plagioclase, chromian diopside, graphite, troilite, and phosphate, all mixed into a FeNi-metal host. The mm-sized chromian diopside crystals have a pronounced green color within the silicate clasts. Recently, 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 from 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 radiogenic 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 causing 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; silicates inclusions in IAB irons are therefore one of the oldest silicates available for study (Schulz et al., 2010).

Following a period of thermal metamorphism, a catastrophic impact 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 persisted 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.

In their study of plagioclase separates derived from individual silicate grains composing different inclusions within Landes, Caddo County, Campo del Cielo, and Ocotillo, Vogel and Renne (2008) found that corrected Ar–Ar ages have a significant range—from as old as ~4.55 b.y., near the probable onset of accretion/differentiation, to as young as ~4.43 b.y., presumably reflecting grains which experienced the latest closure of the K–Ar system (i.e., following reassembly). It was suggested that this wide range of ages represents silicate grains which were cooled at different rates and at different depths within the IAB parent body. Only after the collisional disruption and mixing of solid-to-partially melted silicates with molten metal, and the subsequent gravitational reassembly of this planetesimal, were the individual silicate grains from different source regions combined to form the composite IAB inclusions we observe today (Benedix et al., 2000). Following this catastrophic disruption, which is calculated to have occurred ~4.47 b.y. ago, reassembly and cooling proceeded rapidly in order to preserve the pre-established Ar–Ar ages of individual grains.

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). In contrast to these meteorites, they demonstrated that the Ar–Ar-based age of Campo del Cielo reflects resetting in a more high-temperature thermal environment, probably at a deeper burial location pre- and/or post-reassembly.

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 preatmospheric 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.