Iron, IID, octahedrite
standby for carbo photo
Found 1923
29° 40' N., 111° 30' W.

A mass of 450 kg was found by cowboys on the Alamo Ranch, which is a station on the Southern Pacific Railroad located 40 miles W of Carbo, Mexico. This meteorite was sold to Harvard in 1928, with further distribution of many sections thereafter. Carbo is a low-Ni member of the IID group. The silicate cristobalite has been identified in Carbo.

Results of age studies conducted by Kruijer et al. (2012, 2013), utilizing noble gas and Hf–W chronometry for those irons that have the lowest CRE ages, indicate core formation began for group IID and IVB irons later than it did for other groups, ~2.0–3.0 m.y. after CAIs. This is compared to ~1.5–2.0 m.y. after CAIs for groups IIIAB and IVA, and as early as ~1.0–1.5 m.y. after CAIs for groups IIAB and IIIAB.

Noble gas studies by Ammon et al. (2008) have revealed details about the pre-atmospheric measurments of the Carbo meteoroid, the in situ geometry of Carbo, and its CRE age. Utilizing noble gas concentration data corrected for cosmogenic Ne production contributions from sulfide and phosphate inclusions, a depth profile was developed for Carbo; this profile translates into a pre-atmospheric meteoroid that was generally spherical in shape. Through measurements of the activity of the radionuclide 36Cl, a diameter of 140 cm was inferred for the Carbo meteoroid. Using this more accurate, corrected data, the team derived a 41K–40K-based CRE age for Carbo of 725 (±100) m.y.

From studies of the 3He distribution in Carbo, it was inferred that at least 5 cm of metal has been removed during atmospheric entry and subsequent terrestrial weathering. The 3He distribution was also utilized in a study by Markowski et al. (2006) to determine the initial tungsten isotopic systematics for Carbo and other irons, which revealed that some iron meteorites formed contemporaneously with CAIs within 1 m.y. of the coalescence of the Solar System.

To learn more of the details of the petrogenesis of the group-IID irons as proposed by Wasson and Huber (2006), click here. The specimen of Carbo pictured above is a 22.5 g deep-etched partial slice. An exquisite etched slice can be seen on display at the Smithsonian Institution, Washington D.C.