Iron, IIIB, octahedrite
standby for tambo quemado photo
Found before 1950
14° 40' S., 74° 30' W.

A mass of 141 kg was found in 1949 in a remote part of the Andes Mountains near the village of Tambo Quemado in Peru. It was brought to the Geological Museum in Lima, Peru in 1950. On average, the mass has lost less than 1 mm from its surface during its terrestrial residence, but has been artificially heated to about 1000°C for ~1 hour at some point in its history (Buchwald, 1975). This heating has melted inclusions of the phosphide schreibersite and the phosphate graftonite. As the graftonite recrystallized, phosphoran-wustite and silica were added to the inclusion to form a new silicophosphate. Migration of lithophile trace elements was also facilitated during this period.

In another study involving Tambo Quemado (Sugiura and Hoshino, 2003), it was suggested that the absence of phosphates was the result of its formation under more reducing conditions (the IIIA Thunda is also phosphate-free). Consistent with such highly reducing conditions is the fact that Tambo contains the highest Mn and Cr in its sulfides compared to other IIIAB irons. Highly reducing conditions have also produced low Mn in chromite and high Cr in metal. The phosphides in Tambo are associated with metal, in contrast to their association with sulfide in the phosphate-bearing meteorites. Also present in Tambo Quemado are many inclusions of the carbide cohenite, which make the cutting of this meteorite very difficult and expensive. Upon etching, the kamacite and taenite laths display a medium bandwidth with well-contrasted plessite fields making for a remarkable appearance.

A formation history was constructed based on isotopic chronometry and metallographic cooling rates. The initial accretion of the parent body must have been completed ~1.7 m.y. after CAI formation to provide an appropriate quantity of heat-producing radiogenic 26Al. Results of age studies conducted by Kruijer et al. (2012, 2013), utilizing noble gas and Hf–W chronometry for those IIIAB and other iron group members that have the lowest CRE ages, indicate core formation occurred ~1.0–1.5 m.y. after CAIs. Eventually, sufficient heat was produced resulting in a differentiated molten core on an asteroid measuring ~28 km in diameter, forming a core with a diameter of ~11 km. After a period of rapid cooling, continued accretion (or development of a significant regolith) reduced the cooling rate, and complete core solidification occurred ~3 m.y. later. The oxidation of phosphides then created the various phosphate minerals present in many IIIAB irons (but not Tambo Quemado). In addition, they established segregation models for groups IIIAB and IVA, which began segregating ~1.5–2.0 m.y. after CAIs, and for groups IVB and IID, which iron–silicate segregation occurred later still, ~2.0–3.0 m.y. after CAIs.

The main-group pallasites have metal compositions and O-isotopes that are nearly identical to those of an evolved, high-Au, high-Ni IIIAB melt following ~80% crystallization of the core. Therefore, it has been considered that both groups originated on a common parent body; however, several recent technical studies have ruled out this scenario (Yang and Goldstein, 2006; Scott, 2007; Ziegler and Young 2007; Greenwood et al., 2008; Yang et al., 2010). The specimen of Tambo Quemado shown above is a 61.8 g etched partial slice.