The iron meteorites, their thermal history and parent bodies

Abstract

Cooling rates are determined for the formation of the Widmanstätten pattern in 193 iron meteorites in order to obtain information on the number and possible types of parent meteorite bodies. About two thirds of the meteorites cooled between 1 and 10°C 10 6 yr although the total variation in cooling rates is 0.4–500°C/10 6yr. These variations indicate that the iron meteorites formed in more than one parent body, the Widmanstätten pattern formed at low pressures, and the maximum sized parent body was about 300 km in radius if the irons formed in its core. Evidence is presented to show that meteorites, within each Ga-Ge group are genetically related. Characteristic plessite structures were observed for each of these groups. The cooling rate variations in both Ga-Ge groups I and IIIb are small (2–3°C/10 6yr and 1–2°C 10 6 yr., respectively) and independent of chemical composition. These meteorites probably formed in the cores of parent bodies about 200 km in radius. The cooling rate variations in both groups IIIa and IVa are large ( 1.5–10°C/10 6 yr and 7–90°C/ 10 6 yr, respectively) and vary with the Ni content, decreasing as the Ni content increases. These meteorites probably formed in isolated regions spread throughout parent bodies 150–200 km in radius. These regions consisted of molten metal at the time of formation.