Chondrites: Their metallic minerals, thermal histories, and parent planets

Abstract

Metal grains in 35 chondrites were studied microscopically and by electron microprobe analysis. Taenite and kamacite crystals in ordinary, unequilibrated ordinary, and type III carbonaceous chondrites exhibit the same compositional inhomogeneities previously observed in octahedrites. It is shown that the metal grains evolved in situ to their present form during cooling from metamorphic temperature. Correlations exist between the dimension and central Ni content of taenite and kamacite crystals in many chondrites, making it possible to deduce the rates at which the chondrites cooled by the same technique previously applied to octahedrites. Ordinary chondrites cooled through 500°C at 2–10°/million years, the other types noted above at 0.2–2°/m.y. Heat flow calculations show that these cooling rates would have obtained in sites respectively 20–150 km deep in planets of ⩾90 km radius, and 40–150 km deep in ⩾150 km radius planets. Some of the processes that would have occurred during metamorphism are discussed, especially losses of gas and certain trace elements. Type II carbonaceous chondrites appear to contain no high-Ni metal grains. Their properties are consistent with the theory that they represent unmetamorphosed, primordial planetary matter. Distinct differences are shown to exist between metal grains in the light and dark components of Pantar-type (gas-rich) chondrites, evidence that they evolved in widely separated sites. Several chondrites that appear to have been reheated (as, for example, by shock compression during interplanetary collisions) are described, and the metallographic changes wrought by various degrees of reheating are noted. A model for chondrite formation and evolution that embraces all these effects is presented (Sec. 7).