On the Formation of Chondrules and Metal Particles by ‘Shock Melting’

Abstract

A study of the Ramsdorf meteorite has shown quite clearly that this meteorite was highly altered by partial melting and rapid cooling, probably produced by shock waves (Begemann and Wlotzka). The silicates of this meteorite are transformed into a more or less continuous intergrowth of euhedral olivines and pyroxenes with a glassy groundmass, which also contains small dispersed droplets of troilite and nickel iron. This structure is exactly the same as found inside certain types of chondrules in many chondrites. It seems likely that a dispersion of this partly shock-molten meteorite into droplets would yield spherules consisting of silicate crystals in a glassy groundmass, i.e. chondrules.

A similar concept of chondrule formation has been proposed by Urey, and also by Fredriksson and Ringwood. The collision of the primary objects postulated by Urey to form chondrites would also yield the shock energy for melting and chondrule formation. The observation in Ramsdorf of a shock-formed silicate structure similar to that in porphyritic chondrules strengthens the feasibility of Urey’s hypothesis and of Fredriksson and Ringwood’s mechanism for chondrule formation. A sudden escape of volatiles as the means to disrupt the molten rock into droplets, as conceived by Fredriksson and Ringwood, does not seem necessary, as acceleration by the collision energy may do the same.

One may even speculate that the nickel-iron particles of chondrites were also formed by shock melting of pre-existing metal grains. In Ramsdorf we find rounded particles of nickel iron which apparently have been molten. They show an increasing Ni-content towards their rim, produced by direct rapid non-equilibrium crystallization of the metal out of the melt (Begemann and Wlotzka), and not by solid-state diffusion of Ni as in the taenite-kamacite transformation in iron meteorites. A dispersion of this melt into free drops may disturb this crystallization and yield separate grains of low and of high Ni-content according to the point of interruption and mode of distribution. Hence, it may be possible to produce the taenite particles in chondrites and their M-shaped Ni- concentration profiles during the rapid cooling process which also formed the partly glassy silicate chondrules. The long cooling times as derived by the diffusion model (Wood) are then not required.