Implications of shock effects in iron meteorites

Abstract

The results of a microscopic and X-ray diffraction study of the shock history of sixtyfive octahedrites are reported. These meteorites include forty-six which have been previously dated by the K 40 K 41 method and sixty-three, classified by their gallium and/or germanium contents. Of the dated specimens, twenty-one have been shocked to pressures between 130 and 750 kb, nineteen have been shocked to less than 130 kb, and six have been thermally recrystallized at some time during their history. The age distributions of the shocked and unshooked dated meteorites are significantly different, the shocked-specimens age distribution peaking over the range 400–800 10 6 yr. The distributions of shocked and unshocked octahedrites among the various Ga-Ge groups are very different. While the shocked meteorites are heavily concentrated in group III, the unshocked meteorites appear randomly distributed among the various Ga-Ge groups. The following conclusions are suggested by the results of this study. 1. (1) On the order of 50 % of all iron meteorites reaching the Earth have been preterrestrially shocked to pressures in excess of 130 kb. 2. (2) A -single collision 650 ± 60 10 6 yr ago was responsible for a majority of these shocked meteorites. 3. (3) One of the partners in this collision seems to have been a Ring asteroid and the parent of the Ga-Ge group III octahedrites. 4. (4) The more massive partner in this collision may be the parent of the hypersthene chondrites. Possibly, it too was a Ring asteroid although its orbit may have crossed that of Mars. 5. (5) The collision resulted in the ejection of meter-sized iron meteoroids and, possibly, hectometer- to kilometer-sized hypersthene chondrite fragments into Mars-crossing orbits. 6. (6) Subsequent perturbations by Mars resulted in the continuous ejection of group III meteoroids into Earth-crossing orbits, at least over the last 10 6yr. 7. (7) Secondary collisions, in concert with Martian perturbations, resulted in the insertion of hypersthene chondrite meteoroids into Earth-crossing orbits. 8. (8) This postulated collision also appears to account for the predominance of the hypersthene chondrites and medium octahedrites among meteoritic falls and finds.