Shock History of Mesosiderites

Abstract

THE shock histories of iron meteorites have been extensively studied1–6 but previous investigation of the 25 stony-iron meteorites classified as mesosiderites7 has been much less complete. About 2/3 of the iron meteorites sampled (and, by extrapolation, about half of those recovered on Earth) have been shock-loaded to pressures>130 kbar during the break-up of their parent bodies by collision, but the percentage of such shocked meteorites is markedly different (Table 1) in the various groups distinguished by other physical and chemical criteria6. The ubiquity of effects arising from shock pressures>130 kbar in meteorites of the various groups III (which probably originated in different portions of the same parent body) might reflect either an unusually violent collision or a biased sampling of the collision fragments by Earth1,2,6. Group III meteorites have low cooling rates, indicating that they originated in their parent body at depths greater than those of other iron meteorites8. In turn this suggests that, as in the Canyon Diablo object9, there is an increased degree of shock loading with depth in a parent body. But the correspondence between formation depth and proportion of shocked iron meteorites is not perfect because group I representatives have lower cooling rates than those of group IVA and yet a smaller proportion of shocked meteorites (Table 1).