Shock History of Mesosiderites

Abstract

THE shock histories of iron meteorites have been extensively studied^1–6 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 Earth^1,2,6. 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).