Shocked minerals at the K/T boundary: explosive volcanism as a source

Abstract

Azimuthal symmetry of P waves, first motion up and diminished S wave amplitude indicates an explosion to be the source of the seismicity of the 18 May 1980 eruption at Mt. St. Helens. Explosion coupling efficiencies and scaling from underground mining, excavation and nuclear explosions imply that peak pressures could have been as high as 1000 kbar, which is more than sufficient for the formation of shocked minerals. In addition, high load rates as well as contamination may lower phase transition pressures. A region of formation of shocked minerals surrounding the magma chamber is indicated to have a thickness on the order of 100 m. The outer portions of this region should be sufficiently cool so that shocked features are not annealed, hence the source of shocked minerals should be the country rock surrounding the magma chamber. It is unlikely any of the magmatic material would be able to retain shocked features due its high temperature. The explosive mechanism may be quench supersaturation which theoretically can yield shock producing pressures but chemical processes should not be dismissed as an explosive source. Tektite distributions show no iridium or mass extinction association. Industrial and present volcanic atmospheric discharge are known to have worldwide distribution. The clays present at the K/T boundary may have had a volcanic origin and it is now known that the Deccan Traps, which have been suggested by others as a source of the iridium, lie athwart the K/T boundary. As there is common intimacy between silicic and mafic volcanism, both should be anticipated at the K/T boundary and their global appearance there may be a manifestation of a surge in whole mantle convection. In short, there is a definite possibility shocked minerals at the K/T boundary have a volcanic source.