Shock metamorphism in artificial impact craters

Abstract

Shock metamorphism has been investigated in artificial impact craters of gabbroic anorthositic and granitic target rocks. Shocked quartz grains with irregularly wavy extinction are found at crater walls and eject a, and show the largest values of density in the fine aggregates of eject a. When shock loaded by a steel projectile, a larger density of shocked quartz has been obtained in target rock of gabbroic anorthosite with smaller homogeneous grains, than in granite rock with larger irregular grains. There are two types of shocked quartz; 1) shocked quartz formed by direct transformation from high-pressure silica, and 2) fine shocked quartz crystallized by chemical change mainly from feldspar composition under the vapour condition of the impact event. The closed system of the experimental chamber produced easily shocked cristobalite high-temperature type silica, but no stishovite high-pressure type. Chemical change of feldspar composition can be proved also by the anomalous composition of low-KNa-Ca elements revealed by electron microprobe data. Shock pressures of shocked quartz are estimated by a regression equation for the density-deviation of shocked quartz from the Charlevoix impact structure.