VESICLE LAYERING IN SOLIDIFIED INTRUSIVE MAGMA BODIES - A NEWLY RECOGNIZED TYPE OF IGNEOUS STRUCTURE

We report a novel type of layering structure in igneous rocks. The lay ering structure in the Ogi picrite sill in Sado Island, Japan, is spat ially periodic, and appears to be caused by the variation in vesicle v olume fraction. The gas phase forming the vesicles apparently exsolved from the interstitial melt at the final stage of solidification of th e magma body. We call this type of layering caused by periodic vesicul ation in the solidifying magma body ''vesicle layering.'' The presence of vesicle layering in other basic igneous bodies (pillow lava at Ogi and dolerite sill at Atsumi, Japan) implies that it may be a fairly c ommon igneous feature. The width of individual layers slightly, but re gularly, increases with distance from the upper contact. The layering plane is perpendicular to the long axes of columnar joints, regardless of gravitational direction, suggesting that the formation of vesicles is mainly controlled by the temperature distribution in the cooling m agma body. We propose a model of formation of vesicle layering which i s basically the same as that for Liesegang rings. The interplay betwee n the diffusion of heat and magmatic volatiles in melt, and the sudden vesiculation upon supersaturation, both play important roles.