Origin of some icelandic pitchstones

Abstract

Reconsideration of analyses of pitchstones from Eastern Iceland suggests that other sources than fractional crystallisation should be considered. One possibility involves the origin of silicic magmas by the partial fusion of basaltic material. This primary aphyric silicic melt was perhaps erupted directly on the surface as well as after a period of cooling in a high-level magma chamber. Modal studies suggest that during the postulated period of cooling phenocrysts developed which remained suspended in the melt. The bulk chemical composition of the crystallising mass can be shown to have changed; vapour phase leaching of silica and, to a lesser extent, of potassium-rich alkali feldspar was perhaps important.