Spontaneous development of concentric layering in a solidified siliceous dike, East Greenland

Abstract

A meter-wide rhyolitic dike near the eastern margin of the Skaergaard Intrusion developed fine concentric layering in a small section where a fault appears to have fractured the dike and enhanced its interaction with meteoric water. Layers two or three millimeters thick consist of quartz alternating with low-temperature albite and potassium feldspar around a core of radiating clots of the same minerals. The layering seems to have resulted from late-stage crystallization and “self-organization” during devitrification in a warm hydrous environment. The development of fine-scale layering can be modeled quantitatively using the model for competitive particle growth (CPG) of Ortoleva and co-workers. Owing to the small differences of surface energy of grains of differing sizes, large crystals tend to grow at the expense of smaller ones, and a repetitive pattern of layering develops from a small initial perturbation of grain size. The spacing of layers predicted by a numerical model using reasonable estimates for the physical parameters, such as diffusivity and the rate of cooling, is in good agreement with the observed geometry and dimensions.