Comb-layering in carbonatite dykes

Carbonatitic dykes from the Tertiary Kaiserstuhl volcano in the Upper Rhinegraben, FRG, commonly show fine-banded layering with skeletal branching calcite crystals oriented perpendicular to the layering. These calcite comb-layers are produced from magmatic in-situ crystallization in specific conditions of supercooling and rapid cooling rates(1,2). We show here that the crystallization features underline the magmatic origin of the observed textures, mineral phases and melt compositions and are a key to the physicochemical nature of the carbonatitic magma.