The effect of cooling rate on immiscible silicate liquid microstructure: an example from the Palaeogene dykes of Northeast England

The migration and accumulation of immiscible silicate liquids may play a significant role in the differentiation of crustal magma bodies and the formation of some economic mineral deposits. However, our understanding of the processes that control the segregation of these liquids is currently limited by the short timescales of petrological experiments. Detailed microstructural investigations of Palaeogene basaltic dykes from Northeast England, coupled with simple 1D thermal models, constrain the effects of cooling rate on the microstructure of unmixed immiscible silicate liquids under natural conditions. The size of unmixed Fe-rich droplets within a continuous silicic phase is related to the cooling rate by a power law, with droplet diameter increasing with decreasing cooling rate, accompanied by an increase in the number of droplets. Fe-rich droplet coarsening is a result of diffusion-controlled growth. The average apparent aspect ratio and grain size of matrix plagioclase crystals indicate that nucleation and growth of these grains probably occurred in a static (or only weakly convecting) fluid dynamical regime.