Magma Mingling in Dikes and Sills

Dikes in which two different liquids flowed simultaneously, usually called composite dikes, naturally fall into two classes depending on which lithology forms the contact with the country rock, and hence, which liquid was the first to enter the fracture. In these two kinds of dikes, the structures formed by the mingling of the two liquids differ. Dikes in which the more basic liquid entered first have contacts between the two lithologies that are nearly planar, and parallel to the dike walls, whereas the more basic lithology forms discrete pillows in those dikes in which the more silicic liquid entered first. Experiments indicate that these pillows probably form from a flow-front instability that develops when a liquid invades another of higher viscosity between two parallel rigid walls. We provide scalings for the critical flow rate for the onset of this instability, the time required for the instability to develop, and the wavelength that is selected. These scalings are consistent with field observations.