Magma solidification effects during sill emplacement: insights from laboratory experiments

Igneous sills and interconnected sill complexes transport magma both vertically through the Earth’s crust and laterally over potentially long distances. Although cooling and solidification of magma are acknowledged to play a major role in the propagation and emplacement of sills, their contributions to sill formation remain poorly understood. Here, the effects of solidification on sill propagation dynamics and the resulting intrusion morphologies are investigated using scaled laboratory experiments. Hot coconut oil (magma analogue) that solidifies during emplacement is injected as a sill into a colder, layered, solid visco-elasto-plastic gel (Laponite RD®, host rock analogue). Molten coconut oil is injected directly into the horizontal interface between two Laponite RD® layers to facilitate sill formation. The injection temperature and volumetric flow rate of the coconut oil are systematically varied between experiments in order to vary the degree of solidification. When solidification effects are relatively weak (high injection temperatures), sill propagation is continuous and forms penny-shaped intrusions that later turn into saucer-shaped sills with marginal segmentation. Conversely, when solidification effects are intermediate to relatively strong (low injection temperatures), sills develop complex elongate morphologies that lengthen parallel to the long-axis of the magma flow direction. Such sills also form in a discontinuous manner and propagate in pulses by growth of discrete marginal lobes, representing periods of tip arrest due to freezing, followed by growth of new lobes at the sill margins. A striking morphological feature that occurs in experiments with intermediate to relatively strong solidification effects is the presence of internal flow channels within sills, which can be: (a) thermally controlled, long-lived channels in intermediate solidification experiments; or (b) structurally controlled, randomly oriented short-lived channels in strong solidification experiments. Our experimental findings are consistent with field and seismic observations of sill geometries, and they highlight that the relative degree of solidification during magma emplacement controls both how sills propagate and their internal flow dynamics.