WHAT MAGMATIC PHENOMENA ARE ENCODED IN DIKES?

As magma is transported through the crust as an actively propagating hydraulic fracture it leaves behind the solidified, no longer transportable, margins. This magmatic snail trail likely comprises the majority of dikes observed in the geological record. Because the morphology of a dilated fracture is the product of fluid overpressure and host material strength, it is often assumed that dike morphology records magmatic overpressure and host rock properties. However, if dike morphology is not equivalent to the active hydraulic fracture, it is not immediately clear what information can be extracted from dike morphology. Presented here are the results of experiments that utilize gelatin and molten paraffin as host rock and magma analogs, respectively. These materials are scaled for a relatively weak host (e.g. poorly consolidated sediment) and a low-viscosity magma (e.g. mafic magma). Because of the transparent nature of gelatin and the solidification of paraffin, the style of propagation and intrusion can be directly observed and compared against the final solidified dike. The cross-sectional length of dikes is weakly correlated with overpressure, perhaps due to the inherently non-linear process of coupled fluid flow and solidification. Low-pressure experiments tended to produce only small dikes, while high-pressure experiments produced both large and small dikes. All small dikes tended to propagate rapidly to the surface, and were relatively thin. Large dikes propagated more slowly to the surface, and thickened appreciably, indicating enhanced storage of magma analog in the subsurface prior to eruption. During eruption these thickened dikes deflate, resulting in solidified dikes that are thicker at their margins than in their interior. Lastly, pulsing appears to be a common feature, and may occur at multiple timescales. In larger dikes, propagation can stall, leading to inflation of the dike and subsequent reactivation of propagation. The stall front is recorded as a thickened rind in the dike that is observable in 3D. In smaller dikes, ridges commonly form perpendicular to flow. Based on their characteristic length and the propagation velocity of the experimental dikes, these pulses occur on the 10s of Hz scale.