Let it flow: forensic analysis of crystals and field observations to retrieve dynamic of empanelment

Abstract:

Magmatic dikes can exhibit different patterns of internal flow and different forms of propagation depending on the forces acting on them. We present a series of analogue experiments, in which the internal flow is measured using tracer particles. We find that dikes with low buoyancy tend to assume a circulating internal flow pattern, which is accompanied by radial growth (the dike grows both vertically and horizontally). As dikes become sufficiently buoyant, the flow pattern transitions to unidirectional flow and the dike propagates vertically. Regardless of the buoyancy, when dikes erupt, the flow pattern transitions so that all residing liquids flows towards the vent, and subsequently injected liquid simply ascends.

These findings lead to two discussions: one on the crystal growth patterns associated to such flow dynamics, and one on identifying feeder and non-feeder dikes. Flow measurements provide valuable information which can be related to how crystals move around inside of dikes and how they can temporarily reside in place, even inside of an actively propagating dike. This yields information on the time scale of periods of movement and rest. Regarding feeder and non-feeder dikes, distinguishing between the two can be difficult due to incomplete knowledge of driving conditions. We demonstrate that flow indicators, like imbrication, vesicle shear and magnetic fabrics can provide an additional line of evidence to differentiate feeder from non-feeder. Highly variable flow directions within a given dike is a good indication of circulating flow, which is uniquely associated to non-buoyant, non-feeder dikes. Buoyant non-feeder dikes can be distinguished from feeder dikes by the gradient of flow orientation, in that feeders have a nearly uniform orientation gradient, gradually transitioning from vertical flow at the bottom to sub-horizontal flow at the top. By contrast, buoyant non-feeder dikes have a sharp change in flow orientation near the upper tip, in that the bulk of the dike has uniform vertical flow and the upper tip has divergent flow. Since dike outcrops are a cross section of the larger structure, the relative location can display different characteristic data. For example, circulating flow is convergent at the dike bottom and divergent flow at the top.