Radial Miscible Viscous Fingering of the Icelandic Plume

The Saffman-Taylor instability, a fluid dynamical phenomenon, occurs when a less viscous
fluid is injected into a more viscous fluid, leading to the development of radial miscible viscous
fingers. Approximately five horizontal fingers radiate away from the Icelandic plume at a
depth range of 100 km–200 km. These fingers are manifest as shear wave velocity anomalies in
full-waveform tomographic models. The best resolved fingers lie beneath the British Isles and
beneath western Norway, extending ∼1,000 km away from the Icelandic plume conduit. The
number and wavelength of miscible viscous fingers are controlled by Péclet number (i.e. the
ratio of advective and diffusive transport rates), mobility ratio (i.e. the ratio of fluid viscosities)
and thickness of the horizontal layer into which the fluid is injected. Observational estimates
for the Icelandic plume suggest the Péclet number is O(10⁴), the mobility ratio is at least 20–
50 and the asthenospheric channel thickness is 100 ± 20 km. Appropriately scaled laboratory
experiments play a key role in developing a quantitative understanding of the spatial and
temporal evolution of the Icelandic plume planform. During laboratory experiments, Péclet
number is varied primarily by changing the flow rate as well as the altering the thickness of
the horizontal layer. Viscosity contrasts are generated by using glycerol and water mixtures
which are miscible, like plume material with ambient mantle. However there is no temperature
contrast in the experiments, which is probably significant in the mantle. Comparison between
scaled analogue experiments and observed values suggests the fluid dynamics may be more
complex than the Saffman-Taylor instability alone. Additional processes including influence
of temperature, interaction with the base of the lithospheric plate or small-scale convection,
along with the Saffman-Taylor instabillity, may be the origin of the fingers imaged by seismic
tomography.