Viscous fingering in miscible fluids and the oceanic asthenosphere: significance for the formation of intraplate seamount chains

Abstract:

Recent global and regional studies of seismic tomography in both oceans and continents show linear bands of low velocity anomalies that are aligned with absolute plate motion and coincident with intraplate volcanic chains. To explain the seismic and gravity anomalies observed beneath the Pacific ocean small scale convection is frequently cited but this model does explain the wide range of wavelengths that are reported. We propose a new hypothesis suggesting that viscous fingering instabilities can form when hot and wet mantle plumes discharge into the upper mantle and displace higher viscosity depleted asthenosphere. Fingers travel laterally through the asthenospheric channel below moving plates and are correlated with melting and intraplate volcanic chains observed on the seafloor. The development of viscous fingering instabilities are investigated in laboratory fluid experiments using high viscosity miscible fluids in a Hele-Shaw cell scaled to the Earth using appropriate non-dimensional parameters. We perform a set of fluid experiments with viscosity ratios 3 to 200. Viscous fingers are observed to form for all viscosity ratios above 3.0 Pa s. After an initial growth period, fingers exhibit a constant wavelength for a given viscosity ratio. Fingering wavelength is strongly dependent on the plate spacing and therefore asthenospheric layer thickness. The average fingering wavelength is shown to increase with increasing viscosity ratio. When the viscosity ratio is high we observe longer fingers that initiate closer to the point of origin. Plate motion will be simulated using a sheet of mylar moving over radial fingers. Preliminary results indicate that fingers align with plate motion both upstream and downstream. This new model for viscous fingering may link off-axis mantle plumes to spreading centers where they contribute to melting, surface volcanism and the growth and formation of new lithosphere.