Viscous Fingering in the Mantle Asthenosphere

Abstract:

Regional seismic tomography studies in the Pacific ocean and continental western U.S show linear bands of low velocity anomalies that are aligned with absolute plate motion and coincident with volcanic lineaments located within the interior of plates far from plate boundaries. Small-scale convection provides one possible explanation for these lineations but does not predict age progressive seafloor volcanism that opposes plate motion. We propose a new hypothesis where viscous fingering instabilities form due to hot and wet mantle plumes which rise and discharge into the upper mantle asthenosphere and displace higher viscosity depleted mantle. We perform laboratory fluid experiments scaled to the Earth's mantle, with stationary and moving surface plates that use fluids with viscosities (μ) from 1 to 300 Pas and viscosity ratios (μ₁/μ₂) from 3 to 400. Viscous fingers are observed to form for all viscosity ratios we consider and after an initial growth period, exhibit a constant wavelength that depends on several parameters. Fingering wavelength is strongly dependent on plate spacing (and therefore asthenospheric layer thickness) but shows weak or no dependence on viscosity ratio and injection rate. The radius, R_o, at which fingers first form varies inversely with increasing viscosity ratio. This indicates that low viscosity mantle may flow long distances before fingers develop if viscosity ratios are small. For mobile plates, a ratio Γ of plume flux to plate velocity is defined where Γ is varied from 3.6x10^-4 to 3.6x10⁵ which considers the range expected in the Earth (6.3x10^-3 to 1.5x10^-2). Results indicate that fingers align with plate motion both upstream and downstream, with longer wavelengths in the downstream direction. Particle imaging successfully resolves particle motion vectors and also indicates the presence of a thin film layer above and below each finger. This new geodynamic model for viscous fingering in the asthenosphere links off-axis and rising mantle plumes indirectly to the spreading centers where they contribute to melting, surface volcanism and the growth and formation of new lithosphere.