The fluid dynamics of plume-ridge and plume-plate interactions: An experimental investigation

Abstract

A number of hotspots occur at or near mid-ocean ridges (e.g., Iceland) causing elevated bathymetry, gravity anomalies, and geochemical enrichment for hundreds of kilometers along otherwise normal ridge axes. This is commonly presumed to be the result of hot plume material from the deep mantle flowing outward from a narrow plume conduit. Here we explore the dynmics of plume-ridge and plume-plate interaction using laboratory experiments. We model chemically buoyant plumes, as opposed to thermally buoyant plumes, so we ignore the effects of thermal diffusion while concentrating on the strictly fluid dynamical aspects of the interaction. We find that the spreading of a continuously fed plume beneath a fixed plate is described by a simple expression derived from a balance between plume buoyancy and viscous resistance to spreading. For plumes spreading beneath ridges we find two remarkably distinct phases of flow: At first the plume spreads approximately as it would beneath a fixed plate, then a rapid transition occurs to a steady-state condition in which the plume width along the ridge (‘waist’) is constant. This width is found to be proportional to a natural length scale that increases with the square root of the plume volumetric flux and the ratio of the mantle/plume viscosities, and decreases with the square root of the plate spreading velocity. Although our experimental conditions, with uniform mantle viscosity and no thermal diffusion, are perhaps oversimplified compared to the real Earth, the results scale reasonably well to observation. Our model suggests that sufficient time has elapsed (∼ 60 Myr) for the Iceland plume to reach steady state in its interaction with the Mid-Atlantic Ridge. Using independent estimates for the plume volumetric flux at the Iceland hotspot, and assuming a viscosity ratio of one, the model predicts a minimum waist width of about 850 km, in close agreement with the observed value of about 920 km. Data from other hotspots (Jan Mayen, Azores, Galapagos) are also consistent with the experimental results for average mantle/plume viscosity ratios of 1–3. Using estimated plume fluxes with a simple conservation of mass equation yields reasonable estimates of plume thickness ranging from 70 to 180 km for these hotspots. Thus the experimental model appears to be reasonably consistent with observation, given that thermal diffusion has been disregarded.