Mixing and entrainment in mantle plumes: A 3D experimental investigation

Significant differences exist between isotopic signatures of typical mid-ocean ridge basalts (MORB) and those associated with many ocean islands, with ocean island basalts (OIB) generally exhibiting more variability in trace element concentrations and a bias towards enrichment in more primitive isotopes as well in some cases. Such observations coupled with other geophysical evidence have been used to suggest that OIB’s are surface manifestations of upwellings originating in the deep interior near the core-mantle boundary that interact with distinct, heterogeneous reservoirs as material is transported from the Earth’s interior to the surface. Although many have studied the chemistry and dynamics of these mantle plumes, fundamental questions remain. Such questions can be grouped into two general issues: a) Plume structure and dynamical interaction with the surrounding mantle, b) The degree of entrainment and mixing in mantle plumes of chemically distinct material from the deep mantle. We address these fundamental questions via detailed fluid dynamical experiments to determine the structure, temperature, velocity, entrained mass origin, and degree of entrainment in thermal plumes. Heat is used as the driving convective mechanism to form a single thermal plume in corn syrup. The experiments are conducted using Stereoscopic Particle Image Velocimetry (SPIV) and Thermochromic Liquid Crystals (TLC’s) to measure the 3D flow and temperature fields within the tank. A finite volume numerical model using SPIV velocities as inputs permits reconstruction of temperature values for warmer regions where the fluid temperature is beyond the working range of the TLC’s. Preliminary results further strengthen arguments that the classical view of plumes having well-developed scroll heads may be more a characteristic of injection-type experiments than a fundamental feature of all thermal upwellings, particularly those sourced from a thermal boundary layer. Hence, such scroll-shaped heads may not be universally representative of thermal upwellings originating from Earth’s core-mantle boundary.