Entrainment Processes during Plume Ascent

Basalts from intraplate or hotspot ocean islands are found to have distinct geochemical signatures. In particular, at strong plumes like Hawaii, Iceland or Galapagos, there is evidence for at least three geochemically distinct components. This diversity in composition is generally believed to result from the upwelling plume entraining shallow mantle material during ascent, while potentially also entraining other deep regions of the mantle. In order to understand the geochemical message brought to the surface by plumes, this thesis presents a comprehensive study on the dynamics of plume entrainment using analogue laboratory experiments and 30 numerical modelling, with the focus on the following three questions: • Which regions of the mantle arc most efficiently sampled by mantle plumes? • Is the heterogeneous nature of mantle plumes inherited at the source, or does it develop through entrainment during plume ascent? • How are the plume and plume entrainment affected by mantle discontinuities? The analogue laboratory experiments are conducted using glucose syrup contaminated with glass beads to visualize fluid flow and origin. The plume is initiated by heating from below or by injecting hot uncontaminated syrup. Results from the laboratory experiments indicate the presence of a sheath of mostly unheated ma.t<'riA.l Pnveloping the core thermal plume structure and rising along with the plume. This 'plume sheath' is chiefly made up from material of the lowermost plume source region. All entrainment into the plume head has its origin in the plume sheath, and all entrainment of ambient material happens between plume sheath and surrounding material. The plume sheath itself is too viscous and too cold to rise under its own thermal buoyancy, which suggests that it owes its rise to drag/pull of the fast ascending plume core material. Investigating the plume sheath model inferred from the laboratory experiments via numerical modelling, it is found that the numerical models readily reproduce plume sheath behaviour in the ascending plume. Furthermore, it is observed that there is little to no vorticity within the ascending plume core, and hence no lateral transport of material found within. Numerical models for mantle conditions introduce depth-dependent viscosity, which dramatically changes the dynamics of plume ascent and plume dynamics. Most notably, the passage of an ascending plume through sharp viscosity interfaces (attributed to mineral phase changes) lead to massive distortion and deformation of the plume head. This results in a remarkably complex compositional structure of the plume, and in increased sampling of specific regions in the mantle through entrainment. Still, despite the massive disturbance of plume shape and structure, there is little to no indication of lateral transport in either plume sheath or plume core. Applying a simple melting model to the numerical calculations, it is seen that deformation of the plume head during plume ascent may result in deep source and mid-source material being entrained into the plume head and ending up in the melting portion of the plume head, thus adding to the heterogeneity of the plume head melts. Lastly, the effects of plume deflection by a moving plate is investigated. It is found that a plume rising from a zonated source region exhibits characteristic patterns in the plume swell, thus potentially allowing for the drawing of conclusions on the composition and layout of the source region from the zonation of the plume track