Mantle Plumes

Principal Investigator

Alison Leitch

Research School of Earth Sciences

Heat and mass are transferred within the Earth's rocky mantle by a convecting system which consists of tectonic plates forming and subducting at the surface and mantle plumes rising from the hot boundary between the molten iron core and the mantle. Mantle plumes consist of a large, spherical head of hot material with an even hotter, narrow "tail" connecting the head to the source region just above the core. Plume heads and tails melt when they get close to the surface, the heads producing millions of cubic kilometers of lava (e.g. Deccan Traps in India), and the tails making strings of volcanic islands (e.g. Hawaiian Islands). This project looks at the way mantle plumes rise and melt as they approach the Earth's surface: we aim to understand the influences of mantle composition and viscosity structure, and the core temperature on the rate and total volume of melting.

Projects

r01 - VPP


What are the results to date and the future of the work?

Just off the east coast of the USA, next to the continental shelf, is a long narrow region of thickened oceanic crust called the East Coast Margin Igneous Province (ECMIP). Oceanic crust is produced by melting the mantle at mid-ocean ridges, and unusually thick crust is often attributed to higher than normal temperatures in the mantle. We proposed that when the Atlantic Ocean opened the rift was underlain by the flattened and extended head of a mantle plume. To test this, we carried out numerical models of mantle convection where we rifted the lithosphere over various model plume heads. To match the observations, we need a thin, flat plume head where the temperature tapers sharply from the top boundary. This structure results if, on its way from the core-mantle boundary to the surface, a plume passes through a step decrease in the background viscosity of the mantle at 660 km depth. Such a step decrease is independently indicated from studies of gravity signals around subduction zones and post-glacial rebound.

Flood basalt eruptions are massive outpourings of lava (105 - 107 cubic km) which take place within very short periods of time (1-3 million years). They occur sporadically over the globe about once every 5-10 million years. It has been suggested that they are due to decompression melting of a mantle plume head. However, if the mantle has a constant viscosity, the head traps a layer of normal mantle between it and the base of the lithosphere (the cold, stiff upper boundary to the mantle), and the top of the head remains too deep to melt significantly. If there is a step reduction in the mantle viscosity at 660 km depth, then the plume head necks down as it passes through the step, and the small, hot central part of the plume rises quickly and impinges directly on the base of the lithosphere. This allows about 105 cubic km of



melting in 3 million years, but to explain the larger, faster eruptions we must look further. Geochemical studies suggest plumes contain a component of ancient oceanic crust, which has a much lower melting point than the "normal" mantle. When we add 15 % by volume of this component to a plume passing through a step viscosity decrease, we can match the melt-rates of the larger eruptions, however, this predicts tail melt-rates that are much too high. Further work will involve closer investigation of the effect of mantle rheology, and the possible concentration of a crustal component in the plume head.

What computational techniques are used?

The code used in this project, CONMG is a multigrid finite-difference code written by Dr Geoff Davies of the Research School of Earth Sciences and developed for use in this project by the author. The code is specialized to study mantle convection problems in two dimensions in a cartesian or cylindrical domain. It rapidly and efficiently solves the equations of conservation of mass, momentum and energy in an incompressible, highly viscous medium with boundary conditions appropriate to the Earth. To study melting problems (the aim of this project) very high resolution is needed, involving 40-240MB of core memory.

- Appendix A