Mantle Plumes

               

Principal Investigator

Alison Leitch

Research School of Earth Sciences

Projects

r01 - VPP, PC

The Earth's mantle is a 3000km deep layer of rock
which extends from just beneath the crust (30km
depth) to the surface of the hot, molten iron core. Over geological timescales of 10's to 100's of millions of years, the mantle convects. The cold and stiff near-surface material forms a layer 100-200km thick called the lithosphere. The lithosphere sinks into the mantle at subduction zones, and hot material rises and partially melts, producing about 6km of oceanic crust at mid-ocean ridge where the lithosphere spreads apart. Hot material also rises as "plumes", thermal instabilities which originate at the core-mantle boundary, and rise through the mantle as large spherical "heads", 1000km diameter, with thinner, low viscosity "tails" connecting the heads with the source region. When plume heads approach the surface, they partially melt, leading to massive, geologically brief outpourings of lava. These "flood basalt" events occur about every 10-20 million years, and they are followed by a trail of smaller scale hot-spot volcanism as the lithosphere moves over the stationary tail. The aim of this project is to simulate the mantle plumes impinging on the bottom of the lithosphere and melting.
 
         
               

   
               

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

We have investigated the melting that follows when the lithosphere rifts over the flattened head of a mantle plume. The warm plume material melts to a greater degree than ordinary mantle, resulting in thicker than normal oceanic crust. We were able to explain a thin band of thickened crust that runs along the margin of North America using this model. Quantitative agreement required a realistic viscosity profile of the mantle and lithosphere, and a "bumpy" lithosphere.

We also looked at the melting that occurs when a plume first approaches the bottom of the lithosphere, and found that to match observed melt rates and volumes of flood basalts required a significant component of easily melted, recycled oceanic crust. Again, the presence of such a component is indicated by independent geophysical and geochemical evidence. Continuing and future work focusses on the effect of the detailed viscosity profile of the mantle and the morphology of the bottom of the lithosphere on plume head melting. We are also looking at heat transfer between the mantle and crust, which involves some code development.

               
- Appendix A

               

       

What computational techniques are used?

The simulations have been carried out using the finite-difference program CONMG written by G.F. Davies and modified by A.M. Leitch. The code has been optimized to run on the vpp and is very efficient. This is important because very high resolution is required to properly resolve the melting region in the plumes (10's of kilometers) within a mantle scale (3000km) simulation.

Publications

A.M. Leitch, G.F. Davies, M. Wells, A plume head melting under a rifting margin, Earth Planet. Sci. Lett., 161, 161-177 (1998).

A.M. Leitch, M.J. Cordery, G.F. Davies, I.H. Campbell, Flood Basalts from Eclogite-Bearing Mantle Plumes, South African Journal of Geology, 100-4, 311-318(1998).

       
Appendix A -