HR: 17:00h
AN: V44B-05 INVITED    [Abstracts]
TI: Depletion of the Continental Lithosphere
AU: * McKenzie, D
EM: mckenzie@madingly.org
AF: Cambridge University, Bullard Labs, Madingley Road, Cambridge, CB3 0EZ, United Kingdom
AB: It has been well known for thirty years that the density of the mantle is controlled by composition as well as by temperature, and that mantle convection is therefore strictly double diffusive. The change in density of the mantle caused by the removal of 25% melt is the same as that produced by a change in temperature of 500°C. Compositional variations are therefore likely to have an important effect on mantle dynamics. But it is difficult to include compositional variations in numerical models of mantle convection, partly because they result from melt extraction and partly because the Lewis number, the ratio of the thermal to the compositional diffusivity, is so large (~ 10¹⁰⁾. From an observational point of view there is a further problem. Though melt extraction has an important effect on density, it has almost no effect on the seismic velocities, at least in the upper mantle. Therefore compositional variations therefore cannot be mapped using seismic tomography. The only available method of doing so is by using geochemistry. Melts sample a larger region of the mantle than do nodules, and can therefore provide geodynamically more useful estimates of the mean density. Depletion by melt removal leaves a residue that is refractory. It is only likely to generate more melt if it is re-enriched by metasomatic melt. If it is then remelted, the resulting melts have easily recognisable signatures of the depletion and re-enrichment. The most enriched melts come from the most depleted residues and have a distinctive heavy rare earth signature. These effects can be used to estimate the composition and density of the source regions of the melt. Such studies show that the lithosphere beneath southern Africa, Tibet, and other regions of thick lithosphere have had their density decreased by the removal of ~ 25% melt, which has stabilised the lithosphere against convective instabilities. Similar effects may be important in the deeper parts of the mantle. However, because such regions do not generate melt, their composition is unknown.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting