HR: 13:40h
AN: V33D-01 INVITED    [Abstracts]
TI: What Determines the Thickness of the Lithosphere?
AU: * Simon, N S
EM: n.s.c.simon@fys.uio.no
AF: Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, Oslo, 0316, Norway
AU: Beuchert, M
EM: m.j.beuchert@fys.uio.no
AF: Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, Oslo, 0316, Norway
AU: Podladchikov, Y Y
EM: y.y.podladchikov@fys.uio.no
AF: Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, Oslo, 0316, Norway
AB: Cratonic continental lithosphere can be thicker than 200 km and survive for billions of years. This is commonly ascribed to greater buoyancy and strength of the cratonic lithospheric mantle compared to younger lithosphere. The boundary between the lithosphere and the asthenosphere is often defined as an isotherm even though it is a rheological boundary that determines where rocks become stiff enough to resist deformation by mantle flow. The viscosity of rocks depends on temperature, but also on composition. Refractory mantle has a significantly higher solidus than fertile mantle, implying that refractory mantle has a higher viscosity than fertile mantle at the same temperature. In fact, a significant part of the cratonic lithospheric mantle might be so highly viscous that it behaves elastically. Another important factor for the rheology of the mantle is the water content. The presence of water reduces the viscosity of olivine and therefore significantly weakens the mantle. Refractory mantle that has experienced large degrees of melt extraction should be depleted in water, which adds to the strength imposed by the high solidus and low temperatures. Numerical simulations show that cratonic keels indeed resist erosion by mantle convection for infinitely long times if realistic viscosity contrasts are applied. However, observations also show that some cratons are destroyed over geological time and loose their thick roots. This destruction of ancient roots appears to be preceded by complete refertilization of the lithospheric mantle through metasomatic melt infiltration, which confirms the notion that composition plays an important role for the ability of continental roots to resist erosion. Whereas the thickness of Archaean cratonic lithospheres might be determined by the initial thickness of refractory mantle created during their formation, younger lithospheres seem to stabilize at much lower, but uniform thicknesses, even though they probably form by different mechanisms. We propose that the inherent layering of the lithosphere that is caused by mantle phase transitions may play an important role for the level at which the lithosphere-asthenosphere boundary is established. The garnet-spinel phase transition is located at ca. 90 km depth at 1330 °C in fertile mantle and is associated with one percent change in density. This density contrast is large enough to trigger gravitational instabilities and aid delamination of the garnet-peridotite part of the lithosphere. At the same time, the lower density of spinel-peridotite acts as a barrier to ascending small scale convection. Inherited compositional layering re-enforces this barrier. In addition to the density contrast the garnet- spinel boundary might also influence the rheology due to changes in the modal proportion of olivine relative to pyroxenes. The amount of pyroxene is reduced in the spinel-peridotite to garnet-peridotite reaction. Pyroxenes can host about 10 times more water than olivine, and redistribution of water between the minerals due to the phase transition will increase the water content in olivine and thereby cause a reduction in viscosity of garnet-peridotite relative to spinel-peridotite. The development of gravitational instabilities depends on contrasts in densities and viscosities, which can possibly be both provided by the layering caused by the phase transition.
DE: 3621 Mantle processes (1038)
DE: 4465 Phase transitions
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting