HR: 17:15h
AN: T34A-06    [Abstracts]
TI: Isostatic Response of Strongly Extended Continental Lithosphere to Refertilization
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: Neumann, E
EM: e.r.neumann@geo.uio.no
AF: Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, Oslo, 0316, Norway
AU: Medvedev, S
EM: sergei.medvedev@fys.uio.no
AF: Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, Oslo, 0316, Norway
AU: Podladchikov, Y
EM: y.y.podladchikov@fys.uio.no
AF: Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, Oslo, 0316, Norway
AB: During stretching of the continental lithosphere, the density of the mantle column changes. In contrast to the simplification often used in geodynamic modeling, mantle density does not linearly decrease with increasing temperature, but is strongly dependent on pressure and bulk rock chemistry, and non-linear due to mineral phase transitions. We computed density as a function of temperature, pressure and composition for extending lithosphere from internally consistent thermodynamic data. If the temperatures are sufficiently high and the crust is thin a significant part of the lithosphere consists of light plagioclase peridotite, which causes the lithospheric mantle to become buoyant at strongly stretched continental margins. This effect is most pronounced for fertile bulk compositions. The amount of plagioclase peridotite in the mantle lithosphere, and therefore the magnitude of the buoyancy (uplift), is mainly determined by the bulk rock aluminum content and the sodium/aluminum ratio. Re-fertilization of the mantle by infiltrating fluids or melts should therefore enhance the formation of plagioclase, decrease the density and increase upward motion. Detailed field work on fossile and current passive margins has confirmed that lithosphere extension is inevitably accompanied by extensive infiltration of the stretched lithospheric mantle by magmatic fluids. This leads to re-fertilization of the mantle, crystallization of new plagioclase and formation of plagioclase at the expense of spinel. The metamorphic reaction is enhanced by the flux provided by the infiltrating melts, shifting ‘metamorphic closure temperatures' to lower values. We therefore propose that mantle phase transitions might play an important role for the exhumation of strongly stretched continental mantle at passive margins.
DE: 3621 Mantle processes (1038)
DE: 3939 Physical thermodynamics
DE: 4465 Phase transitions
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting