HR: 11:20h
AN: T21F-05    [PDF]
TI: Chemical relaxation and seismic attenuation across a phase transition
AU: * Matas, J
EM: jan.matas@ens-lyon.fr
AF: ENS Lyon / CNRS, 46 allee d'Italie, Lyon, 69007 France
AU: Guyot, F
EM: guyot@lmcp.jussieu.fr
AF: Universite Paris 6 /IPGP, 4 place Jussieu, Paris, 75005 France
AU: Ricard, Y
EM: yanick.ricard@ens-lyon.fr
AF: ENS Lyon / CNRS, 46 allee d'Italie, Lyon, 69007 France
AB: Values of mantle thermodynamic properties are often deduced indirectly through seismological and geodynamical experiments that cover a large range of characteristic times. We show that due to phase transitions, the incompressibilities obtained by these experiments could be very different. Experimental probes faster than phase transformation kinetics sample unrelaxed properties at constant compositions. On the contrary, slow experiments sample relaxed properties at compositions maintaining thermodynamic equilibrium. We illustrate these concepts by using the example of the olivine-wadsleyite phase change that takes place around 410 km depth. We compute the incompressibility, attenuation and Bullen parameter as a function of kinetic rate of the transformation. We show that the compressional quality factor $Q_K$ of mantle material undergoing a phase change can be as low as 0.6 when characteritic times of the phase kinetics and geophysical probes are comparable. Assuming that the characteristic time of olivine to wadsleyite transformation under average mantle temperatures is between 3 hours and 3 months, we show that the chemical relaxation can be a significant source of dissipation and of energy sink for slow processes (normal modes, tides or Chandler wobble). Using the example of normal modes we show that the compressional quality factor can be significantly low ($Q_K \sim$100) even if the sampled values of $K_S$ are close to the unrelaxed ones. If the phase change kinetics is faster than 3 hours, the chemical dissipation can significantly affect probes even on seismic periods.
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3939 Physical thermodynamics
DE: 5144 Wave attenuation
SC: Tectonophysics [T]
MN: 2003 Fall Meeting