COMPUTATION OF SEISMIC PROFILES FROM MINERAL PHYSICS - THE IMPORTANCEOF THE NON-OLIVINE COMPONENTS FOR EXPLAINING THE 660 KM DEPTH DISCONTINUITY

The recent increasing number of experimental works leads us to review the elastic properties and mineralogical transformations of mantle min erals. The updated data set is used to compute seismic profiles for tw o petrological models along three adiabatic temperature profiles. Thes e profiles are chosen to stress out the influence of non-olivine miner als on seismic parameters, and to represent cold and horizontally aver aged temperature profiles of the Earth's mantle. In a first part, star ting compositions of pyrolite and piclogite and a single layer convect ion are assumed. The results clearly point out the importance of the n on-olivine part of the mineralogy. Two scenarios are found to explain the 660 km depth discontinuity, whatever the starting composition. (1) Ilmenite appears at the expense of garnet at 660 km depth, and then t ransforms into perovskite and a small amount of garnet at pressures re levant to the lower mantle (case of a 1500 K adiabat with current phas e diagrams). The cumulative effects of the breakdown of gamma-spinel a nd of the reactions involving ilmenite lead to strong seismic disconti nuities at 660 km depth followed by relatively small seismic gradients at the top of the lower mantle. (2) Ilmenite is not stable around 660 km depth, and the breakdown of gamma-spinel is the only sharp reactio n to occur (case of a 1600 K adiabat). Smaller seismic discontinuities are found at 660 km depth, and higher seismic gradients are obtained at the top of the lower mantle. Taking into account experimental uncer tainties, the comparison of our calculations with reference seismic mo dels strongly suggests that ilmenite is present at the upper-lower man tle boundary. Along the cold temperature profile (1000 K adiabat), the reactions involving ilmenite appear at separate depths, leading to a complex upper-lower mantle transition with three separate discontinuit ies. A case with a stratified convection (thermal boundary layer and d ifferent compositions between the upper and the lower mantle) is also studied. The strength of the discontinuity induced by the chemical bou ndary is in a good agreement with seismic observations when a horizont ally averaged temperature profile is used for the mantle. This result implies that it is not possible to discriminate between layered or sin gle cell convection in the Earth's mantle. Along the cold temperature profile, the appearance of ilmenite in the upper mantle leads to a sec ond discontinuity in addition to the one imposed by the chemical bound ary. Hence, subduction zones should be characterised by a multiple-ste p transition from the upper to the lower mantle, whatever the chosen s tyle of convection. The proposed explanation of the 660 km discontinui ty is comforted by recent seismic observations: a complex behaviour of the 660 km discontinuity has indeed been found in subducting slabs, a nd broadband studies of converted waves have suggested a multiple disc ontinuities pattern to explain this complexity. (C) 1998 Elsevier Scie nce B.V. All rights reserved.