Multiple seismic discontinuities near the base of the transition zone in the Earth's mantle

The seismologically defined boundary between the transition zone in the Ear th's mantle (410-660 km depth) and the underlying lower mantle is generally interpreted to result from the breakdown of the gamma-spinel phase of oliv ine(1) to magnesium-perovskite and magnesiowustite(2). Laboratory measureme nts of these transformations of olivine have determined that the phase boun dary has a negative Clapeyron slope and does indeed occur near pressures co rresponding to the base of the transition zone(2,3). But a computational st udy has indicated that, because of the presence of garnet minerals, multipl e seismic discontinuities might exist near a depth of 660 km (ref. 4), whic h would alter the simple negative correlation of changes in temperature wit h changes in the depth of the phase boundary. In particular, garnet mineral s undergo exothermic transformations near this depth, acting to complicate the phase relations(5-9) and possibly effecting mantle convection processes in some regions(9). Here we present seismic evidence that supports the exi stence of such multiple transitions near a depth of 660 km beneath southern California. The observations are consistent with having been generated by garnet transformations coupling with the dissociation of the gamma-spinel p hase of olivine. Temperature anomalies calculated from the imaged discontin uity depths-using Clapeyron slopes determined for the various transformatio ns(4)-generally match those predicted from an independent P-wave velocity m odel of the region.