The 660-km discontinuity within the subducting NW-Pacific lithospheric slab

The 660-km seismic discontinuity (660) in Earth's mantle is generally attributed, to the breakdown of the ringwoodite phase of olivine, but other mineral reactions are also thought to occur near 660-km depth. Recently, complex arrivals of P660s waves (converted from P to s at the 660) in active and recently active subduction zones have been interpreted as evidence for additional seismic discontinuities caused by the garnet-perovskite and garnet-ilmenite-perovskite phase transformations (gt --> pnu, gt --> il --> pnu) at relatively low temperatures. Here we show that the P660s phases converting at the 660 within the subducting NW-Pacific slab beneath the station MDJ in Northeast China are clear and coherent, with no additional arrivals in the vicinity. P660s waves that convert near the boundaries of the area where the 660 occurs within the slab produce distinctly more complex, multiple arrivals, but they are more likely to be caused by small-scale topography rather than 'multiplicity' of the 660. Our observations suggest that the gt --> pnu transformation and the gt --> il --> pnu, if it occurs in the mantle, are spread over tens of kilometers and do not have sharp onsets visible to short-period seismic waves. (C) 2002 Elsevier Science B.V. All rights reserved.