PLASTIC-DEFORMATION OF SILICATE SPINEL UNDER THE TRANSITION-ZONE CONDITIONS OF THE EARTHS MANTLE

The dynamics of the Earth's deep interior are controlled to a large ex tent by rheological properties(1,2). Until recently, however, experime ntal studies on the rheological properties of materials thought to be present in the Earth's deep interior have been limited to relatively l ow pressures. Most previous estimates of rheology have therefore been based on either large extrapolations of low-pressure experimental data (3,4) or inferences from geodynamical observations(5-7). Such studies have provided only weak constraints on the complicated rheological str ucture expected in the transition zone of the Earth's mantle (between 410 and 660 km depth) where a series of phase transformations occur in silicate minerals(8). Here we report the results of a direct experime ntal study of deformation, under transition-zone conditions, of the sp inel phase of (Mg,Fe)(2)SiO4 (ringwoodite; thought to be present in th e Earth's transition zone). Relatively coarse-grained samples show evi dence of dislocation creep with dislocation structures similar to thos e observed in oxide and germanate spinels(9,10), which have significan tly higher creep strengths than olivine(10,11). In contrast, a fine-gr ained sample shows evidence for grain-size-sensitive creep. These obse rvations suggest that a ringwoodite-rich layer of the transition zone is likely to have a higher viscosity than the olivine-rich upper mantl e(3), whereas a subducting slab in the deep transition zone may lose i ts strength if significant grain-size reduction occurs(12-14).