The effects of stress on phase reaction progress and position in depth-temperature space: implications for seismically observable mantle dis

At nominal depths of 410 and 660km in the Earth, there are distinct changes in density and seismic wave speed. These features arise from phase changes in (Mg,Fe)2SiO4. At 410km olivine transforms to wadsleyite and at 510km into ringwoodite. At 610km ringwoodite dissociates to bridgmanite + ferropericlase. The depths of these two discontinuities varies laterally by a few 10s of km. This variation is often explained in terms of variations in temperature, water, or iron content in the mantle. However, these explanations are incomplete and are frequently invoked in contradictory ways for different parts of the Earth. An alternative hypothesis is that convection in the Earth’s mantle induces differential stress which may have a significant influence on the depth and reaction rate of the phase transition. We propose here to test this hypothesis by measuring the progress of phase transitions in (Mg,Fe)2SiO4 polymorphs and analogues under high and low stress conditions in the same experiment.