Slow Si–Al interdiffusion in garnet and stagnation of subducting slabs

Si4++M2+↔2Al3+ (M=Mg+Fe+Ca) interdiffusion rates in majoritic garnet have been determined at pressure of 17 GPa and temperatures of 1550–1700 °C using a Kawai-type high-pressure apparatus. The diffusion profiles were measured across the interface between pyropic garnet and majoritic garnet samples by an analytical transmission electron microscope, and the rate of the Si4++M2+↔2Al3+ interdiffusion was determined to be a DSi–Al (m2 s−1)=2×10−10 exp [−364 (kJ mol−1)/RT]. We found that the diffusion rate is low compared to the Si diffusion rate in other silicate minerals relevant to the deep mantle. Slow kinetics of the pyroxene–garnet transformation due to the low diffusion rate indicates that large low-density metastable regions may exist in the slab, greatly contributing to the slab stagnation around the transition region. Stagnant slabs possibly stay in this region over 108 yr because of the sluggish pyroxene–garnet transformation, but may eventually fall into the lower mantle after the completion of the transformation.