Role of garnet shaping the 660-km seismic discontinuity

The 660-km discontinuity is commonly attributed to the post-spinel transition of ringwoodite to bridgmanite and ferropericlase, yet it does not fully explain observed structural complexity. Here, we examine the influence of majorite garnet, the second most abundant mantle mineral, on the post-spinel transition using high-pressure and high-temperature experiments. We find that the post-spinel transition pressure in garnet-bearing systems differs from that in garnet-free systems but remains consistent with the post-garnet transition of majorite garnet to bridgmanite. This suggests that the post-spinel transition in a multi-component mantle occurs as a linked reaction induced by the post-garnet transition. Moreover, the occurrence of garnet transformations consistently explains the roughness structures and dynamics of the 660-km discontinuity observed near cold and hot mantle flows. These results indicate that the average mantle comprises not a mechanical mixture of heterogeneous lithologies but rather a homogeneous lithology with a pyrolite-like bulk composition.