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Experimental and theoretical studies have shown that the iron spin transition alters the properties of lower mantle minerals. This may have important implications for mantle dynamics. In particular, the vigor of convection is enhanced, which in turn may impact the stability of large primordial reservoirs at the base of the lower mantle. Here we performed numerical experiments of thermochemical convection in 2-D annulus geometry including the change of density induced by iron spin transition. Our results show that this density change only slightly affects mantle dynamics by increasing the convection vigor. This, in turn, slightly increases the entrainment of primordial dense materials by plumes and leads to smaller reservoirs with sharper edges. However, this effect is small compared to those of the intrinsic density contrast between the dense primordial and regular mantle materials, which is the dominant parameter controlling the long-term stability of the large primordial reservoirs.

Plain Language Summary Experimental and theoretical studies suggested that transition in the spin state of iron occur in the lower mantle minerals at lower mantle pressure and that it may have important implications for mantle dynamics. In this study, we performed 2-D numerical experiments of thermochemical mantle convection implemented with a recent mineral data set as density input. We find that the dynamic effects of the iron spin transition on the lower mantle's thermochemical structures are small. The main parameter controlling the long-term stability of large primordial reservoirs in the lower mantle remains the chemical density contrasts between the primordial material and ambient mantle material that controls.