Narrow, fast, and "cold" mantle plumes on Earth explained by strain-weakening rheology in the lower mantle

The rheological properties of­­­ Earth’s lower mantle materials are key for mantle dynamics and  planetary evolution. The main rock-forming minerals in the lower mantle are bridgmanite (Br) and smaller amounts of ferropericlase (Fp). Bridgmanite minerals are intrinsically much stronger than ferropericlase minerals, resulting in significant variations in lower-mantle rheological behavior depending on the quantity and degree of interconnectivity of the weak phase. The resulting effective bulk rock viscosity decreases with accumulating strain when the weaker Fp minerals become elongated and eventually interconnected. This implies that strain localization may occur in Earth’s lower mantle, which would in turn influence the pattern of mantle flow and could potentially aid the preservation of compositionally distinct, “hidden” reservoirs. So far, there have been no studies on global-scale mantle convection in the presence of such strain-weakening (SW) rheology.

Here, we present 2D numerical models of thermo-chemical convection in spherical annulus geometry including a new strain-weakening (SW) rheology formulation for lower-mantle materials. This macro-scale SW rheology is based on micro-scale rheological behavior found in prior studies, and combining rheological weakening and healing terms. We determine the effects of SW rheology on the planform of mantle flow, the mixing of chemical reservoirs, and the dynamics of mantle plumes.

We find that, in particular, plume conduits are weakened and act as lubrication channels which allow for the rapid ascent of mantle material. Their thermal anomalies and geometries are significantly different than those of mantle plumes which are not rheologically weakened. Moreover, larger thermochemical piles at the base of the mantle are stabilized by SW rheology, with implications for preservation of chemically-distinct materials over long timescales. Finally, we put our results into context with observations and existing hypotheses on the style of Earth's mantle convection and mixing. Most importantly, we suggest that the new kind of plume dynamics may explain the discrepancy between expected and observed thermal anomalies of deep-seated mantle plumes on Earth.