Recycled basaltic material in mantle plumes explains the appearance of the X-discontinuity in the upper mantle beneath the Hawaiian hotspot: 2D geodynamic numerical models

Mantle plumes have a widely known role in recycling materials, but the quantities of the latter are not well constrained. Important indications come from seismic discontinuities like the X-discontinuity, interpreted as the coesite—stishovite phase transition, and clearly detected around 300 km depth beneath the Hawaiian hotspot. Seismology indicates that, for the X-discontinuity to be observed, 40% or more eclogite needs to be present. However, classical geodynamics studies predict that plumes can only carry up to 15-20% denser basalt to the surface.

To resolve this controversy, we run two sets of 2D geodynamics numerical models. Our first series features the recycled material in the plume conduit as discrete heterogeneities with diameters of 30-40 km, such that they do not mix with the background pyrolite and material segregation is allowed. Depending on the background viscosity, we observe three regimes. For its lowest values, the heterogeneities are too dense to be entrained: basalt segregation is directed downwards and no accumulation is present. At the highest viscosities, the heterogeneities rise together with the background. For intermediate viscosities, we detect cyclical ponding and material accumulation around and above the 410 km depth discontinuity, with occasional peaks up to 40-50% recycled material.

To explore the effect of the entrained basalt on plume dynamics we extend our models, now assuming mixing between the mantle and the recycled material. We also overcome limitations of our previous study by accounting for the energy balance, allowing the plume velocity to develop dynamically and the plume to spread laterally. These regional models feature a background mantle — made up of 82% harzburgite and 18% basalt — and mantle plumes with 1-15% additional basalt. Our preliminary results show that, in the range of plume excess temperatures between 200—300 K, the plume can still rise with up to 30% recycled basaltic material in the whole model, provided that it is hot enough to counterbalance the increased density.

Our models suggest that mantle plumes have the potential to accumulate higher percentages of denser material than previously thought. These results also provide a viable mechanism to explain the regional appearance of additional mantle phase transitions like the X-discontinuity.