The role of mantle viscosity heterogeneities on the development of secondary plumes in the upper mantle

Recent seismic observations have revealed diverse plume morphologies beneath hotspots that are characterized by multiple secondary plume branches with different sizes and temperatures. However, how the structures of these secondary plumes have developed and what controls the properties of these secondary plumes have not been well explored. Here, we performed 3-D numerical models to systematically investigate the effects of mantle viscosity structures and the strength of the 660-km phase change on the development of secondary plumes. We find that the characteristics of the secondary plumes, in terms of their numbers, excess temperatures and sizes, are controlled by the Clapeyron slope of the 660-km phase change (gamma(660)), the properties of the low-viscosity layer beneath the mantle transition zone (viscosity reduction ratio, thickness, depth or phase change dependency of viscosity) and the presence of the weak asthenosphere. For gamma(660) between -2.0 and -3.0 MPa K-1, the weak layer viscosity reduction ratio plays a first-order role in controlling secondary plume properties, that is, a smaller viscosity reduction ratio results in more secondary plumes, higher plume excess temperature and larger plume size. The thickness and viscosity dependencies of the weak layer have a secondary effect on secondary plume properties. However, a relatively small gamma(660) (e.g. -1.0 MPa K-1) would result in a single secondary plume irrespective of the weak layer viscosity reduction ratio, the viscosity dependency, and whether with or without the weak asthenosphere. However, our models also show that the presence of the weak asthenosphere has a prominent influence in increasing the number of secondary plumes but reducing the size of the plumes. Our results provide new insight into plume dynamics and suggest that the diverse plume structures observed in the upper mantle likely reflect different mantle viscosity structures beneath the hotspots.