Deep mantle structure and the plumes within

Please join us for a special seminar this Wednesday at 1:30 PM in room 155 to be given by Keely O’Farrell from the Department of Earth and Environmental Sciences at the University of Kentucky.

Deep mantle structure and the plumes within

The journey of a plume through the mantle has the potential to tell us about the structure and dynamics of the deep interior. Hotspot volcanism provides a direct link between the deep mantle and the surface, but the location, depth and source of the mantle plumes that feed hotspots remain highly controversial. In order to address these issues, it is necessary to try and track plumes along their journeys. The location and behaviour of plumes can tell us about the vigour of mantle convection, net rotation of the mantle and role of thermal versus chemical anomalies as well as important bulk physical properties of the mantle such as the viscosity profile. To address these questions, we developed a new algorithm to trace plume-like features in shear-wave (Vs) seismic tomography models based on picking local minima in the velocity and searching for continuous features with depth. Applying this new method to recent tomography models has produced 60+ continuous plume conduits that are > 750 km long. Around a third of these can be associated with known hotspots at the surface.

We study the morphology of these plume chains and find that no net rotation is apparent in the mantle based on net deflection of the plumes. We analyzed the preferred orientation of the plume deflections and their gradient to infer large scale mantle flow patterns and the depth of viscosity contrasts in the mantle respectively. We find good agreement with previous studies showing a viscosity increase around 1000 km depth based on the common depth of plume deflections. To further explore the viscosity in the deep mantle, we compute synthetic geoids from different viscosity and density structures and compared with observations. We find evidence for a gradual increase in viscosity starting from below the transition zone (660km depth) to about 1000 km depth. The structure of the deep mantle will be explored through these two techniques.