Hydrous upwelling across the mantle transition zone beneath the Afar Triple Junction

Abstract:

The upwelling of material from the lower mantle to the base of the lithosphere is hypothesised as

being a primary geodynamic process and the mechanisms that drive upwelling (e.g. thermal vs. compositional

buoyancy) are key to our understanding of whole mantle convection. We address these issues with

new seismic data from recent deployments located on the Afar Triple Junction. The detailed images of

deep structure beneath this large igneous province illuminate features that give insights into the nature of

upwelling from the deep mantle. A seismic low velocity layer directly above the mantle transition zone,

interpreted as a stable melt layer, along with a prominent 520 km discontinuity suggest the presence of a

hydrous upwelling. Coincident with these features is a tomographically determined low velocity feature

within the mantle transition zone, and relatively uniform transition zone thickness associated with this

implies little variation in temperature. This suggests that upwelling is driven by compositional as opposed

to thermal buoyancy. The results are consistent with volatile rich, chemically distinct upwellings rising

from a heterogenous lower mantle source within the African Superplume.