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

Abstract

The mechanisms that drive the upwelling of chemical heterogeneity from the lower to upper mantle (e.g., thermal versus compositional buoyancy) are key to our understanding of whole mantle convective processes. We address these issues through a receiver function study on new seismic data from recent deployments located on the Afar Triple Junction, a location associated with deep mantle upwelling. The detailed images of upper mantle and mantle transition zone structure illuminate features that give insights into the nature of upwelling from the deep Earth. 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. A relatively uniform transition zone thickness across the region suggests a weak thermal anomaly (<100 K) may be present and that upwelling must be at least partly driven by compositional buoyancy. The results suggest that the lower mantle is a source of volatile rich, chemically distinct upwellings that influence the structure of the upper mantle, and potentially the chemistry of surface lavas.

Why is Africa rifting?

“…The architecture of mantle upwelling from the core -mantle boundary to the surface beneath NE Africa is still being determined and there is considerable debate as to its thermo-chemical nature. Smaller plumes may initiate from instabilities in a deeper super-plume that may or may not be chemically distinct from the ambient mantle (Civiero et al 2015;Thompson et al 2015). However, it is hard to see how this anomaly could not be buoyant given the significant dynamic topography of Africa.…”

Multiple mantle upwellings in the transition zone beneath the northern East‐African Rift system from relative P‐wave travel‐time tomography

“…However, Thompson et al . [] find little variation in depth of “410” and “660” below the region, and consequently suggest that the temperature anomaly is less than 100°C, to explain the relatively constant thickness of the transition zone similar to the global average (as also found below two stations in the region by Nyblade et al . []).…”

Stochastic Inversion of Geomagnetic Observatory Data Including Rigorous Treatment of the Ocean Induction Effect With Implications for Transition Zone Water Content and Thermal Structure

“…Although an accurate characterization of the melt layer is beyond the scope of this work, our results nonetheless suggest that the water filter hypothesis is only likely to be operative on a local/regional scale. The locations include western, central, and eastern United States (Courtier & Revenaugh, 2006;Jasbinsek & Dueker, 2007;Song et al, 2004;Song & Helmberger, 2006), Siberian platform (Vinnik & Farra, 2007), Arabian Plate (Vinnik et al, 2003), off the coast of Japan (Bagley et al, 2009;Revenaugh & Sipkinf, 1994), Southwest Pacific (Courtier & Revenaugh, 2007), and Afar Triple Junction in East Africa (Thompson et al, 2015). The locations include western, central, and eastern United States (Courtier & Revenaugh, 2006;Jasbinsek & Dueker, 2007;Song et al, 2004;Song & Helmberger, 2006), Siberian platform (Vinnik & Farra, 2007), Arabian Plate (Vinnik et al, 2003), off the coast of Japan (Bagley et al, 2009;Revenaugh & Sipkinf, 1994), Southwest Pacific (Courtier & Revenaugh, 2007), and Afar Triple Junction in East Africa (Thompson et al, 2015).…”