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

D A Thompson,D Keir,B Goitom,J O S Hammond,J.-M Kendall,A Ayele,G W Stuart,G R Helffrich

doi:10.1002/2014gc005648

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.

Highlights

The upwelling of material from the lower mantle to the base of the lithosphere is hypothesized as being a primary planetary geodynamic process [Morgan, 1971], and it is widely believed that the driving force behind these upwellings is thermal convection initiated due to heating at the core-mantle boundary [Beier et al, 2008]
The extent to which upwellings and their associated surface volcanism are driven by temperature or compositional variations in the mantle is currently poorly constrained by observations [Ito and van Keken, 2007]
This observation is supported by recent estimates from P wave tomography and joint seismic, geochemical, and numerical modeling that suggest a mild thermal anomaly of 100 K exists in the mantle transition zone (MTZ) and upper mantle beneath the Afar Depression(Armitage et al, Upper mantle temperature and the onset of extension and break-up in Afar, Africa, submitted to Earth and Planetary Science Letters, 2015; Civiero et al, submitted manuscript, 2015)

Summary

Introduction

The upwelling of material from the lower mantle to the base of the lithosphere is hypothesized as being a primary planetary geodynamic process [Morgan, 1971], and it is widely believed that the driving force behind these upwellings is thermal convection initiated due to heating at the core-mantle boundary [Beier et al, 2008]. Due to the opposite signs of the Clapeyron slopes associated with the 410 and the 660, regions of warmer than average mantle, a likely situation if upwelling is driven by thermal buoyancy, should produce a thinner than expected MTZ. This has been observed in certain localities (e.g., 20 km beneath Iceland suggesting an excess temperature of 150 K) [Shen et al, 1998], but other global observations have suggested that there is little correlation of a thinner MTZ thickness with the location of mantle upwellings [Tauzin et al, 2008]

Results

Discussion

Conclusion