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Abstract Constraints on chemical heterogeneities in the upper mantle may be derived from studying the seismically observable impedance contrasts that they produce. Away from subduction zones, several causal mechanisms are possible to explain the intermittently observed X‐discontinuity (X) at 230–350 km depth: the coesite‐stishovite phase transition, the enstatite to clinoenstatite phase transition, and/or carbonated silicate melting, all requiring a local enrichment of basalt. Africa hosts a broad range of terranes, from Precambrian cores to Cenozoic hotspots with or without lowermost mantle origins. With the absence of subduction below the margins of the African plate for >0.5 Ga, Africa presents an ideal study locale to explore the origins of the X. Traditional receiver function (RF) approaches used to map seismic discontinuities, such as common conversion‐point stacking, ignore slowness information crucial for discriminating converted upper mantle phases from surface multiples. By manually assessing depth and slowness stacks for 1° radius overlapping bins, normalized vote mapping of RF stacks is used to robustly assess the spatial distribution of converted upper mantle phases. The X is mapped beneath Africa at 233–340 km depth, revealing patches of heterogeneity proximal to mantle upwellings in Afar, Canaries, Cape Verde, East Africa, Hoggar, and Réunion with further observations beneath Cameroon, Madagascar, and Morocco. There is a lack of an X beneath southern Africa and strikingly, the magmatic eastern rift branch of the southern East African Rift. With no relationships existing between depth and amplitudes of observed X and estimated mantle temperatures, multiple causal mechanisms are required across a range of continental geodynamic settings.
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Linking Geodynamic Models of Basalt Segregation in Mantle Plumes to the X‐Discontinuity Observed Beneath Hotspots
Abstract Mantle plumes are thought to recycle material from the Earth's deep interior. One constraint on the nature and quantity of this recycled material comes from the observation of seismic discontinuities. The detection of the X‐discontinuity beneath Hawaii, interpreted as the coesite‐stishovite transition, requires the presence of at least 40% basalt. However, previous geodynamic models have predicted that plumes cannot carry more than 15%–20% of high‐density basaltic material. We propose this contradiction can be resolved by taking into account the length scale of chemical heterogeneities. While previous modeling studies assumed mechanical mixing on length scales smaller than the model resolution, we here model basaltic heterogeneities with length scales of 30–40 km, allowing for their segregation relative to the pyrolitic background plume material. Our models show that larger basalt fractions than previously thought possible—exceeding 40%—can temporarily accumulate within plumes at the depth of the X‐discontinuity. Two key mechanisms facilitate this process: (a) The random distribution of basaltic heterogeneities induces large temporal variations in the basalt fraction with cyclical highs and lows. (b) The high density contrast between basalt and pyrolite below the coesite‐stishovite transition causes ponding and accumulation of basalt within the rising plume at that depth. Because the statistical effect dominates, large values of 35%–40% basalt are only sustained temporarily. These results further constrain the chemical composition of the Hawaiian plume. Beyond that, they provide a geodynamic mechanism that explains the seismologic detection of the X‐discontinuity and highlights how recycled material is carried toward the surface.
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- Award ID(s):
- 1925677
- PAR ID:
- 10426708
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 128
- Issue:
- 6
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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