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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 $>$0.5\,Ga, Africa presents an ideal study locale to explore the origins of the X. Traditional receiver function (RF) approaches used to map seismic discontinuities, like common conversion-point stacking, ignore slowness information crucial for discriminating converted upper mantle phases from surface multiples. By manually assessing depth and slowness stacks for 1$^\circ$ 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.

The contribution of mantle upwellings of varying spatial extent to Cenozoic magmatism across Africa is debated because geochemical and seismological tools used to interrogate them are primarily sensitive to either composition or temperature. Thermochemical conditions control the depth at which mantle materials undergo phase changes, which cause seismic discontinuities. Mapping seismic discontinuities across the mantle transition zone (MTZ) and below provides insight into the variable thermochemical nature of upwellings. We present observations of seismic discontinuities beneath Africa obtained from a compilation of P-to-s receiver functions (using Pds, PPds, and PKPds phases), recorded at seismograph networks across Africa between 1990-2019. We exploit a recent high-resolution African continental P-wavespeed model to migrate our receiver functions to depth in a common conversion point stack. Cenozoic magmatism along the East African Rift is largely underlain by a thin MTZ implying a contribution to rift magmatism from sources at or below MTZ depths. The Ethiopian rift is underlain by a depressed d410 and uplifted d660 indicating a moderate positive thermal anomaly at MTZ depths (~100-150K). The southern East African Rift displays a greater d410 depression and a regional d660 depression, suggesting a stronger thermochemical anomaly at MTZ depths. Here, seismic conversions at ~1025km depth are collocated with slow wavespeeds within the African Superplume, corroborating evidence for a compositional anomaly. We suggest that the contribution of a purely thermal plume directly below Ethiopia augments conditions for mantle melting and rifting. Distinct upwellings may also affect the MTZ below Cenozoic magmatism in Cameroon and Madagascar.

Africa’s Cenozoic tectonism is often attributed to mantle plumes, particularly below East Africa, but their morphology, number, location, and impact on the African lithosphere are debated. The broad slow wavespeed African Superplume, ubiquitous in large-scale tomographic models, originates below South Africa, reaching the surface somewhere below East Africa. However, whether the diverse East African mantle geochemistry is best reconciled with one heterogeneous upwelling, or current tomographic models lack the resolution to image multiple distinct plumes, remains enigmatic. S-wavespeed tomographic images of Africa are legion, but higher-frequency P-wavespeed whole-mantle models possessing complementary diagnostic capabilities are comparatively lacking. This hinders attempts to disentangle the effects of Cenozoic hotspot tectonism and Pan African (and older) tectonic events on the East African lithosphere. Here we develop a continental-scale P-wave tomographic model capable of resolving structure from upper-to-lower mantle depths using a recently-developed technique to extract absolute arrival-times from noisy, temporary African seismograph deployments. Shallow-mantle wavespeeds are δVP ≈–4% below Ethiopia, but less anomalous (δVP ≥–2%) below other volcanic provinces. The heterogeneous African Superplume reaches the upper mantle below the Kenyan plateau. Below Ethiopia/Afar we image a second sub-vertical slow wavespeed anomaly rooted near the core-mantle boundary outside the African LLVP, meaning multiple disparately sourced whole-mantle plumes may influence East African magmatism. In contrast to other African cratons, wavespeeds below Tanzania are only fast to 90–135km depth. When interpreted alongside Lower Eocene on-craton kimberlites, our results support pervasive metasomatic lithospheric modification caused by subduction during the Neoproterozoic Pan-African orogeny.

The Hawaiian Island chain in the middle of the Pacific Ocean is a well-studied example of hotspot volcanism caused by an underlying upwelling mantle plume. The thermal and compositional nature of the plume alters the mantle phase transitions, which can be seen in the depth and amplitude of seismic discontinuities. This study utilises >5000 high quality receiver functions from Hawaiian island stations to detect P-to-s converted phases to image seismic discontinuities between 200 to 800 km depth. Common-conversion point stacks of the data are used to map out lateral variations in converted phase observations, while slowness stacks allow differentiation between true conversions from discontinuities and multiples. We find that the 410 discontinuity is depressed by 20 km throughout our study region, while the main 660 is around average depth throughout most of the area. To the southwest of the Big Island we observe splitting of the 660, with a major peak at 630 km, and a minor peak appearing at 675 km depth. This is inferred to represent the position of the hot plume at depth, with the upper discontinuity caused by an olivine phase transition and the lower by a garnet phase transition. In the upper mantle, a discontinuity is found across the region at depths varying between 290 to 350 km. Identifying multiples from this depth confirms the presence of a so-called X-discontinuity. To the east of the Big Island the X-discontinuity lies around 336 km and the associated multiple is particularly coherent and strong in amplitude. Strikingly, the discontinuity around 410 km disappears in this area. Synthetic modelling reveals that such observations can be explained by a silica phase transition from coesite to stishovite, consistent with widespread ponding of silica-saturated material at these depths around the plume. This material could represent eclogite enriched material, which is relatively silica-rich compared to pyrolite, spreading out from the plume to the east as a deep eclogite pool, a hypothesis which is consistent with dynamical models of thermochemical plumes. Therefore these results support the presence of a significant garnet and eclogite component within the Hawaiian mantle plume.

Iceland sits astride a mid‐ocean ridge underlain by a mantle hot spot. The interplay of these two geological processes has the potential to generate a complex and laterally variable crustal structure. The thickness of the Icelandic crust is a long running and controversial debate, with estimates ranging from a thin 20‐km crust to a thick 40‐km crust. We present new images of the first‐order seismic discontinuity structure of the Icelandic crust based on a joint inversion of receiver function and ambient noise‐derived surface wave dispersion data. Inversion results are validated through comparison to receiver functions multiphase common conversion point stacks across the densely instrumented Northern Volcanic Zone. We find a multilayered crustal structure consisting of a 6‐ to 10‐km‐thick upper crust underlain by either one or two discontinuities. The shallower discontinuity is found at depths of ≈20 km throughout Iceland. The deeper discontinuity is only present in some regions, defining the base of a lens‐like lower layer with maximum depths of 44 km above the center of the mantle plume. Either of these two discontinuities could be interpreted as the seismic Moho, providing an explanation why previous estimates of crustal thickness have diverged. Such structure may form via underplating of a preexisting oceanic crust as has been hypothesized in other ocean island plume settings. However, we demonstrate with a simple petrological model that variability in seismic discontinuity structure can also be understood as a consequence of compositional variation in melts generated with distance from the plume center.\ud \ud Plain Language Summary\ud \ud When tectonic plates pull apart, magma wells up between them forming new oceanic crust. Iceland sits astride one of these mid‐ocean ridges, but unlike most others which are found on the ocean floor, it is raised above sea level. This is caused by a hot area of the Earth's mantle raising the area up, thought to be caused by a mantle plume (a convective upwelling rising from the Earth's core). In this study we try and understand what crust formed in this special setting, where mid‐ocean ridge and mantle plume interact, looks like. We make observations of the Icelandic crust using distant earthquakes that are recorded in Iceland, extracting information that earthquake signals carry about the material they travel through on their journey through the Icelandic crust. This gives us a new picture of Iceland's crust: it is much thicker than normal mid‐ocean ridge crust, thickest in the center above the plume and thinning outward, and is made up of several layers. By analyzing crystal content of lavas erupted in Iceland at different distances from the plume, we construct a model that explains the structure we observe by variation in the types of magma available for crustal formation in different locations.

Funder: Royal Commission for the Exhibition of 1851; Id: http://dx.doi.org/10.13039/501100000700, Funder: University of Cambridge; Id: http://dx.doi.org/10.13039/501100000735, Funder: Geological Society of London; Id: http://dx.doi.org/10.13039/100008066, Geophysical analysis of the Earth's lower mantle has revealed the presence of two superstructures characterized by low shear wave velocities on the core‐mantle boundary. These Large Low Shear Velocity Provinces (LLSVPs) play a crucial role in the dynamics of the lower mantle and act as the source region for deep‐seated mantle plumes. However, their origin, and the characteristics of the surrounding deep mantle, remain enigmatic. Mantle plumes located above the margins of the LLSVPs display evidence for the presence of this deep‐seated, thermally and/or chemically heterogeneous mantle material ascending into the melting region. As a result, analysis of the spatial geochemical heterogeneity in ocean island basalts provides constraints on the structure of the Earth's lower mantle and the origin of the LLSVPs. In this study, we focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of LLSVP material in the underlying plume. We show, using spatial variations in the major element contents of high‐MgO basalts, that the isotopically enriched south‐western region of the Galápagos mantle—assigned to melting of LLSVP material—displays no evidence for lithological heterogeneity in the mantle source. As such, it is unlikely that the Pacific LLSVP represents a pile of subducted oceanic crust. Clear evidence for a lithologically heterogeneous mantle source is, however, found in the north‐central Galápagos, indicating that a recycled crustal component is present near the eastern margin of the Pacific LLSVP, consistent with seismic observations.