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 230-350km 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 >0.5Ga, 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° 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 233-340km 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.

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 the percentage of high-density basaltic material that mantle plumes can carry to the surface is no higher than 15–20%. 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 accumulate within plumes at the depth of the X-discontinuity. Two key mechanisms facilitate this process: (1) The random distribution of basaltic heterogeneities induces large temporal variations in the basalt fraction with cyclical highs and lows. (2) The high density contrast between basalt and pyrolite below the coesite-stishovite transition causes ponding and accumulation of basalt at that depth, an effect that only occurs for intermediate viscosities of pyrolite. 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 towards the surface.

Previous studies of the East African upper mantle have invoked one or more mantle upwellings with varying thermochemical nature to underly the distribution of surface volcanism. For example, Boyce and Cottaar (2021) suggest that a hot, chemically distinct upwelling beneath the southern East African Rift (EAR) is sourced from the African Large Low Velocity Province (LLVP), while magmatism in Ethiopia may lie above an additional purely thermal upwelling. 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, two causal mechanisms are possible to explain the X-discontinuity (X; 230-350km): the coesite-stishovite phase transition and/or carbonate silicate melting, both of which require entrainment of basalt from the lower mantle. Intriguingly, carbonate silicate melt was invoked by Rooney et al., (2012) to explain the discrepancy in upper mantle temperature anomalies predicted by seismic wavespeed and petrological estimates beneath East Africa. Further, active carbonatite magmatism occurs along the edge of the Tanzanian craton (Muirhead et al., 2020). Several recent regional to continental receiver function (RF) studies have identified potential observations of the X in East Africa. These studies are not focused on the presence of these upper mantle phases or lack the spatial sampling needed to robustly identify the X and its causal mechanism. Targeted high-resolution observations of the X are required to confirm the presence of exotic converted phases in the East African upper mantle and their relationship to mantle upwellings. We capitalise on the new TRAILS dataset from the Turkana depression (Bastow, 2019; Ebinger, 2018) and an adjacent network in neighbouring Uganda (Nyblade, 2017), to supplement our existing RF database and characterise the X across active continental rift setting in unprecedented detail. The prevalence of the X is mapped beneath East Africa, and subsequently compared to other areas of the African continent.

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. However, the thermal and compositional nature of the plume is still uncertain. The depth and amplitude of seismic discontinuities can show how the plume effects phase transitions in mantle minerals, providing insights into the plume’s thermo-chemical properties. This study utilises >5000 high quality receiver functions from Hawaiian island stations to detect P-to-s converted phases. These receiver functions are stacked in a variety of ways in order to image seismic discontinuities between 200 to 800 km depth. In the mantle transition zone, we find that to the southwest of the Big Island the 660 discontinuity is split. 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, the so-called X-discontinuity, which has an enigmatic origin, is found across the region at depths varying between 290 to 350 km. To the east of the Big Island the X-discontinuity lies around 336 km and is particularly strong in amplitude, to such an extent that the discontinuity around 410 km disappears. Synthetic modelling reveals that such observations can be explained by a silica phase transition from coesite to stishovite. This suggests there is widespread ponding of silica-saturated material (such as eclogite, which is silica-rich relative to pyrolite) spreading out from the plume to the east, a hypothesis which is consistent with dynamical models. We suggest that this seemingly thermochemical plume could be sampling recycled basalt, now in the form of eclogite, from lower in the mantle. Therefore these results support the presence of a significant garnet and eclogite component within the Hawaiian mantle plume. We will briefly highlight further work comparing Hawaii with other hotspot locations around the world to consider whether this is also occurring in other plumes and what heterogeneous plumes may imply about the recycling of material in the mantle.

Mapping absolute P-wavespeeds in the Canadian and Alaskan mantle will further our understanding of its present-day state and evolution. S-wavespeeds are relatively well constrained, especially across Canada, but are primarily sensitive to temperature while complimentary P-wavespeed constraints provide better sensitivity to compositional variations. One technical issue concerns the difficulties in extracting absolute arrival-time measurements from often-noisy data recorded by temporary seismograph networks. Such processing is required to ensure that regional Canadian datasets are compatible with supplementary continental and global datasets provided by global pick databases. To address this, we utilize the Absolute Arrival-time Recovery Method (Boyce et al., 2017). We extract over 180,000 new absolute arrival-time residuals from seismograph stations across Canada and Alaska that include both land and ocean bottom seismometers. We combine these data with the latest USArray P-wave arrival-time data from the contiguous US and Alaska. Using an adaptively parameterised least-squares tomographic inversion we develop a new absolute P-wavespeed model, with focus on Canada and Alaska (CAP21). Initial results suggest fast wavespeeds characterise the upper mantle beneath eastern and northern Canada. A sharp transition between the slow wavespeeds below the North American Cordillera and the fast wavespeeds of the stable continental interior appears to follow the Cordilleran Deformation Front (CDF) in southwest Canada. Slow wavespeeds below the Mackenzie Mountains may extend further inland of the CDF in northwest Canada. In Alaska, CAP21 illuminates both lithospheric structure and the along strike morphology of the subducting slab. The newly compiled data may also improve resolution of subducted slab remnants in the mid-mantle below the North American continent, crucial to help constrain the formation of the Alaskan peninsular at ≥50Ma.