Zooming in on the core-mantle boundary

Project Information

ZoomDeep

Grant agreement ID: 804071

DOI

10.3030/804071

Project closed

EC signature date 30 August 2018

Start date 1 January 2019

End date 30 June 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 407 784,00

EU contribution

€ 1 407 784,00

Coordinated by

THE CHANCELLOR MASTERS AND SCHOLARS OF THE UNIVERSITY OF CAMBRIDGE
United Kingdom

Periodic Reporting for period 4 - ZoomDeep (Zooming in on the core-mantle boundary)

Reporting period: 2023-07-01 to 2025-06-30

Summary of the context and overall objectives of the project

The boundary between the liquid metal outer core and the solid silicate mantle nearly 3000 km deep, is the strongest internal boundary within our planet. Heat leaves the core into the mantle across this interface. The cooling of the core is a driving force for vigorous convection in the liquid outer core causing the magnetic field. The heating at the base of the mantle creates hot upwelling plumes causing volcanism at the surface. Strong variations in composition on top of the core-mantle boundary lead to variation in conductivity and thus in heat flux, affecting patterns of convection on either side of the boundary. The presence of heterogeneity will also influence compositional transfer across the interface, topography on the boundary, and the nature of dynamics in the lowermost mantle.

The strongest heterogeneities observed by seismic waves are so-called ultra-low velocity zones (ULVZs). These are 'thin' patches (10s of km high) of extremely reduced seismic velocities and thus very anomalous compositions. The largest of these have been discovered beneath locations of major hotspot volcanism, such as Hawaii, Iceland and Samoa, and are thus speculated to be the base of the mantle plumes. Currently only a fraction of the core-mantle boundary has been mapped for the presence of ULVZs. Many questions remain about the nature of these zones, their origin, and their role in the evolution of our planet and present-day dynamics. These questions can be further explored with better imaging of the geometry of the ULVZs, and their velocity and density characteristics.

Our proposal objectives were to resolve the layering near the core-mantle boundary to finer resolution than before; build global maps of ULVZ prevalence and its implications for heat flux variations across the core-mantle boundary; understand the relationship between the ULVZs and larger scale heterogeneity in the deep mantle; and determine the potential compositional and/or partially molten nature of ULVZs. Our novel maps of the geology at the core-mantle boundary would lead to new insights into the nature of the heterogeneities and the role they play in the dynamics.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Project results include a great number of newly discovered ULVZs as well as more detailed constraints on their internal structure and morphology. In the process, we developed several new techniques to resolve ULVZs.
We have pushed previous observations of diffracted waves to higher frequencies, finding thinner ULVZs (on the order of ~10 km, Martin et al. 2024) as well as layering within larger ULVZs (~2 km, Li et al. 2022).
We have discovered new observations of postcursors to core-diffracted shear waves that have let us map ULVZs on the CMB near Galapagos (Cottaar et al. 2022), St Helena (Davison et al. 2023), Pitcairn (Li et al. 2023), as well as two in the Pacific that are less easily linked to a hotspot (Martin et al. 2024, and Martin et al. 2025, in revision). We are now in the process publishing our approach to pick a global data set of core-diffracted shear waves, identifying the largest ULVZs around the globe, and also focussing on the extensive areas with no large ULVZs (Atkins et al., in revision). ULVZs in this data set appear to correlate with hotspot volcanism at the surface and can also be related to larger scale structure present in the lowermost mantle.
We have developed a Bayesian approach to mapping the shape and location of ULVZs (Martin et al. 2023a). We have applied this to the Hawaiian ULVZ, showing it is elongated which shows a relationship to the surrounding dynamics (Martin et al. 2023b). The technique is also good to show trade-offs between locations and ULVZ size, particularly when the data available only illuminate a ULVZ from a single direction. To illustrate this trade-off the Bayesian mapping approach has been applied to central Pacific ULVZ (Martin et al. 2024) and a ULVZ near Vanuatu (Martin et al. 2025, in revision).
Additionally, we provided complimentary constraints on the nature of ULVZs. We provided the first ever constraints on the ULVZ using postcursors to Pdiff waves, which are complimentary to Sdiff (Jagt et al. 2024). Our initial interpretation provides potential insights to the compositional nature of the ULVZ. Further complimentary came from the application of vertically scattered shear waves (ScS), which allowed us to map the morphology of the Hawaiian ULVZ (Jenkins et al. 2021). The results could be linked to the surrounding impinging subducted slabs.
We have explored the potential of ULVZs to be caused by present-day partial melting processes (Dannberg et al. 2021). We found that under most-reasonably assumed conditions any melt would sink to the bottom and create a 100% melt layer. Viscous forces are not sufficient to create partial melt zones that are consistent with seismically observed parameters.
We have also zoomed in on the finest-scale structure at the CMB, testing our limits of knowledge here. Russell et al. 2022a showed that the exotic PKKPdf phase would be most sensitive to a layer of melt at the CMB. Global observations show significant scatter, and trade-offs with structures elsewhere means that a layer of several kms could be present without being seen. Russell et al. 2023 shows that modes are okay with the presence of a global slow layer of several km a the CMB, although this layer would have to be solid.
Summaries of our work have been published in short pieces for Astronomy and Geophysics (Cottaar et al. 2024) and Nature Communications (Russell et al. 2024).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

• We developed a Bayesian inversion setup to map ULVZ anomalies on the core-mantle boundary using core-diffracted waves (Martin et al. 2023a) and applied this in various locations (Martin et al. 2023b, 2024, 2025 in rev.). The approach is previously been applied to the surface where it could confirm the rough geology at the surface. Applying to the CMB has uncovered the new unknown ‘geology’ nearly 3000 km deep.
• We pushed frequency of observations of postcursors to shear core-diffracted waves higher than before, uncovering anomalies that are 10 km (Martin et al. 2024) or even 2 km (Li et al. 2022) in height above the CMB. At higher frequencies, we have also found the first observations of postcursors to compressional core-diffracted waves related to the Hawaiian ULVZ (Jagt et al. 2024).
• We have explored the limits of our resolution on the finest scale layering at the core-mantle boundary, revealing the possibility of a global layer of several kilometres, likely solid, hidden within the data (Russell et al. 2022a, 2023).
• We are currently publishing a global map of the geology of the CMB, also focusing on regions without ULVZs (Atkins et al. in rev. )
• We have developed a dynamical model of partial melt at the CMB testing against seismic constraints (Dannberg et al. 2021). This is leading to further collaboration to understand the CMB from an inter-disciplinary perspective.

ULVZ at the base of the mantle upwelling beneath Hawaii