Periodic Reporting for period 3 - ZoomDeep (Zooming in on the core-mantle boundary)
Berichtszeitraum: 2022-01-01 bis 2023-06-30
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 are 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 results lead to novel maps of the geology at the core-mantle boundary, and new insights into the nature of the heterogeneities and the role they play in the dynamics on either side of the boundary.
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 are 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 results lead to novel maps of the geology at the core-mantle boundary, and new insights into the nature of the heterogeneities and the role they play in the dynamics on either side of the boundary.
In our project we are advancing imaging of the ULVZs in various ways. Firstly, we are pushing observations of waves that diffract along the core-mantle boundary to higher frequencies, which are noisier and computationally expensive to model. However, this data allows us to map the internal layering within the ULVZs, increasing the resolution from 10s of kilometres to several kilometres. Unique data were found sampling the ULVZ at the base of the Hawaiian mantle plume, indicating internal layering with a localised basal layer of several km thick and 40% reduced in seismic shear velocity compared to surroundings (Li et al. 2021, preprint). The characteristics of the basal layer puts new bounds on the potential compositions of the ULVZs, which is likely to be caused by strong layering in iron enrichment. The existence of such strong heterogeneity at the core-mantle boundary puts new perspective on the boundary conditions of core and mantle convection and the coupling between the two.
Secondly, we have detected new large ULVZs beneath the Pacific using core-diffracted waves. One is located near the Galapagos hotspot, and another one or two might be located near the Macdonald hotspot. These are slightly smaller than the ULVZs beneath Hawaii and Iceland. The ocean island basalts at these hotspots all have interesting geochemical isotope signature, which might indicate ULVZs have formed early in Earth's history, and give potential evidence of compositional interaction with the core.
Thirdly, we have developed new methods to use reflected waves off the tops of ULVZs to map their morphology. These results confirm the presence of the large and broad zone, but also show there are small anomalous "hills" and "ridges" in the same region, and morphology of these appears quite varied (Jenkins et al. 2021). These results illustrate that variation in anomalies referred to as ULVZs, and argue for the dubbing of 'mega-ULVZ's for the broad scale features that we are detecting using core-diffracted waves near major hotspots.
Lastly, working across disciplines, we have built dynamical models for the hypothesis that the ULVZs are a result of partial melting of the material at the core-mantle boundary (Dannberg et al. 2021). Such models do not reproduce the morphologies and seismic velocities observed, thus making ULVZs more likely to be purely compositional features.
Secondly, we have detected new large ULVZs beneath the Pacific using core-diffracted waves. One is located near the Galapagos hotspot, and another one or two might be located near the Macdonald hotspot. These are slightly smaller than the ULVZs beneath Hawaii and Iceland. The ocean island basalts at these hotspots all have interesting geochemical isotope signature, which might indicate ULVZs have formed early in Earth's history, and give potential evidence of compositional interaction with the core.
Thirdly, we have developed new methods to use reflected waves off the tops of ULVZs to map their morphology. These results confirm the presence of the large and broad zone, but also show there are small anomalous "hills" and "ridges" in the same region, and morphology of these appears quite varied (Jenkins et al. 2021). These results illustrate that variation in anomalies referred to as ULVZs, and argue for the dubbing of 'mega-ULVZ's for the broad scale features that we are detecting using core-diffracted waves near major hotspots.
Lastly, working across disciplines, we have built dynamical models for the hypothesis that the ULVZs are a result of partial melting of the material at the core-mantle boundary (Dannberg et al. 2021). Such models do not reproduce the morphologies and seismic velocities observed, thus making ULVZs more likely to be purely compositional features.
The work described so far has involved detailed modelling of individual data sets, which is computationally intensive and time consuming. We are currently working towards a more automated and Bayesian imaging method to map the geometries of ULVZs employing larger data sets. We have collected global data of over 7 million event-station geometries to build a map of ULVZs on the core-mantle boundary.
Additionally, we are moving beyond using shear waves, which only tell us about the shear velocity anomaly for the ULVZs. We are collecting a global data set of compressional waves with unique sensitivity to layering close to the core-mantle boundary.
Over the course of ZoomDeep, we are developing new state-of-the-art data analyses techniques and novel inverse methods to target the core-mantle boundary 3000 km deep, and in particular the thin anomalous zones called ultra-low velocity zones that are present on top. We are uncovering their internal structure, pushing observations and modelling to higher frequency; their morphology, using new techniques to combine core-reflected phases; and general prevalence, developing new techniques to apply global data sets. We are working towards a global geological map of the core-mantle boundary.
Our new constraints are used in collaboration with scientists working in geodynamics, geomagnetism, mineral physics, and geochemistry, to further uncover the nature of the ULVZs and their role in mantle and outer core dynamics.
Researchers on project: Dr Jennifer Jenkins, Dr Florian Millet, Zhi Li, Carl Martin, Stuart Russell
Collaborators: Dr Jessica Irving, Dr Thomas Bodin, Dr. Juliane Dannberg, Dr Robert Myhill, Dr Rene Gassmoeller, Dr John Rudge
Additionally, we are moving beyond using shear waves, which only tell us about the shear velocity anomaly for the ULVZs. We are collecting a global data set of compressional waves with unique sensitivity to layering close to the core-mantle boundary.
Over the course of ZoomDeep, we are developing new state-of-the-art data analyses techniques and novel inverse methods to target the core-mantle boundary 3000 km deep, and in particular the thin anomalous zones called ultra-low velocity zones that are present on top. We are uncovering their internal structure, pushing observations and modelling to higher frequency; their morphology, using new techniques to combine core-reflected phases; and general prevalence, developing new techniques to apply global data sets. We are working towards a global geological map of the core-mantle boundary.
Our new constraints are used in collaboration with scientists working in geodynamics, geomagnetism, mineral physics, and geochemistry, to further uncover the nature of the ULVZs and their role in mantle and outer core dynamics.
Researchers on project: Dr Jennifer Jenkins, Dr Florian Millet, Zhi Li, Carl Martin, Stuart Russell
Collaborators: Dr Jessica Irving, Dr Thomas Bodin, Dr. Juliane Dannberg, Dr Robert Myhill, Dr Rene Gassmoeller, Dr John Rudge