Superplumes, superpiles or superpuddings? Understanding the thermochemical dynamics of the mantle with waveform seismology

Lead Research Organisation: Cardiff University
Department Name: School of Earth and Ocean Sciences

Abstract

We understand plate tectonics. What we still do not understand is how the mantle and plates work together to generate this unique behaviour. This is an outstanding fundamental problem. In particular we do not understand the dynamics of mantle convection including what role composition plays. Seismological studies have shown us that the mantle has two giant mysterious structures at its base, one beneath Africa the other beneath the Pacific. We do not understand their role in mantle convection.

There are three hypotheses for these structures, each with different dynamical implications. First that they are the result of thermal convection e.g. result from a cluster of plumes (super-plumes); second that they represent dense detrital pile (super-piles); and third that they are thermo-compositional, resulting from recycling of oceanic crust, leading to distributed heterogeneity (equated by some to plums in a pudding) through the mantle, which maybe more concentrated in these regions (super-puddings).

We will produce computer simulations of mantle circulation to investigate each class of hypothesis. We will apply plate motion history to these to produce models that can be compared with the real Earth. Earlier work of our team has shown that both preliminary superplume and superpile models produce structures similar to the two large structures imaged with seismic tomography. The different hypotheses though will have different internal and top structures, which cannot be resolved with current seismic tomography. They can be differentiated seismically, but it requires more advanced methods.

This project will bring these more advanced methods to bear. Models of the predicted seismic structure will be produced from the present-day stage of the resulting mantle circulation models. This will be done using a world-class thermodynamic database of the mineralogy and its elastic properties, derived from hundreds of laboratory experiments.

The models will be tested using seismic probes that can look inside the structures, and another set that focus at their upper edge. The predictions of the probes for the different hypotheses will be produced by accurately directly simulating the propagating seismic waves. This will be done using the spectral finite-element code SPECFEM3D_GLOBE on the National Supercomputer, HECToR (and soon ARCHER). One set of probes is the so called 'ScS' seismic wave. This is a wave that reflects off the core mantle boundary - this provides a tool to look inside the structure with high lateral resolution. The second set of probes will be body-waves that bottom around the top of the structures. If the structures extend high above the core mantle boundary then waves that only sample them will be affected.

The distinctive predicted seismic signatures of the different models will then be compared to the large datasets now available allowing the hypotheses to be exactingly tested. The weak signatures in the data will be amplified by using the advanced techniques of observational seismology of stacking waveforms (adding multiple seismograms), which are best done using arrays of seismometers. They will also provide a good test for current approximate methods used to image and model the mantle structure.

The research team has the resources (access to high performance computing), tools (code to model mantle circulation (TERRA, Fluidity) and seismic wave propagation (SPECFEM3D_GLOBE)), data, track record and expertise (including partners for plate motion histories, mineral physics, modelling, and seismic data analysis) in place to undertake this ambitious project.

Planned Impact

Despite the fundamental science focus of the project, there is a broad range of potential beneficiaries and end-users of the knowledge generated beyond academia.

Industry knowledge exchange

The hydrocarbon industry are interested in a better understanding of mantle dynamics and how it has affected surface motions - vertical and horizontal - and near-surface temperatures - through Earth history. These are of value in understanding basin evolution and prospectivity. We note that the geological history of oil and gas reservoirs is also important in considering sequestration of CO2. Multiple companies are funding Mantle Geodynamics studies at Cardiff, including Shell International Exploration and Production (SEIP) who are funding a geodynamic project testing plate motion histories. The Global Frontiers group of SEIP, and other sponsors will be directly informed of the new knowledge at our ongoing regular meetings.

Such industries will also be interested in technical aspects of the project, such as developing improved seismic imaging. This can be applied to, for example, using non-traditional data sources such as passively recorded natural seismicity and linking dynamic models to seismic observables. Bristol has a long pedigree of applying global scale imaging to reservoir scale problems, through BUMPS, a Bristol consortium project (involving PI Wookey). This is funded by not only multiple industrial partners but also government stakeholders such as UKERC and NERC, for applications such as the monitoring of CO2 sequestration.

Public engagement

While this project clearly focuses on part of the Earth beyond people's direct experience, its fundamental nature as the very force which shapes the planet we live on makes it a compelling subject to engage the public. Given the potential for its visual and dynamic nature, with the correct presentation this work can help us forge an appreciation in the public of the significance of the deep interior, and the geosciences in general. The National Centre for Public Engagement based at Bristol, and 'Beacon for Wales' hosted at Cardiff University will facilitate this. These both organise and provide access to events at which our science can be showcased.

The fundamental nature of this kind of research and its capacity to fire the imagination of the public is reflected in the media attention it draws. In recent years, project investigators have been contacted by journalists (e.g., New Scientist, S4C, BBC and online media outlets) in relation to their research. For example PI Davies' work has recently been covered by major popular publications New Scientist, Earth (American Geological Institute's news stand publication); and he has taken part in TV news and documentaries for S4C and BBC. PI Wookey has written articles for general audience publications such as NERC's PlanetEarth, and consulted on a major upcoming BBC series, 'The Story of the Continents'. We will engage politicians / policy makers by attending the Annual 'Science and the Assembly' meeting organized by the Royal Society of Chemistry which occurs in May every year at the Wales Millennium Centre and Senedd, Cardiff Bay. PI Davies has already presented his work to Wales's First Minister and we will continue to pursue such opportunities.
 
Description We have developed a computer modelling program that allows us to simulate the evolution of the solid Earth including melting. We are also developing methods to test the structures produced by using these models in a computer program that allows us to simulate seismic wave propagation. In principle this will allow the original evolution program predictions to be predicted by comparing with observed seismograms. Through this we can better understand the evolution of Earth's interior and how it controls the surface.
Exploitation Route This work will provide a test of plate motion histories and properties of Earth's interior. This will therefore give others a quantified measure of the confidence they should have to use such properties and histories in their simulations and work. Such models could be used by industries that need to understand the influence of the interior on the surface over geological timescales.
Sectors Energy

 
Description The findings have indirectly stimulated work by energy companies to help incorporate such findings in their exploration systems. This has included jointly preparing applications for future projects.
First Year Of Impact 2014
Sector Energy
Impact Types Economic

 
Description NERC GW4+ DTP Studentship
Amount £10,041,450 (GBP)
Funding ID NE/L002434/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 10/2017 
End 10/2021
 
Description Research Leave Scheme
Amount £15,000 (GBP)
Organisation Cardiff University 
Sector Academic/University
Country United Kingdom
Start 09/2014 
End 09/2015
 
Description Research Programme - Volatiles, Geodynamics & Solid Earth Controls on the Habitable Planet - Mantle volatiles: processes, reservoirs and fluxes
Amount £221,000 (GBP)
Funding ID NE/M000397/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 07/2014 
End 06/2019
 
Description Research Programme - Volatiles, Geodynamics & Solid Earth Controls on the Habitable Planet - Volatile legacy of the Early Earth
Amount £123,512 (GBP)
Funding ID NE/M000400/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 09/2014 
End 08/2019
 
Description Rhodri Davies - ANU 
Organisation Australian National University (ANU)
Country Australia 
Sector Academic/University 
PI Contribution We have modelled mantle convection in spherical geometry focussing on thermo-compositional simulations.
Collaborator Contribution Our partner has provided end-member simulation outputs from (i) purely thermal and (ii) strictly compositionally layered simulations. Partner has also provided guidance and support on the conversion of model outputs to seismic structure.
Impact No peer-reviewed publications yet, just conference abstracts.
Start Year 2013
 
Title Mantle thermo-compositional and melting simulation software 
Description The product is a significant extension on the mantle modelling software TERRA. This is an open research software but not licenced, and therefore not Open Source. The product now allows 3D spherical mantle models to be run, incorporating compositional as well as thermal buoyancy. The model also models melting, producing compositional variations and tracking them; and also tracking resulting changes in trace elements including all the major heat producing elements. 
Type Of Technology Software 
Year Produced 2013 
Impact One peer-reviewed publication has been produced and 3 further funding applications succeeded. We expect many more impacts over the coming years. 
 
Description Earth Science Teacher Association Conference - Presentation 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact A presentation was made to around 40 Secondary School teachers attending the Earth Science Teacher Association's Conference, using the 3D projection facility, Helix, to describe mantle geodynamics research and the way that all STEM subjects contribute to such research. The presentation was followed by a lot of questions and discussion. I was also requested to prepare an article related to the presentations to ESTA's Journal. The presentation was also attended by Iain Stewart who twitted the event to his significant followers.
Year(s) Of Engagement Activity 2015
 
Description Ffeil - Contribution to TV Programme produced by BBC for S4C 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Media (as a channel to the public)
Results and Impact I was answered questions to camera regarding sinkholes for the Welsh language TV Programme - Ffeil - aired on S4C - produced by BBC. Target audience is 8 to 16 years old.
Year(s) Of Engagement Activity 2016