Testing mantle dynamics : Constraining high resolution numerical spherical convection models with geochemistry and geophysics

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

Abstract

Mantle convection is important since it drives (i) plate tectonics (the ultimate process behind seismicity and mountain building); and (ii) melting, (critical for volcanism and producing crust, cryo/hydrosphere and atmosphere) but we do not know how it 'works'. In convection, for example water heated in a saucepan, the movement of the light (hot) material from the base to the surface where it cools and sinks back down to restart the cycle again provides a very efficient heat transfer mechanism. The differences in buoyancy that drive flow, with lighter material rising and denser material sinking, can be due to differences in temperature and/or composition. Amazingly the solid mantle deforms by creep on the geological time-scale allowing Earth's mantle to also lose its heat by convection. While we know that the ocean plate is the manifestation of the surface element of this cycle on Earth, and we have incomplete knowledge from seismic imaging for the present-day geometry of this process, we have no direct evidence of the geometry in the past. The field of mantle convection is now ready to yield a significant advance using the combination of the improvements in mantle convection modelling, the maturity of geophysics and geochemical observables, and mineral physics constraints. Convection in the mantle is more complex than convection in simple systems, such as water in a saucepan, since as hot mantle reaches the surface it melts. The melt rises to the surface forming a crust, and degasses to give an atmosphere and hydrosphere, and leaves behind a residue. The combination of these processes make the modelling more interesting since the crust and residue have a different buoyancy to the starting material. Significantly it also gives us the means to constrain the process. For example the rate of melting and degassing is related to the vigour of convection. The known amount of Argon40 that has collected in the atmosphere, produced at a known rate in the mantle from Potassium40, provides an integrated constraint on the rate of degassing. We will also use the flux of primordial Helium3 and alpha particle produced He4 as further constraints. We will also look at the isotopes of lead which are the stable daughters of radioactive U and Th parents. These are further useful stopwatches on mantle convection, but are different to the inert gases since they are not degassed but are recycled. They are returned to the mantle where the convection stirs the crust, residue and starting material together. When they are melted again their Lead isotopic signature is dependent on the proportion of the various components, the stirring and the time that has elapsed since it last melted. To understand mantle stirring one needs models in the right geometry (we will model it correctly as a spherical shell) and at the right vigour (we can reach Earth-like vigour even for early Earth). The geophysics evidence suggests that present-day the mantle convects as a whole body, while geochemical evidence requires ancient isolated reservoirs. There are a large number of hypotheses in play (usually motivated by one discipline alone) trying to reconcile these constraints. We will test these hypotheses. The geochemical data-sets we will use have been collected over very many decades, by countless research teams across the globe, utilizing data whose value at collection easily exceeds £1 billion (>2000*500k). Understanding mantle convection is a zeroth order problem for solid Earth science and the project proposed will allow us to make a significant long-lasting advance. The numerical geodynamic approach allows the broadest range of constraints to be brought to bear in a quantitative manner - the basic conservation laws of physics, geophysics (including integrative ones such as size of inner core - and very high spatial resolution seismic tomography) and geochemistry observables; providing the meanest test of this proliferation of hypotheses.
 
Description We have developed a computer program to simulate evolution of the solid Earth interior on geological timescales including melting. We also developed the means to predict the goechemical predictions of such models for geochemical observations of such melts. This allows us to test the models with this wide-range of geochemical data.
Exploitation Route Our findings will be taken forward by others as starting points for their models of Earth evolution. This will help a wide range of disciplines to better understand their specific data and provide the context for their work. This will include academic and industrial geodynamicists, geochemists, seismologists and geologists.
Sectors Energy

 
Description Our findings have been used to constrain models of how the inside of the Earth has evolved. This has led to scientists approaching us to further use and test these models. This has led to three other projects being funded and another proposal with industrial links being submitted. We are currently undertaking a project for an industrial partner that grows out of this work.
First Year Of Impact 2013
Sector Energy
Impact Types Economic

 
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 Standard Research Grant
Amount £192,000 (GBP)
Funding ID NE/K004824/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 10/2013 
End 09/2015
 
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 3D Presentation to Ysgol Tre-Gib (Llandeilo) Secondary School 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Presentation to 26 Year 8 Secondary children and 2 Teachers of Mantle Geodynamics - using 3D visualisation - on 13 March 2013; as part of a longer session in collaboration with the National Museum of Wales.

After a visit to the Education Dept. National Museum of Wales looking at fossil evidence for evolution and plate tectonics, I presented the mantle, and how we can use plate tectonics to develop mantle circulation models. The presentation led to a large number of questions from the School children - showing their obvious interest and heightened awareness of Earth dynamics and the uses of science.
Year(s) Of Engagement Activity 2013
 
Description Presentation to Applied Geoscientists 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Oral presentation was made at the leading Industry Conference in Europe. This introduced methods (and ensuing results) which we use in our research, and how they could translate and help industry. Over 100 attended the presentation.

Discussions were had with many industry representatives following the presentation regarding mantle modelling
Year(s) Of Engagement Activity 2013
URL http://www.earthdoc.org/publication/publicationdetails/?publication=68911