Understanding how the mantle transition-zone 'valve' controls slab fate

Subduction is the process where tectonic plates descend into Earth's deep interior, the mantle. Subduction is critically important since it drives (i) plate tectonics (the ultimate process behind seismicity and mountain building); (ii) melting, (critical for volcanism, and producing crust and atmosphere) and (iii) mantle circulation. Yet, we do not fully understand how it 'works'.

Subducting plates ('slabs') form the downwelling limb of mantle convection. Mantle convection differs in several important ways from the familiar convection of water boiling in a saucepan on a stove. Firstly, mantle rocks are solid, but they can creep on long time scales. Secondly, in the 'transition zone', 400 to 800 km down into the 3000 km deep mantle, mantle minerals undergo high-pressure phase changes to more tightly-packed and denser structures.

Creep varies strongly with temperature, making cold subducting plates much stiffer than the surrounding warm mantle. Exact creep style varies with, amongst others, pressure and stress, and controls how rapidly slabs lose their strength as they heat up while sinking. How easily a slab deforms again influences its sinking speed. The style of creep is also affected by the changes in mineral structure and grain size that occur at phase transitions. The interaction between creep and phase changes in the transition zone complicate the subduction of plates from the upper mantle into the mantle below the transition zone. Of special importance is the transition around 660 km depth where mantle viscosity increases by a factor of 10-100 and a delay of the phase transformation in the cold slabs makes them temporarily lighter than the mantle. This can lead to stalling of slabs in the transition zone. In this way, the transition zone controls how efficiently heat and material are cycled through the mantle, (including water and CO2 which have affected the evolution of climate).

Observed rapid changes in plate motions indicate that there are episodes in which slabs sink through the transition zone quite readily ('valve' open), and others in which they stall there and pile up ('valve' shut). Seismic tomography images of the Earth's interior, reconstructed from seismogram recordings, show that at the moment, many slabs, including those below Tonga, Japan and Sumatra pool in the transition zone, while a few others, for example below Central America, descend straight to great depths.

Different explanations have been proposed. One end-member hypothesis (put forward by co-I Dr. Goes) is that the oldest, coldest plates are stiffest and tend to flatten at the base of the transition zone rather than sink straight through, while young warm slabs form piles that sink through the transitions more easily. Partner Karato in contrast hypothesises that slabs emerge from the major phase transition at 400 km consisting of small, weak new grains. While in young slabs, warm temperatures encourage grain growth and the slabs quickly regain strength allowing them to push through, old slabs remain weakened and are hence unable to open the valve.

Recently, co-I Davies, together with colleagues at Imperial developed a numerical code that allows models with grids that adapt to the scale of model complexity, i.e. high resolution in regions with changes over small scales, like near changes in phase or creep mechanism, and, computationally-less-expensive, coarser resolution in regions with low variability. This allows us to model for the first time, the complex interplay between the thermal, phase and creep effects on subducting slabs.

We will make a set of subduction models incorporating the most recent data on phase change properties (from co-I Lithgow-Bertelloni) and creep laws (from partner Karato). By comparing model predictions with geophysical observations we will be able to determine if either of the two end-member hypotheses or combined or alternative mechanism explains the crucial workings of the transition zone 'valve'.

Specific users this work might be of interest to and how they will benefit:

The Hydrocarbon industry are interested in a better understanding of subduction and more broadly 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.

Shell International Exploration and Production (SEIP) are already funding a geodynamic project at Cardiff testing plate motion histories. The Global Frontiers group of SEIP will be directly informed of the new knowledge at our ongoing regular meetings. We also propose to attend the leading exploration geophysics conference (Society of Exploration Geophysicists Annual Meeting - SEG) in the final year of the project, to disseminate our results and tools to a wider audience from industry

Geodynamics modellers
The numerical code, Fluidity, will be formally released at the end of this project and made available to the global Earth Science community. We note that numerically this is a leading edge tool, especially in being able to dynamically adapt the grid effectively on parallel architectures. In addition though it will incorporate even more leading physics, partly as a result of this project. A proper manual will be written for the code, and the AMCG web-site extended to release the code. A proper stable release will be prepared for a number of platforms; and all the necessary libraries will be provided also. The AMCG group at Imperial College already have much experience and success in this with their other codes. We will hold a 3-day workshop at Imperial College London, covering all necessary aspects for use of the code (i) overview; (ii) obtaining and building Fluidity; (iii) mesh-generation; (iv) the input file/Graphical User Interface; (v) running in parallel; (vi) visualization; (vii) pedagogical examples. Through this we expect the projects impact to build through the academic community. In this way not only will our science outputs but also the methodology developments will have a long term impact.

Wider user interest
Secondary School Students - The clarity that this project will bring to our understanding of the interior dynamics will ultimately allow textbooks to present an exciting and accurate picture of the driving processes. It shows students the power and need for interdisciplinary science (in this case Physics, Chemistry (Mineralogy), Computer Science, Maths, Geology) to answer significant questions. It also shows a 'big' application of 'simple' Physics. In particular to aid teachers, we will prepare and upload a series of power-point presentations, providing a simplified summary of the latest scientific results.

Public - While this project clearly addresses part of the Earth (ocean crust and mantle) that is out of sight it is so fundamental and intrinsically fascinating and visual (detailed thermodynamics aside possibly!) that with the correct presentation - this work will help us forge an appreciation in the public of the significance of the deep interior.

Over recent years, all project investigators have been contacted by journalists (e.g. New Scientist, S4C, BBC) 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. 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. The PI has already presented his work to Wales's First Minister.

We will continue to pursue such openings. We will use the web-site to not only release Fluidity but also to present the exciting results to the public as well as the academic beneficiaries.