The strength of the lower mantle

Lead Research Organisation: University College London
Department Name: Earth Sciences

Abstract

The internal behaviour of the Earth is controlled by convection hence knowledge of mantle viscosity is critical to our understanding. The viscosity of the mantle on large scales has been measured from post-glacial rebound and geoid inversions. These have shown that there is a significant contrast in viscosity between the upper and lower mantles which has been invoked to explain the varied behaviour of subducting slabs at the 660 km discontinuity. The slabs show a range of behaviour from ponding just above the 660 km discontinuity to penetrating the discontinuity and thickening as they enter the lower mantle.
It is apparent therefore that the size and impermeability of the 660 km viscosity contrast affects the style of mantle convection and cooling rates but the resolution of the geophysical observations is not sufficient for a complete understanding of the fate of material around the 660 km discontinuity. Here we propose to measure the rheology of the minerals and assemblages present in the transition-zone and lower-mantle and so provide the constraints needed for a fuller understanding of the viscosity contrast between the upper and lower mantles.

Planned Impact

Academic users: We will integrate our work into the recently established Paris-UCL Research Exchange (PURE) programme which aims to foster collaboration between scientists working on the deep Earth (both mantle and core) at UCL and at the Institut de Physique du Globe de Paris. Broader engagement with material scientists will be made through the UCL Centre for Materials Research, on which Dobson is a board member.

Training: The project will result in the training of one PDRA and the continued training of the Research Co-I Hunt. The PDRA will be trained in high-pressure/high temperature experimental techniques (mostly Liverpool University), measurement of physical properties and diffraction studies, and will also gain experience working at international large-scale facilities (e.g., NSLS); as a result s/he will be in a position to gain employment in a variety of materials science based occupations. As part of their training, both PDRAs will be required to participate in courses, as part of the staff development programmes at their respective universities, which continues the Robert's Agenda in our institutions.

Industry Dobson will continue his work with the fine-arts industry to develop improved paper registration techniques. One day meetings will be held annually to discuss progress and further developments.

Outreach - There is an existing outreach programme run by UCL Earth Sciences to which the investigators from UCL already contribute; the PDRAs working on this project will be expected to participate in this activity. In addition, we shall our research activity, "Diamonds from Dirt", aimed at key-stage 3 students. Diamonds can be readily grown from any source of carbon; we have already grown diamonds from toast with work-experience placements. We believe that growing diamonds from 'snot' will engage with younger students. While this might seem flippant we genuinely believe it is of vital importance to capture the imaginations of students at as young an age as possible and this should appeal broadly, involving a 'girl's best friend' and, possibly, a young boy's best friend. Nasally secreted mucus is a mixture of water and glycosylated peptides: once dried it has a high concentration of carbon (about 30 mole%) and, after reducing pyrolysis, will readily form diamonds via the standard solvent catalysis route. It will not be possible, for health-and-safety reasons, for key-stage 3 students to attend the synthesis experiments so we propose to make a web-cast of the diamond growing process which we will use in conjunction with school visits to discuss the process and present the grown diamonds. These outreach activities will be performed in collaboration with Dr Andrea Sella (UCL Chemistry) who is an EPSRC senior media fellow and has contacts with a network of primary schools in London, including being a governor of Gilmore Primary School and running several science clubs. We also strongly suspect that a webcast about growing diamonds from snot might be picked up by the news media, allowing us to discuss more serious aspects of high-pressure research and public outreach.

Publications

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Bull C (2017) High-resolution neutron-diffraction measurements to 8 kbar in High Pressure Research

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Crichton W (2016) High-temperature equation of state of vanadium in High Pressure Research

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Dobson D (2016) The phase diagram of NiSi under the conditions of small planetary interiors in Physics of the Earth and Planetary Interiors

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Dobson D (2016) Geophysics: Earth's core problem. in Nature

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Hunt S (2016) In-situ measurement of texture development rate in CaIrO3 post-perovskite in Physics of the Earth and Planetary Interiors

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Hunt SA (2017) The thermal expansion of (Fe Ni )Si. in Journal of physics. Condensed matter : an Institute of Physics journal

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Hunt SA (2017) Note: Modified anvil design for improved reliability in DT-Cup experiments. in The Review of scientific instruments

 
Description We have shown that the grain size of mantle material is reduced to approximately 2 microns during the upwelling reaction at 660 km depth. This should result in transient weakening in upwelling regions of the mantle just above 660 km and might enhance viscous decoupling between upper and lower mantles.We have shown that there is a large effect of pressure on the strength of grain boundaries and this effect alone can explain the change in seismic attenuation with depth without having to invoke a very large grain size in the lower mantle.
Exploitation Route Further research to look at the effects of viscous decoupling and to investigate grain growth.
Sectors Other

 
Description talks to schools and other non-expert groups. I have been appointed as UCL's first ever scientist in residence- in the Slade School of Fine Art. This is an entirely new concept in cross-disciplinary residences where I spend one day per week interacting with artists to help improve and inform their practice with aspects of the scientific method and modern technologies. One concrete outcome of this is the development of a new pigment mentioned in one of my other grants. This impact draws from across all of my research so shuold be associated with all of my grants, although it appears that this is not possible in the current Research Fish incarnation.
First Year Of Impact 2016
Sector Education
Impact Types Cultural

 
Description NERC fellowship scheme _ Simon Hunt my postdoc applied
Amount £711,084 (GBP)
Funding ID NE/P017525/1 
Organisation Natural Environment Research Council 
Department NERC Postdoctoral Fellowship
Sector Academic/University
Country United Kingdom
Start 08/2017 
End 08/2022
 
Title X-ray Imaging through carbide anvils 
Description A modification to anvil geometries in order to allow enhanced X-radiographic imaging and X-ray diffraction studies in high-pressure large-volume experiments. 
Type Of Material Improvements to research infrastructure 
Year Produced 2015 
Provided To Others? Yes  
Impact This modification has been adopted at the GSE CARS beamine at the APS and is enhancing their multi-anvil studies with full Debeye-ring data collection. 
 
Description Diffusion creep in post perovskite 
Organisation University of Bayreuth
Department Bavarian Research Institute of Experimental Geochemistry and Geophysics
Country Germany 
Sector Academic/University 
PI Contribution We have proposed the idea, synthesised and characterised the starting materials
Collaborator Contribution They are measuring the rheology of fluoride post-perovskite in the diffusion creep regime. 3 weeks of experiments in 6-axis deformation multi-anvil press.
Impact None yet
Start Year 2016
 
Description Japan deformation press 
Organisation Ehime University
Country Japan 
Sector Academic/University 
PI Contribution We have designed a new deformation press and provided access to the prototype, and help and support in performing experiments at UCL during August-November 2016. Dobson visied Japan during March 2016 to advise on the new press (based on our deformation design) being commissionned to go on a beamline at Photon Factory synchrotron in Tsukuba, Japan.
Collaborator Contribution A group of 4 researchers Visited UCL for 2 months in 2016 (August, November) to perform deformation experiments at the P-T conditions of the lower mantle. They provided all cells and hard anvils to reach the extreme pressures necessary. They have won a grant (approximately £200k) to install one of our (UCL design) presses on an existing multi-anvil beamline at the Photon Factory.
Impact The dfevelopment of a new high-pressure deformation beamline on Photon Factory synchrotron in Japan.
Start Year 2016
 
Description Art Symposium - UK 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Other audiences
Results and Impact approximately 60 artists attended the 'Clearwell Caves Symposium' where I gave a lecture entitled 'The colours of Iron'
multiple URLs which are not allowed in the URL box:
http://www.vasw.org.uk/events/deep-material-encounters-symposium.php
http://hardwickgallery.org/event/deep-material-encounters/
Year(s) Of Engagement Activity 2016
URL http://www.ucl.ac.uk/slade/events/deep-material-encounters-clearwell-caves
 
Description Meeting in Japan 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact A talk about the high-pressure deformationa nd imagimng techniques which my group has developed.
Year(s) Of Engagement Activity 2016
 
Description Seminar Zurich 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact A general colloquium about my research into the deep Earth and viscoelastic effects in the mantle.
Year(s) Of Engagement Activity 2016