Understanding the D'' zone: novel fluoride analogues to MgSiO3 post perovskite

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

Abstract

The thermal boundary layers of a convecting system control many aspects of its style of convection and thermo-chemical history. For the silicate Earth these boundary layers are the lithosphere, whose low temperature and high rigidity induces slab-style downwellings, and the D'' region on the mantle side of the core-mantle-boundary (CMB). The D'' region is the source of plume-style convection and regulates heat exchange from the core to the silicate Earth. The lower thermal boundary is made more complex by the existance of a phase transition in the most common mineral in the lower mantle (magnesium-silicate perovskite) which changes the properties of the D'' region at the CMB. Unfortunately, most of these properties cannot be measured at the extreme pressures (120 GPa) of stabilisation of the post-perovskite phase. The best chance of constraining them is through a combination of measurements on low-pressure analogue materials (which have the same crystal structure but a different chemical composition) and ab initio simulations of both the analogue and natural systems. We have recently developed a set of ABF3 analogues whose properties are much more similar to MgSiO3 than are those of the CaBO3 analogues currently in use. We propose, therefore, to use these improved fluoride analogues to determine the properties of post-perovskite which control the dynamics of D'' (phase diagram, pressure-temperature-volume relations, viscosity, slip systems and thermal diffusivity). These measurements will allow models to be developed which accurately predict the behaviour of the lower thermal boundary layer of the mantle. This will place coinstraints on (1) the heat budget, dynamo power and start of crystallisation of the inner core, (2)the vigour of plumes, (3) the ratio of underside heating to internal heating in the mantle and, (4) the radioactive element budget of the silicate Earth.

Planned Impact

Academic users: We will integrate our work into the recently established Paris-UCL Research Exchange 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. This collaboration was established because the group in Paris have similar scientific goals, with a complimentary experimental programme to that at UCL; however, UCL is much stronger in computational methods, so both parties benefit greatly from this synergism. 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 two PDRAs. The first PDRA will be trained in high-pressure/high-temperature experimental techniques, measurement of physical properties and diffraction studies, and will also gain experience working at international large-scale facilities (e.g., ESRF); as a result s/he will be in a position to gain employment in a variety of materials science based occupations. The second PDRA will gain further expertise in simulation methods; this is an area which has been identified as having a national shortfall of trained researchers. The importance of computer-enabled research is recognised by the UK Research Councils; for example, the International Review of Research Using High Performance Computing in the UK commissioned by EPSRC in 2005 reported that "Computation has now become essential for the advancement of all research across science and engineering". As part of their training, both PDRAs will be required to participate in courses, as part of the UCL staff development programme, which continues the Robert's Agenda in our institution.

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 test prototypes and discuss 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 set up a new outreach 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

10 25 50
publication icon
Ammann M (2014) Variation of thermal conductivity and heat flux at the Earth's core mantle boundary in Earth and Planetary Science Letters

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

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

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

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

publication icon
Jakymiw C (2018) The phase diagrams of KCaF and NaMgF by ab initio simulations. in Physics and chemistry of minerals

publication icon
Lord OT (2015) The equation of state of the phase of NiSi. in Journal of applied crystallography

publication icon
Walker A (2018) The anisotropic signal of topotaxy during phase transitions in D ? in Physics of the Earth and Planetary Interiors

 
Description We have measured a whole range of physical properties just like we said we were going to do in the grant.
They have shown that there are new ways to interpret seismic anisotropy in the lowermost mantle which are much better than the old ways.
Exploitation Route Geodynamic models and interpreting seismic observations
Sectors Other

 
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 Transformational textures in the perovskite post-perovskite system 
Organisation University of Lille
Department Earth Sciences
Country France 
Sector Academic/University 
PI Contribution Synthesised samples and characterised them
Collaborator Contribution Synchrotron experimenst investigating texture inheritance during transformation. Analysed using bespoke Lille software to follow evolution of individual grains during the transformation.
Impact Conference abstract: European Geosciences Union 2017 Study Of The Perovskite to Post-Perovskite Transformation Using Multigrain Crystallography
Start Year 2015
 
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