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

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.

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.