Calcium Perovskite: the forgotten mantle phase

The lower mantle, extending from approximately 660 to 2900 km depth, is a vast and inaccessible layer of the Earth. There are no direct samples from the lower mantle, so everything we know about this region is inferred from the speed that seismic sound waves transit this region. By constraining the acoustic properties of candidate mineral assemblages using experiments, Earth Scientists can infer the chemistry of the lower mantle. Additionally, seismic data can be used in an analogous way to medical ultrasound, to image lateral variations, which reveal that the lower mantle is full of heterogeneity. Two massive regions (> 1000 km in diameter) of slow acoustic velocity sit on top of the core beneath Africa and the Pacific Ocean. Much smaller fragments of anomalously slow material are observed pervasively throughout the remainder of the lower mantle.

It is believed that much of this anomalous material is recycled oceanic crust, which has been subducted and mixed back into the Earth's lower mantle. The distribution of this heterogeneity, if it is indeed recycled crust, combined with knowledge of mechanical properties will tell us about the vigour and style of mantle convection. However, both whether or not the seismic heterogeneities are recycled crust and what they tell us about Earth processes currently remains uncertain. This is because the acoustic and rheological properties of calcium perovskite, which makes up almost a third of oceanic crust at lower mantle conditions, are not known. Indeed, even the most basic property of calcium perovskite, its crystallographic structure, is not known because it cannot be recovered to room conditions without decomposing during pressure release. This property of calcium perovskite makes it extremely challenging to study, and requires that we measure its properties whilst the sample remains at high pressure and temperature conditions. If I can determine the structure, acoustic and rheological properties of calcium perovskite, I will be able to unlock many secrets about the way the deep mantle works. My research aims to do exactly this by using experiments performed in two different apparatuses, the multi anvil and the diamond anvil cell.

A multi-anvil is a large hydraulic press that can apply a force of up to 1000 tonnes, the equivalent of ~ 170 African elephants, to a millimetre sized sample. It allows simulation of conditions up to ~ 700 km depth in the Earth (30 million bar), which is the very top of the lower mantle. Using this equipment, in combination with additional "microphone-like" sensors, it is possible to measure the speed that sounds waves traverse through a calcium perovskite sample whilst it is at lower mantle conditions. It is also possible to deform a sample of calcium perovskite, by shortening it in one direction once it is at lower mantle pressure. This allows determination of the strength, or rheology of the sample. The diamond anvil cell, consisting of two opposing gem-quality diamonds with flat tips compressed together, can generate much higher pressures, more than 200 million bar. But the samples are tiny, with a diameter thinner than a human hair (approximately 100 microns) and a thickness of 5 microns. However, strangely, such a tiny sample has some big advantages because it is transparent to optical light. This allows, using spectroscopy (called Brillouin) and x-rays, for the structure and acoustic velocities of calcium perovskite to be measured simultaneously at lower mantle conditions.

Together, knowledge of the acoustic velocity and rheology of calcium perovskite will allow identification of whether heterogeneity in the lower mantle is made from recycled ocean floor, and to predict how it would be stirred back into the mantle. It is currently unknown whether slabs remain intact because they are rigid, or whether they get rapidly stirred into the mantle because are soft and malleable. Ultimately these behaviours control the habitability of our planet.

The proposed research has potential benefits for those outside the immediate scientific field of the geosciences. The techniques and equipment supported by research such as that proposed here are often of utility to other scientific groups, especially materials scientists and crystallographers. Resistive internal heating in the diamond anvil cell will be of great interest to both groups, who are interested in studying the mineralogical and physical properties of industrial materials and various crystallographic groups at extreme conditions. The ability to heat a sample to > 1000 K without lasers will hugely simplify many potential measurements at high PT conditions, e.g. pair-distribution-functions, synchrotron x-ray absorption fine structure, single crystal diffraction and x-ray scattering techniques.

In addition to promoting multidisciplinary research, the proposed project also has the potential to engage members of the general public, especially school pupils. The nature of this research, exploring a seemingly remote and unimaginable place using a high-tech experimental approach at 'extreme conditions' can be awe inspiring to those with curious minds. The potential for interest from the popular scientific press and the wider media in the results generated during this project is significant and is something I will pursue alongside the usual pathways of publication in peer-reviewed journals. More specifically, this kind of research is ideally suited for use as a method of encouraging pupils in school to take an active interest in STEM (Science, Technology, Engineering and Mathematics) subjects and ultimately to study them. This is especially true of GCSE and A-level students who are actively deciding on the next step in their future educational path. As part of my pathways to impact I have request funds to buy an educational SEED cell. This is a lever-operated sapphire anvil cell. Using this device, it is possible for pupils to perform a scientific experiment and observe water freezing to form ice VI at room temperature and pressures of up to 3 GPa. Pressure is increased or decreased simply by turning a dial, allowing students to grow and melt water crystals, observed by an integrated camera. I intend that this equipment be incorporated into the GeoBus outreach project based at UCL, which runs educational workshops at local schools. This will educate students about not just the nature of the Earth and the findings of the proposed project but also to try and actively encourage students to study science, especially the geosciences at A-level and university (see the Pathways to Impact plan).