Calcium Perovskite: the forgotten mantle phase

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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.