A mineral called CaSiO₃ perovskite—stable in two different crystal shapes—is the key to reading the hidden history of subducted oceanic crust as it sinks thousands of kilometres into Earth’s mantle. The problem is that no one knows which crystal structure, cubic or tetragonal, actually exists at the pressures and temperatures deep inside the planet. The two forms have very different seismic wave speeds and viscosities, so without knowing which one is present, geophysicists cannot interpret the seismic images that reveal where subducted crust accumulates, nor understand how it influences mantle flow and surface volcanism. This project will use novel experiments to pin down exactly where and under what conditions the cubic-to-tetragonal transition occurs. The team will then measure the physical properties of both forms and build synthetic seismic models to predict how the transition would appear in real seismograms. If successful, the work will give researchers a reliable seismic fingerprint for tracking subducted crust through the entire mantle. That would transform our understanding of global material cycles—how carbon and other elements are carried down and later returned to the surface—which ultimately shapes Earth’s long-term habitability. This is fundamental science; practical applications are not immediate, but past work on mantle minerals has underpinned everything from earthquake hazard models to the discovery of deep-Earth water reservoirs.
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Material properties and processes in Earth's mantle govern our planet's inner dynamics and control surface plate tectonics. Subduction of oceanic crust is a key process to deliver crustal material into Earth’s deep mantle, where it may reside for deep time or travel back to the surface via volcanic processes, such as hot spots. Understanding these global surface to deep Earth material cycles is pivotal to modelling our planet’s dynamic evolution, including the long-term chemical interactions between the mantle and atmosphere that are vital to the development of habitability on Earth. CaSiO3 perovskite (CaPv) is a major mineral in Earth’s mantle, in particular in deep subducted crust. Recent works have shown that CaPv has unique physical properties among the mantle minerals, including its elasticity (seismic wave velocities) and viscosity. These unique characteristics promise the ability to map CaPv, and hence subducted oceanic crust, throughout Earth’s mantle using seismic data. Among others, it has been proposed that the characteristically low seismic shear wave speeds of CaPv explain the unique seismic properties of the Large Low Velocity Provinces in Earth’s lower mantle, continent-sized enigmatic structures that have puzzled geophysicists for decades and have been linked to surface volcanism. Moreover, the very low viscosity of CaPv has been linked to key geodynamic mantle processes, including the accumulation of subducted crust at in the shallow lower mantle and close to the core-mantle-boundary. However, CaPv can exist in two different crystal structures, cubic or tetragonal, mostly depending on pressure, temperature, and chemical composition. The physical properties between cubic CaPv (c-CaPv) and tetragonal CaPv (t-CaPv) are largely different and the seismic signature of CaPv, as well as its quantitative impact on large-scale geodynamic processes is entirely dependent on its stable crystal structure at depth. Unfortunately, no consensus has been reached as to whether c-CaPv or t-CaPv exists throughout Earth’s mantle, and whether a transition between the mineral phases occurs in the mantle. Here, we propose to use novel experiments pioneered in our group to tightly constrain the phase boundary and hence depths of the cubic-tetragonal transition in CaPv in Earth’s mantle. We will further determine physical properties of both c-CaPv and t-CaPv, and their changes across the phase transition, focusing on properties that govern seismic wave speeds and mantle dynamic processes, including plastic strength and viscosity. Based on these new experimental data, we will construct synthetic seismic models of the Earth and model the propagation of seismic waves. The outcome will be detailed knowledge on the effects of the cubic to tetragonal transition in CaPv on full seismic waveforms, informing attempts to prospect for the transition signature in real-world seismograms. In addition, our novel viscosity data will further allow us to constrain the effects of the transition on large-scale material transport in future geodynamic models. Overall, the results will impact our understanding of slab subduction into Earth’s deep interior, and hence global material cycles.
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