A comprehensive laboratory-based model for interpretation of the seismic structure of the Earth’s upper mantle

The Earth’s upper mantle is being imaged with increasing fidelity with the various methods of modern seismology. However, such images are of limited value without a laboratory-based model for their robust interpretation in terms of the many factors (e.g., chemical composition, temperature, and partial melting) that influence the seismic properties of mantle materials. Over decades, we have progressively developed a unique facility for high-pressure-temperature measurement of seismic properties of rock cylinders by forced oscillation at seismic periods (1-1000 s) rather than the much shorter periods (ns-µs) of laboratory wave propagation methods. Recently, we have used such seismic-period methods to test whether ‘water’, present as hydrous defects within nominally anhydrous minerals, may strongly affect seismic wave speeds and attenuation. Surprisingly, however, we found that the more oxidizing environment required for the stability of such hydrous defects, rather than the dissolved water, may be the key factor (Cline et al., Nature, 2018). A further PhD student Tongzhang Qu is currently using the same methods to clarify the nature of the transition from elastic to anelastic behaviour in fine-grained polycrystalline olivine, the grain-size sensitivity of such anelastic behaviour, and the properties of olivine-orthopyroxene mixtures. 

Now, with the funding by the Australian Research Council of a proposal by Ian Jackson, Uli Faul, and Katharina Marquardt, we are seeking a suitably qualified student to participate in an extension of this distinctive experimental work to:

•    explore more fully the interplay between redox conditions, hydrous defects, and an aqueous fluid phase in the anelastic behavior of polycrystalline olivine;
•    further document the role of dislocations in anelastic relaxation (Farla et al., Science, 2012) through studies of pre-deformed single-crystal and/or polycrystalline olivine specimens;
•    re-assess the role of partial melting (Jackson et al., J. geophys. Res., 2004; Faul et al., J. geophys. Res., 2004) especially in short-period anelastic relaxation. 

The goal is a comprehensive laboratory-based model to provide a robust understanding of the mechanical behaviour of the Earth’s upper mantle at periods ranging from those of seismic waves, and the solid-earth tides raised by the Moon and Sun, to lake and glacial loading. The outcome will be better understanding of the complex three-dimensional architecture of the Earth’s upper mantle and the processes involved in its long-term tectonic evolution. 

Accordingly, we invite applications for PhD study from students, preferably with some prior experience in experimental mineral/rock physics and able to start in late 2021/early 2022, by which time international travel to/from Australia is likely to again be feasible.