On June 6, 2024, Federic Rochira defended her thesis! She presented her work for 30 minutes, followed by 60 minutes of oral examination, in front of an international committee composed of Tine Thomas as first examiner, Sebastian Rost (University of Leeds) as second examiner, and Alexander Kappes (University of Münster) as third examiner.

In her work, Federica investigated the presence of seismic structures in the Earth’s mantle and their effect on seismic wave paths, using a combination of seismic signals that are thought to travel along the predicted great circle path, between the source and the receiver, as well as signals traveling off the great circle path direction. To better constrain depth and location of mantle reflectors, she took into account directivity parameters of seismic signals, not yet widely used, as well as their traveltime.

Results show the advantages and necessity of using directivity information of seismic signals to detect regional-scale structures in the mantle associated with both subducted slabs and upwelling plumes, as well as mantle discontinuities. Ignoring the directional information, estimated location and depth of reflectors might be inaccurate and contribute to a misleading interpretation of mantle structures in terms of Earth’s dynamics and mineralogy.

She is now a Doctor of the University of Münster. Congratulations!

Yet another publication from the TIMEleSS team! Former TIMEleSS PhD student Jeff Gay has a new paper entitled Depth Dependent Deformation and Anisotropy of Pyrolite in the Earth’s Lower Mantle in the latest issue of Geophysical Research Letters.

Seismologists rely on observable data to construct models that describe the dynamic state of the Earth’s lower mantle. These models, however, require constraints such as mantle composition and material behavior at high pressures and temperatures, which can be provided through experimental mineral physics.

In this study, we use a high pressure devices and X-rays to impose deformation and image the state of our sample with increasing pressure and temperature. We are able to extract information of individual mineral grains within our assemblage, such as the number of grains per phase and their orientations.

Using this experimental data, we identify three regimes of grain orientations in bridgmanite in the lower mantle, corresponding to

1. transformation from lower pressure phases,
2. deformation below ~50 GPa,
3. deformation above ~50 GPa.

With this information, we are able to make predictions about how seismic waves travel and behave based on the deformation state of the lower mantle.

Former TIMEleSS post-doc John Keith Magali has a new publication: Pressure-dependent large-scale seismic anisotropy induced by non-Newtonian mantle flow. The work was published in May 2024 in Geophysical Journal International.

In this work, John combines the team expertise in both mineral microstructures and seismic measurements to make predictions of seismic anisotropy development induced by mantle flow. In fact, observations of large-scale seismic anisotropy can be used as a marker for past and current deformation in the Earth’s mantle. Nonetheless, global features such as the decrease of the strength of anisotropy between ∼150 and 410 km in the upper mantle and weaker anisotropy observations in the transition zone remain ill-understood.

In this work, we integrate pressure-dependent microscopic flow properties in mantle minerals particularly olivine and wadsleyite into geodynamic simulations, compute the crystallographic preferred orientation and anisotropy in the upper mantle and upper transition zone, and apply a tomographic filter that accounts for finite-frequency seismic data, with the aim of providing mantle models comparable with seismic tomography observations.

Our results show that anisotropy observations in the upper mantle can be well understood by introducing gradual shifts in strain accommodation mechanism with increasing depths induced by a pressure-dependent plasticity model in olivine. Across the upper transition zone, our models predict reasonably low anisotropy, in agreement with observations.

These calculations show that, despite the relatively primitive geodynamic setup, models of plate-driven corner flows can be sufficient in explaining first-order observations of mantle seismic anisotropy. This requires, however, incorporating the effect of pressure on mineralogy and mineral plasticity models.

The TIMEleSS team has a new publication! In the December 2023 issue of American Mineralogist, former TIMEleSS student Estelle Ledoux present an In-situ study of microstructures induced by the olivine to wadsleyite transformation at conditions of the 410 km depth discontinuity.

The olivine-wadsleyite transformation is believed to occur at depths of about 410 km in the Earth, producing a major seismic discontinuity in this region of the Earth’s mantle. The mechanism of this phase transition controls the microstructures of the newly nucleated wadsleyite, the major phase of the upper part of the mantle transition zone, and thus impacts seismic observations in the region.

In this work, we study the microstructures produced by the olivine-wadsleyite transformation using in situ laboratory experiments at pressures and temperatures relevant for the mantle transition zone and measure the orientation and size distribution of individual sample grains using multigrain crystallography at synchrotron radiation sources. We find that the olivine to wadsleyite transformation is incoherent at the conditions of the mantle transition zone, and is probably dominated by nucleation of wadsleyite at grain boundaries of the parent olivine.

Thus, we expect that seismic anisotropy near 410 km would drop significantly due to the randomized lattice preferred orientation of newly nucleated wadsleyite induced by the incoherent transformation.