The TIMEleSS project aims at studying interfaces in the Earth’s mantle combining observations from seismology, mineral physics experiments, microstructures, and wave propagation modeling. It is supported through a bilateral grant, from the ANR in France and the DFG in Germany. The project is led by Sébastien Merkel and Nadège Hilairet at the Université de Lille, Christine Thomas and Carmen Sanchez-Valle from the Westfälische Wilhelms-Universität, Münster, and Sergio Speziale from the Deutsche GeoForschungsZentrum, Potsdam.
Project launch: March 2018
Duration: 36 months, extended until December 2022
Total ANR-DFG funding: 700 000 €
The active research period of TIMEleSS is now over, but we are still working on our datasets and publications so stay tuned for more!
New year, and new publication for the TIMEleSS team! Former timeless PhD student Jeff Gay is the first author of Transformation microstructures in pyrolite under stress: Implications for anisotropy in subducting slabs below the 660 km discontinuity, published in the February 15, 2023, issue of Earth and Planetary Science Letters. The publication is a result of a collaboration between partners at the Université de Lille (J. Gay, E. Ledoux, J. Chantel, S. Merkel), WWU Münster (N. Krug, C. Sanchez-Valle) with measurements at the Deutsches Elektronen-Synchrotron (A. Pakhomova, H.-P. Liermann).
The ‘660’ discontinuity marks the boundary between the upper and lower mantle and is located 660 km below our feet. The is discontinuity often associated with a phase transitions in pyrolite, a model rock composition for the Earth’s mantle. In addition, there are ubiquitous reports of seismic anisotropy below the ‘660’ which are difficult to explain from a mineralogical point of view.
In this study, we implement multigrain crystallography X-ray diffraction in the laser-heated diamond anvil cell in order to track microstructures induced by phase transitions at the pressure and temperature conditions of the discontinuity, around 24 GPa and 1900 K. Before the onset of transformation, pyrolite minerals such as garnet and ringwoodite are isotropic and do not contribute to seismic anisotropy. After the transformation, bridgmanite, the most abundant mineral in the Earth, displays a strong preferred orientation, which we attribute to growth under stress. Other minerals such as davemaoite and ferropericlase are also considered.
The results are used to model anisotropy in a subducting slab, with a prediction of no anisotropy above the ‘660’ and up to 1.28% (0.08 km/s) shear wave splitting below the ‘660’ and provide details on how detailed wave forms can be used to understand the geometry of stress at those depths.
On Nov 24th, 2022, Jeffrey Gay defended his thesis entitled Microstructures and anisotropy of pyrolite in the Earth’s lower mantle: insights from high pressure/temperature deformation and phase transformation experiments at the Université de Lille.
The PhD committee was composed of
- Motohiko Murakami, ETH Zürich, Rapporteur
- Denis Andrault, Univ. Clermont Auvergne, Rapporteur
- Ana Ferreira, Univ. College London, Examinateur
- Angelika Rosa, European Synchrotron Radiation Facility, Examinateur
- Huges Leroux, Univ. Lille, Examinateur and President
- Sébastien Merkel, Univ. Lille, PhD Advisor
Jeff presented his work for 45 minutes, followed by an hour of discussion with the committee. After deliberation, the committee decided to award the Doctoral Degree to Jeffrey Gay.
Time to meet again! On Friday Oct 28th, 2022, TIMEleSS members met to discuss anisotropy in the upper mantle and transition zone.
John Keith Magali presented his latest results combining mantle flow models and results from the experiments in TIMEleSS.
Nice work, John!
Yet a second publication in Geochemistry, Geophysics, Geosystems: coesite-stishovite transition and the X-discontinuity
September 2022 is a good month for the TIMEleSS project: we have a second publication in the journal Geochemistry, Geophysics, Geosystems by the American Geophysical Union! TIMEleSS PhD student Matthias Krug is the first author of Textures induced by the coesite-stishovite transition and implications for the visibility of the X-discontinuity. The work is a result of the TIMEleSS collaboration and involves Morvarid Saki, Estelle Ledoux, Jeffrey P. Gay, Julien Chantel, Anna Pakhomova, Rachel Husband, Arno Rohrbach, Stephan Klemme, Christine Thomas, Sébastien Merkel, and Carmen Sanchez-Valle as co-authors.
Coesite is a high pressure polymorph of SiO2 formed from quartz at pressures above 2 to 3 GPa. In the Earth coesite is formed at approximately 70 km depth or more, depending on the exact temperature conditions. At pressures of 8 to 11 GPa, corresponding to approximately 250 to 300 km, yet another transformations occurs and coesite becomes stishovite. Stishovite was discovered experimentally in 1961 by Sergey M. Stishov and later found in Meteor Crater in Arizona. In parallel, seismic studies report widespread occurrence of velocity anomalies at ∼300 km depth in the Earth’s mantle, whose origin is still not well understood. In this work, Matthias Krug performed experiments to check whether the phase transition in SiO2 from coesite to stishovite could explain these observations and the reasons for the widespread but not global occurrence of the X-discontinuity at 300 km depth.
We reproduced the pressure and temperature conditions at 300 km depth in the laboratory and applied an advanced X-ray diffraction technique to monitor changes in the orientation of grains (i.e., microstructure) in the sample across the transition. We observe that the randomly oriented grains in the low-pressure phase coesite display strong preferred orientation when transformed to stishovite after the transition. In order to relate the experimental observations of grain orientations to the seismic detection of the X-discontinuity, we then computed the effect of grain orientations on the propagation of seismic waves and the velocity changes across the phase transitions. We conclude that 10 – 50 vol.% of crustal rocks embedded in the mantle are needed to explain the observed anomalies and propose that the intermittent observation of this anomaly is related to the seismic sampling strategy rather than to lack of silica anomalies (and hence the absence of the transition) in some specific mantle settings.