Publications – TIMEleSS

Seismic anisotropy of pyrolite in the Earth’s lower mantle

9 June 2024 Sébastien Merkel People news, Publications, TIMEleSS news

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

transformation from lower pressure phases,
deformation below ~50 GPa,
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.

Pressure-dependent large-scale seismic anisotropy induced by non-Newtonian mantle flow

13 May 2024 Sébastien Merkel Publications, TIMEleSS news

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.

Publication on olivine-wadsleyite transformation microstructures

10 December 2023 Sébastien Merkel Publications, TIMEleSS news

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.

New publication in The Seismic Record

8 September 2023 Sébastien Merkel Publications, TIMEleSS news

TIMEleSS PhD student Federica Rochira has a new paper! Along with TIMEleSS PI C. Thomas, Federica is the first author of On the Importance of Using Directional Information in the Search for Lower Mantle Reflectors. The paper is available in Gold Open Access in The Seismic Record.

In this work, Federica focuses on SS precursor signals. SS precursors are signals from S-waves reflected from structures in the Earth’s mantle. Those waves are interesting because they are sensitive to the local thermal and compositional structure deep in the Earth’s mantle.

She analyses how to model the location of the reflections of the wave and shows that a full account of the wave travel path can lead to differences of up to ∼150 km in regard to more simple geometrical calculations. These results hence indicate that the travel‐path deviations of SS precursors should be considered to avoid misinterpretation of mantle discontinuities and potentially reduce previously observed scatter in discontinuity depth.

Congratulations to Federica for the nice work!

New publication in Earth and Planetary Science Letters

3 February 2023 Sébastien Merkel Publications, TIMEleSS news

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.

Yet a second publication in Geochemistry, Geophysics, Geosystems: coesite-stishovite transition and the X-discontinuity

4 October 2022 Sébastien Merkel Publications, TIMEleSS news

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.

New publication in Geochemistry, Geophysics, Geosystems

4 October 2022 Sébastien Merkel Publications, TIMEleSS news

The TIMEleSS project has a new paper in September 2022! TIMEleSS PI C. Thomas is the first author of a publication entitled D” Reflection Polarities Inform Lowermost Mantle Mineralogy in the journal Geochemistry, Geophysics, Geosystems from the American Geophysical Union.

In this work, C. Thomas and co-authors L. J. Cobden, A. R. T. Jonkers investigate the polarities of seismic waves reflecting at structures in the Earth’s mantle and how they can be affected by seismic velocity changes at the interface. For the lowermost mantle reflector, a velocity increase generates a polarity that is the same for the main wave and the core-reflected wave. If, however, the percentage change of the velocity of the S wave increases at least three times as much as that of the P wave velocity (expressed as the R-value, the ratio dVs/dVp), the polarity of the D”-reflected PdP wave changes polarity, becoming opposite to both the main P wave and the reflection from the core-mantle boundary below it.

In the publication, they analyze sets of 1 million models with variable compositions of mantle material and mid-ocean ridge basalt and use an advanced statistical method to identify those combinations of minerals that produce large positive R-values. They distinguish four cases and find that previous explanations for three of these cases concur with our analysis. For regions where velocities decrease over the D” reflector, the analysis shows that enrichment with the lower-mantle mineral bridgmanite is responsible for the observed polarity behavior of P and S waves. This means that for regions such as large low-velocity anomalies in the lowermost mantle, primitive or bridgmanite-enriched material is the preferred explanation.

For more details, have a look at the full publication: C. Thomas, L. J. Cobden, A. R. T. Jonkers, D” Reflection Polarities Inform Lowermost Mantle Mineralogy, Geochemistry, Geophysics, Geosystems, 23, e2021GC010325 [doi: 10.1029/2021GC010325]

New publication in Frontiers in Earth Science

9 May 2022 Sébastien Merkel Publications, TIMEleSS news

We have a new publication! On May 9th, 2022, former TIMEleSS PhD student Estelle Ledoux published a new paper in Frontiers in Earth Science: Deformation of Polycrystalline MgO Up to 8.3 GPa and 1270 K: Microstructures, Dominant Slip-Systems, and Transition to Grain Boundary Sliding.

The work is a result of a collaboration between the Université de Lille and the University of Utah. We focus on polycrystalline periclase, the pure Mg end-member of the second-most abundant mineral in the Earth lower mantle, ferro-periclase, for which mechanical properties are important to understand flow and the dynamics of the Earth mantle.

we deform polycrystalline periclase at conditions ranging from 1.6 to 8.3 GPa and 875–1,270 K. We analyse the flow laws and microstructures of the recovered samples using electron microscopy and compare our observations with predictions from the literature. We identify a first mechanism for samples deformed at 1,270 K, attributed to a regime controlled by grain boundary sliding accommodated by diffusion, and characterized by a small grain size, an absence of texture, and no intracrystalline deformation. At 1,070 K and below, the deformation regime is controlled by dislocations. The samples show a more homogeneous grain size distribution, significant texture, and intracrystalline strains. In this regime, deformation is controlled by the ⟨110⟩{110} slip system and a combined ⟨110⟩{110} and ⟨110⟩{100} slip, depending on pressure and temperature.

Based on these results, we propose an updated deformation map for polycrystalline MgO at mantle conditions and discuss the implications for ferropericlase and seismic observations in the Earth’s lower mantle.

More details can be found in the open-access full reference of the study: E. E. Ledoux, F. Lin, L. Miyagi, A. Addad, A. Fadel, D. Jacob, F. Béclin, and S. Merkel. Deformation of Polycrystalline MgO Up to 8.3 GPa and 1270 K: Microstructures, Dominant Slip-Systems, and Transition to Grain Boundary Sliding. Front. Earth Sci. 10, 849777 (2022) [doi: 10.3389/feart.2022.849777]

New publication in Physical Review Materials

4 May 2022 Sébastien Merkel Publications, TIMEleSS news

We have a new paper release ! On May 3rd, 2022, TIMEleSS student Matthias Krug, PI Carmen Sanchez-Valle, and PI Sébastien Merkel published a paper in Physical Review Materials, along with Binbin Yue and Fang Hong from the Center for High Pressure Science & Technology in Beijing, China.

The paper focuses on deformation in MnO₂ and particularly its high pressure phase, in the CaCl₂ structure. Why do we care about MnO₂ ? MnO₂ is not a deep Earth material, but it can be used as an analogue for SiO₂ stishovite, which crystalizes in a rutile structure at low pressure and transform to a denser CaCl₂ structure under high pressure. In SiO₂, the stishovite to post-stishovite transformations occurs at ~ 50 GPa and is not always easily reached in deformation experiments. In MnO₂, this same structural transition occurs at much lower pressure, around 4 GPa.

Our results show that, after phase transition to a ${CaCl}$ ₂ structure above 3.5 GPa, the dominant (010)[100] and secondary {110}[001] and {011}[0-11] slip systems induce a 121 texture in compression. Further compression increases the activity of the ${011} ⟨ 0 - 11 ⟩$ slip system, with an enhanced 001 texture at $\sim 50 GPa$ . Finally, ${MnO}_{2}$ transforms back to a rutile structure upon pressure release, with a significant orientation memory, highlighting the martensitic nature of the ${CaCl}_{2}$ to rutile structural transformation. Overall, these results help us understanding plasticity and microstructures of ${CaCl}_{2}$ -structured dioxides, with implications in materials and Earth and planetary science.

The full reference for the study: B Yue, M. Krug, C. Sanchez-Valle, S. Merkel, and F. Hong, Deformation and slip systems of CaCl2-type MnO2 under high pressure, Phys. Rev. Mater., 6, 053603 (2022) [doi: 10.1103/PhysRevMaterials.6.053603]

New publication in Geophysical Journal International

25 March 2022 Sébastien Merkel Publications, TIMEleSS news

On March 23rd, 2022, TIMEleSS student Federica Rochira, published a new paper in Geophysical Journal international: Mapping the edge of subducted slabs in the lower mantle beneath southern Asia.

In this work, Federica Rochira, Lina Schumacher, and Christine Thomas from the Westfälische Wilhelms-Universität, Münster, investigate the presence of seismic structures in the Earth’s mantle by searching for seismic signals, and in particular signals from the edges of subducted slabs. They rely on an original approach that uses was that travel off the great circle path direction and are reflected or scattered off structures in the lower mantle and focus on areas of current and past subduction beneath Eurasia by using events from Indonesia and Japan recorded at the broad-band stations in Germany, Morocco and Namibia. Applying seismic array techniques, they measure the direction and traveltime of the out-of-plane arrivals and backtrace them to their location of reflection/scattering.

The results of the work indicate that most of the backtraced reflectors are located beneath southern Asia and are found shallower than 1500 km depth. They correlate well with the edges of prominent high velocity anomalies in tomographic inversions beneath southern Asia, which have been interpreted as remnants of fossil slabs of the subduction of the Tethys Oceans. They also observe few reflectors deeper than 1600 km that are located away from subducting regions and their positions coincide with the eastern edge of the African low velocity anomaly.

These observations suggest that the presence of reflectors in the mid-lower mantle is not exclusively related to current or past subducting regions, but widespread throughout the mantle.

The full details are in the following publication: F. Rochira, L. Schumacher, C. Thomas, Mapping the edge of subducted slabs in the lower mantle beneath southern Asia, Geophysical Journal International, 230, 1239–1252 (2022) [doi: 10.1093/gji/ggac110]

Publication in the European Journal of Mineralogy!

30 September 2021 Sébastien Merkel Activities, Publications

A new publication from a TIMEleSS student in the European Journal of Mineralogy : Deformation of NaCoF₃ perovskite and post-perovskite up to 30 GPa and 1013 K: implications for plastic deformation and transformation mechanism.

Jeff Gay uses a resistively heated diamond anvil to study the plastic deformation and phase transformation mechanisms in NaCoF₃. Under ambient pressure, NaCoF₃. crystallizes in the perovskite structure, and later transforms to the post-perovskite. It is hence an excellent analogue to understand the physical properties of bridgmanite, the most abundant mineral on Earth, and dominant component of the Earth’s lower mantle between 660 and 2900 km depth.

These results from a collaboration between the Université de Lille, the University of Utah, University College London, and the PETRA III / DESY synchrotron source were published on 30 Sep 2021 in the European Journal of Mineralogy.

Full reference: J. P. Gay, L. Miyagi, S. Couper, C. Langrand, D. P. Dobson, H.-P. Liermann, S. Merkel, Deformation of NaCoF₃ perovskite and post-perovskite up to 30 GPa and 1013 K: implications for plastic deformation and transformation mechanism, European Journal of Mineralogy, 33, 591–603 (2021), abstract [doi: 10.5194/ejm-33-591-2021].

Paper out in Nature Communications !

12 December 2019 Sébastien Merkel Publications, TIMEleSS news

First publication for the TIMEleSS team: Kinetics and detectability of the bridgmanite to post-perovskite transformation in the Earth’s D″ layer.

Bridgmanite is a magnesian-iron mineral ((Mg,Fe)SiO₃) with a crystal structure that is not stable under ambient conditions. It forms about 660 kilometers below the surface of the Earth, and transforms to a new structure at even greater depth, approximately 2700 km depth, just above the Core-Mantle boundary.

During his PhD, C. Langrand, PhD student at the Université de Lille studied the kinetics of such transformation. It is fast on geological timescales: about 10 to 10,000 seconds, depending on pressure and temperature. Thanks to the collaborations in the TIMEleSS project, the authors realized that this includes the timescales of seismic waves. As such, seismic waves can trigger the transformation and, in turn, the transformation can amplify the seismic signal from D” seismic reflections.

These results from a collaboration between the Université de Lille, the université Clermont-Auvergne, the université de Lyon, the Westfälische Wilhelms-Universität, Münster, CNRS, and the PETRA III / DESY synchrotron source were published on 12 decembre 2019 in Nature Communications.

Full reference : C. Langrand, D. Andrault, S. Durand, Z. Konôpková, N. Hilairet, C. Thomas, S. Merkel, Kinetics and detectability of the bridgmanite to post-perovskite transformation in the Earth’s D″ layer, Nature Communications, 10, 5680 (2019) [doi: 10.1038/s41467-019-13482-x].