Deep Earth Mantle Phase Transition Maps: Studied by Time-Resolved Experiments

Project Information

DEEP-MAPS

Grant agreement ID: 864877

DOI

10.3030/864877

EC signature date 17 February 2020

Start date 1 October 2020

End date 31 March 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 261 100,00

EU contribution

€ 2 261 100,00

Coordinated by

THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF OXFORD
United Kingdom

Periodic Reporting for period 2 - DEEP-MAPS (Deep Earth Mantle Phase Transition Maps: Studied by Time-Resolved Experiments)

Reporting period: 2022-04-01 to 2023-09-30

Summary of the context and overall objectives of the project

Processes in Earth's lower mantle, the region between 600 and 2900 km depth, govern our planet's inner dynamics and control surface plate tectonics. As such, a quantitative understanding of the physical and chemical properties of the lower mantle is pivotal to model Earth’s dynamic evolution, including the long-term chemical interactions between mantle and atmosphere that are vital to the development of habitability on Earth, and possibly other planets. While models based on the propagation characteristics of Earthquake waves through the Earth, i.e. seismic tomography, are providing increasingly detailed three-dimensional maps of the lower mantle, the interpretation of these models to elucidate key factors such as mantle geochemical heterogeneity or dynamic mantle flow processes has proven to be highly ambiguous.
All evidence points to phase transitions in minerals being the missing link needed to converge to a consistent interpretation of seismic observations. The same phase transitions also play a key role in governing mantle dynamics. But even fundamental properties, such as the location of major phase transitions in Earth’s mantle, are poorly constrained. This is because the parameter space (pressure-temperature-composition) is huge and experimental measurements at planetary interior conditions are extremely slow and difficult to perform.
The ERC-funded project DEEP-MAPS employs a novel class of time-resolved high-pressure/-temperature experiments that reduce by several orders of magnitude the time for key experiments. This allows DEEP-MAPS to map lower mantle phase transitions, their impact on physical properties and their seismic signature with practically continuous coverage in relevant pressure-temperature-composition-space. DEEP-MAPS further probes the time-dependence of phase transitions, transforming our understanding of how to scale from laboratory measurements to geophysical processes. DEEP-MAPS will provide a step-change in our ability to interpret mantle seismic observables and to quantify the geodynamic impact of mantle phase transitions, ultimately leading to a holistic picture of Earth’s deep mantle processes and their surface expressions.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We have shown within the first years of the ERC-funded project, that phase transitions can be tracked from time-resolved x-ray diffraction data collected during continuous compression of mantle minerals. We have also demonstrated that the quality of our data allows for measuring subtle effects related to phase transitions that were undetectable previously.

One example include results on (Mg,Fe)O, the second most abundant mineral in Earth’s lower mantle, which undergoes a change of electronic structure at pressures corresponding to the mid-lower mantle. This spin crossover is predicted to cause changes in a range of physical properties of the lower mantle, with major impacts on geophysics and geodynamic. Within DEEP-MAPS we used novel time-resolved x-ray diagnostics combined with a dynamically-driven DAC to constrain the softening of P-wave velocities in (Mg,Fe)O throughout the iron spin crossover, for the first time at high temperature. High-temperature results are pivotal to distinguish between different proposed theoretical descriptions of the spin crossover. We use our result to benchmark new theoretical calculations and combined them with an expected temperature field inside the Earth based on a large-scale geodynamic model to predict the seismic signature of the spin crossover. After applying a filter to simulate a realistic resolution of geophysical techniques in the lower mantle, we compare our prediction to results from real Earth observations (seismic tomography model SP12RTS) and find strong evidence for the presence of the spin crossover. We also show that the seismic changes associated with the spin crossover might allow us to constrain mantle temperatures once seismic resolution improves.

Another example is work on a phase transition in SiO2 stishovite, a major phase in subducted oceanic crust, that occurs in Earth’s lower mantle. Detecting the transition in seismic data is of relevance for tracking SiO2 cycling from the surface through the deep mantle. In this study, we used the novel experimental setup to continuously compress a sintered sample of SiO2 stishovite to high pressures across the tetragonal-orthorhombic phase transition in the lower mantle. Different experiments have been performed under different sample stress states. The dense pressure-coverage achieved allowed us to (1) pinpoint the phase transition pressure and quantify the effect of stress on the phase transition pressure, and (2) directly calculate the bulk modulus (incompressibility) from our experimental data. We found a very strong effect of stress on the phase transition pressure, suggesting that the different mantle depths where scattering of Earthquake waves is observed could be partially explained by different stress states in the mantle. In addition, and surprisingly, we found that the polycrystalline stishovite is less compressible than single-crystal stishovite, an observation we explain by grain-grain interactions across stiff grain boundaries. This finding has implications for our understanding of how to scale from laboratory measurements on single crystal samples to rocks in the Earth.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our works on the spin crossover in ferropericlase are a significant advancement beyond the state of the art. The softening of the bulk modulus across the spin crossover has been first reported in 2008, but only at room temperature. While theoretical models have since then predicted the bulk modulus softening to shift to higher pressures and broaden, quantitative differences between models have been substantial, and no experimental benchmark existed. As such, our measurement of the bulk modulus softening at high temperatures, facilitated through the methodological developments within DEEP-MAPS, is a breakthrough. The robust detection of the spin crossover in a global geophysical data is another major advancement, with a range of implications for geophysics, geochemistry, as well as geodynamics.

Our results on the compression behaviour of sintered stishovite have been surprising to some extent as the material was less compressible than anticipated. We are aiming to make some more progress in understanding this behavior with the DEEP-MAPS project, noting that the results are not only of interest to Earth sciences, but also for materials science research on ceramic properties and the possibility to tailor these.

Having established the time-resolved x-ray diffraction methodology, we expect that we will be able to collect and analyse a large set of data on different mineral phase transitions in the lower mantle until the end of the ERC-funded project. These data will be of enormous relevance for the interpretation of geophysical observables and the design of geodynamic models.

Within the DEEP-MAPS project, we also started building a novel system to measure elastic properties based on using laser-based spectroscopy and performed first successful test experiments. We plan that this system will be fully operational soon and enable measurements of the elastic properties of mantle minerals at mantle conditions.