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The effect of Fe, Al and Ti on the density, elasticity and the post-stishovite transition of eclogitic H2O-bearing stishovite: Implications on the observability of seismic anomalies in Earth’s mantle

Project description

The changing elasticity of subducted crustal silica may account for seismic anomalies in Earth's mantle

Silicon dioxide or silica occurs in nature as several distinct polymorphs, with quartz in sand being the most common. At high pressures and temperatures, stishovite and, later, post-stishovite form as very dense polymorphs and are the chief water-bearing silicate phases in subducted oceanic crust (eclogite) as it descends into the vast and inaccessible layers of the Earth's deep mantle. The aims of the EU-funded ELASTIC project are to establish the effect of the incorporation of defects typical of eclogitic systems (Al, Ti, Fe, OH) on the high-pressure-temperature phase diagram, density, structure and elasticity of defective stishovite. This will be completed by the application of high-tech methods using the large volume press at the European Synchrotron Radiation Facility. The results will allow improvements to fundamental models used to predict and determine seismic anomalies caused by recycled subducted material through the Earth's mantle, particularly in the upper part of the lower mantle.


Al-bearing stishovite is considered to be the main H2O-bearing silicate phase present in recycled subducted oceanic crust (i.e. eclogite) at uppermost lower mantle conditions, equivalent to >660 km mantle depths, and pressures above 23 GPa, where it can form 25% of the bulk. The incorporation of Al into stishovite leads to significant changes to its phase diagram and its ability to take up H2O. For instance, it halves the pressure required for its rutile- to CaCl2-type transformation, which exhibits significant elastic softening and may be seismically observable.

We currently lack any information on the influence of Fe and Ti on the elastic properties and post-stishovite transitions of Al-and/or H2O-bearing stishovite - even though we know that these defects and solid-solutions are important features of other silicate phases in the mantle; e.g. Ca(Si,Ti)O3 perovskite.

We propose a comprehensive synthesis programme, combined with on-line elastic measurements by ultrasonic interferometry and x-ray diffraction to determine the effects of defect type and concentration on the density, phase diagram and elastic response of this important suite of stishovites. This work, based at ID06LVP, The European Synchrotron, will be supplemented by examination by microscopic and spectroscopic methods at the forefront of microbeam techniques.

Understanding elastic properties of Fe-, Al-, Ti-, and H2O -bearing eclogitic stishovite will allow improvements in fundamental models used to predict and determine the observability of seismic discontinuities and velocity anomalies caused by the subduction of eclogitic material through the Earth’s mantle, particularly in the upper part of the lower mantle. This study will also help to better assess the effects of incorporation of minor amounts of FeO, Fe2O3, Al2O3 and H2O in stishovite on the lower mantle’s density.


Net EU contribution
€ 295 061,76
71 avenue des martyrs
38000 Grenoble

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Auvergne-Rhône-Alpes Rhône-Alpes Isère
Activity type
Research Organisations
Other funding
€ 0,00