impractical. Answers to open questions surrounding the Earth’s deep water cycle thus rely on secondary tools, including experiments to understand the physical and chemical properties of


candidate


hydrous phases that plausibly carry water at depth, geodynamic models and seismic observations. Seismic waves propagating in the Earth’s interior are indeed the primary source of information about the inaccessible Earth. Because water changes the physical properties of many rocks, it slows the speed of seismic waves and generates seismic wave anisotropy (i.e., variation of seismic wave velocities as a function of the direction of propagation in anisotropic materials) when passing through. These parameters are the major diagnostic


features for the remote


identification of hydrous regions in the Earth’s interior. A group of dense hydrous silicate phases


discovered in laboratory experiments in the mid-1960s,


the so-called alphabet


phases (phase A, E, D and superhydrous B), are plausible candidates


for the


transport of water at depth due to the large stability field. The physical and chemical properties of


these materials,


obtained through mineral physics studies, are fundamental to revealing the deep water cycle. Therefore, seismic signature of subducting degree


plates of to


identifying the these phases determine


hydration (water


which means that brilliant x-rays produced by synchrotron sources and laser analysis can be used to probe the physical and chemical state of samples while they are submitted to extreme pressure and temperature conditions,” adds Sanchez-Valle. “These experimental simulations provide us with a virtual window into the deep Earth.” Fortunately,


simulated within the team’s


seismic waves can be facilities.


Using a unique laser spectroscopy called Brillouin scattering spectroscopy (Figure 2), the speed of seismic waves and elasticity of materials can be monitored under pressure, divulging their water- bearing qualities. The team also use the brilliant X-rays produced at synchrotron sources to monitor the development of textures in hydrous materials deformed at conditions subducting


slabs


that mimic those of penetrating


lower mantle. The deformation mechanism and strength of materials can be monitored using unconventional X-ray diffraction measurements.


in their storage


capacity) at depth has been one of the goals of Sanchez-Valle’s work. Recent studies have focused on the role of phase D, a dense hydrous phase made up of silicon, magnesium and oxygen (with minor amounts of iron and aluminium), in recycling water into the Earth’s interior via subduction. Phase D is the ultimate water carrier in subducted slabs, as it is the only known hydrous phase stable under the immense pressures


“These experimental simulations provide us with a virtual window into the deep Earth” combined


“Our studies and


temperatures of the mantle. A device called a diamond-anvil cell is


the primary tool used by researchers to replicate extreme conditions that exist at the Earth’s interior, and explore how hydrous phases behave. By powerfully compressing micrometric-size


samples


between the flat surfaces of quarter-carat diamonds, the apparatus authentically simulates pressure conditions down to the Earth’s core (Figure 1). To recreate the infernal


temperatures present in these


realms, heating elements or infrared lasers


are introduced to the tests. “Crucially, the diamonds are transparent, www.projectsmagazine.eu.com


AT A GLANCE Project Information


Project Title: Elasticity and rheology of hydrous phases at high P-T conditions: Implications for anisotropy in subducted slabs and the deep water cycle


Project Objective: The aim of this work is to identify the seismic signature of hydration in hydrous tectonic plates descending into the mantle at subduction zones to elucidate the deep water cycle. We use unconventional laser spectroscopies and synchrotron X-ray techniques to probe the physical properties of small samples of hydrous materials compressed in diamond anvil cells. The results are combined with seismic observations from deep regions to estimate the amount of water recycled and stored into the deep Earth.


in the


Project Duration and Timing: 3 years, October 2009 to September 2012


Project Funding: ETH Zürich – ETHIIRA project ETH-2009-2/ 200.0 kCHF


Project Partners: European Synchrotron Radiation Facility (ESRF), Grenoble, France University of Lille, Lille, France


MAIN CONTACT


on hydrous


phases has allowed us for the first time to interpret seismic anomalies observed in deep subducted slabs in terms of the degree of hydration, placing constraints on the amount of water transported below the transition zone (region in the mantle that spans depths of 400 to 700 km),” says Sanchez-Valle. “The work has shown that hydrous slabs penetrating below the transition zone in areas such as Tonga could contain at least 1.2% in weight of water bound to dense hydrous phase D. The dehydration of phase D at greater depths


is a potential mechanism to


activate very rare (and less damaging) deep focused earthquakes, and the water released


important consequences 


geodynamical and geochemical evolution of the deep Earth.”


53


into the lower mantle has for the


Carmen Sanchez-Valle Carmen Sanchez-Valle is Assistant Professor for Experimental Geochemistry & Mineral Physics at ETH Zurich, Zurich, Switzerland. She holds a Bachelor and a Master Degree in Physics (Condensed Matter) and earned a PhD in Earth Sciences in 2003. Her research covers a broad range of topics aimed at understanding the evolution and dynamics of the deep Earth and planets through experimental investigations of the physical and chemical properties of Earth’s and planetary materials under high pressure and temperature conditions.


Contact: Tel: +41-44-632-4319 Email: carmen.sanchez@erdw.ethz.ch Web: www.geopetro.ethz.ch/research/geo- chemistry/people/scarmen


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