Shear modulus and attenuation Qs of olivine from forced-oscillation measurements under mantle conditions and seismic frequencies

Abstract:

Interpretation of seismic profiles and tomographic models of the Earth’s mantle requires exploring the viscoelastic behaviour of rocks / minerals at low strains and seismic frequencies. These physical properties can be investigated by cyclic loading experiments that measure the dynamic response of materials to an applied stress. To date, these data were only accessible by torsional forced-oscillation tests performed in gas vessel apparatus and limited to ~ 0.3 GPa. Here, we demonstrate the feasibility of forced‐oscillations tests up to the Earth’s upper mantle conditions thanks to the development of a torsion module in the Paris‐Edinburgh press and the use of X‐ray radiation.

The experimental device is based on the frame of a V7 Paris-Edinburgh press where both of the two opposed anvils can rotate through two sets of gear reducer and thrust bearings located at the end of each anvil. The accurate rotation of the anvils is monitored by stepper motors and encoders, and enables to apply a low-frequency sinusoidal torque on the cell assembly. The sample and an elastic standard of known modulus and negligible dissipation are loaded in series in a boron-epoxy gasket. The measurement of the relative amplitudes and phases of the torsional mode displacements in standard and sample provides for the determination of the shear modulus and of the strain energy dissipation for the sample. The main challenge in applying the forced-oscillation method at high pressure is the need of a technique for measuring stress and strain with sufficient precision, and with sufficiently short measurement time, while the sample is embedded in the high-pressure vessel. These technical difficulties can be overcome through the use of synchrotron radiation. The strain of the sample is measured in situ and in real-time through X-ray radiography, while the stress is estimated from the strain of the elastic standard (a corundum sample in our case). Hence, coupling the new RoToPEc device with synchrotron radiation provides a unique facility for investigating the viscoelastic behaviour of materials under extreme conditions. The potentials of this new equipment will be illustrated by shear modulus and attenuation measurements on olivine.