In this paper, appearing in volume 219 (still in progress) of Acta Materialia, we characterized the room temperature rheology of amorphous olivine (hereafter a-olivine) by the use of nanoindentation relaxation experiments.
This study has been motivated by our recent observations of stress-induced grain boundary amorphization in polycrystal olivine (results publish in Nature ). It is proposed that the presence of this amorphous layer act as a lubricant between grains when the temperature get close to its glass transition temperature, therefore promoting grain boundary sliding. TEM observations highlight a transition of deformation mechanisms, from brittle grain boundary to viscous flow around 1000 °C, in agreement with the lithosphere-asthenosphere boundary isotherm.
Until now, only extrinsic and qualitative evidences of the rheological behavior of a-olivine has been shown. The present study aims at measuring the intrinsic rheological properties of amorphous olivine.
Amorphous olivine cannot be produced at large scale due to the extreme quenching rate necessary to obtain such a glass. The only known option is to use pulsed laser deposition technique which allows the deposition of thin amorphous films (around 300 nm thick) . Nanoindentation appears to be one of the best techniques to probe such a small-scale sample.
Figure 1 Nanoindentation of a-olivine: (a) typical diamond Berkovich tip observed in a scanning electron microscope; (b) 3D view of a residual imprint in a-olivine thin film, measured with an atomic force microscope (AFM) and (c) top view and profile of the residual imprint, measured by AFM.
We use a recently developed method called nanoindentation relaxation test which allows the measurement of the load relaxation over long period (typically 1 hour or more) [3,4]. This technique uses the continuous measure of the contact stiffness to keep the area between the tip and the sample surface constant, while measuring the load decrease (similarly to a uniaxial relaxation experiment). Combining this measurement with classical indentation tests, we can access a large range of strain rates, typically from 3.10-1 to 10-7 s-1, which is hardly reachable for most micro and macro laboratory techniques. This represents a significant step toward natural strain rates.
Figure 2 Stress vs. strain rate response of a-olivine measured by constant strain rate indentation and long-term relaxation indentation. The colored arrows highlight the improvements made over the years to reach lower strain rates.
We show, that even at room temperature, a-olivine is highly strain rate-sensitive with , indicating a complex out-of-equilibrium structure. The physical activation volume extracted from the measurements correspond to 10 to 20 Mg and Fe metallic sites in the (Mg,Fe)2SiO4 crystalline structure. Furthermore, the rate sensitivity of a-olivine is significantly higher than single crystal olivine (5 times higher) at room temperature (see the figure below).
Figure 3 Stress vs. strain rate response of a-olivine (this study) and single crystal olivine (data from Kranjc et al. [5,6]).
All these measurements support the high rheological heterogeneity of the polycrystalline olivine containing amorphous grain boundaries which will localize strain along the boundaries. It also emphasizes the importance of applied strain rate on the rheological behavior of an aggregate: As tectonic strain rates can be as low as 10-12 to 10-16 s-1 in the Earth’s upper mantle, the rheological contrast between amorphous olivine and the crystalline grains should be even greater that what measured at 10-7 s-1. We can expect that in natural conditions, grain boundary sliding will be more promoted than in laboratory experiments.
In further studies, we need to investigate high temperature mechanical behavior of a-olivine up to 1000 °C by nanoindentation, in order to get closer to natural conditions and better assess the model proposed in the work of Samae et al. .
 V. Samae, P. Cordier, S. Demouchy, C. Bollinger, J. Gasc, S. Koizumi, A. Mussi, D. Schryvers, H. Idrissi, Stress-induced amorphization triggers deformation in the lithospheric mantle, Nature. 591 (2021) 82–86. https://doi.org/10.1038/s41586-021-03238-3.
 R. Dohmen, H.-W. Becker, E. Meißner, T. Etzel, S. Chakraborty, Production of silicate thin films using pulsed laser deposition (PLD) and applications to studies in mineral kinetics, Eur. J. Mineral. 14 (2002) 1155–1168. https://doi.org/10.1127/0935-1221/2002/0014-1155.
 P. Baral, G. Guillonneau, G. Kermouche, J.-M. Bergheau, J.-L. Loubet, Theoretical and experimental analysis of indentation relaxation test, J. Mater. Res. 32 (2017) 2286–2296. https://doi.org/10.1557/jmr.2017.203.
 P. Baral, G. Guillonneau, G. Kermouche, J.-M. Bergheau, J.-L. Loubet, A new long-term indentation relaxation method to measure creep properties at the micro-scale with application to fused silica and PMMA, Mech. Mater. 137 (2019). https://doi.org/10.1016/j.mechmat.2019.103095.
 K. Kranjc, Z. Rouse, K.M. Flores, P. Skemer, Low‐temperature plastic rheology of olivine determined by nanoindentation, Geophys. Res. Lett. 43 (2016) 176–184. https://doi.org/10.1002/2015GL065837.
 K. Kranjc, A.S. Thind, A.Y. Borisevich, R. Mishra, K.M. Flores, P. Skemer, Amorphization and Plasticity of Olivine During Low-Temperature Micropillar Deformation Experiments, J. Geophys. Res. Solid Earth. 125 (2020) 0–3. https://doi.org/10.1029/2019JB019242.
To learn more:
P. Baral, A. Orekhov, R. Dohmen, M. Coulombier, J.-P. Raskin, P. Cordier H. Idrissi & T. Pardoen (2021) Rheological properties of amorphous olivine thin films measured by nanoindentation. Acta Materialia, https://doi.org/10.1016/j.actamat.2021.117257