Olivine constitutes a major component of the Earth’s mantle. The mechanical characteristics of olivine-rich rocks are critical in determining the rheology of the Earth’s mantle and controlling the mechanical coupling between the lithosphere and the ductile asthenosphere [1]. Extensive research has been focused on the dislocation-based plasticity mechanisms in olivine. However, due to the limited slip systems in orthorhombic olivine, alternative mechanisms have been proposed which include grain boundary sliding (GBS) with a limited understanding of these processes. Recently, it was experimentally demonstrated that the grain boundaries can undergo stress-driven amorphization which could be a potential mechanism to accommodate the plasticity processes within the Earth’s interior subjected to high-stress conditions [2]. In particular, these processes become relevant at the lithosphere-asthenosphere boundary (LAB), where olivine approaches glass transition temperature leading to a reduction in its viscosity, thereby promoting GBS [2,3]. As a step towards understanding these structural transformations, we propose in situ investigations of microstructural changes of amorphous olivine from nano- to atomic-scales by increasing temperature inside TEM.   

A thin film of amorphous olivine was transferred in situ on a MEMS-based heating chip inside a focused ion beam. In situ TEM heating experiments were conducted by increasing the temperature in steps of 50 °C /100 °C in different imaging modes: selected area electron diffraction (SAED), automated crystal orientation in TEM (ACOM-TEM), and high-resolution TEM (HRTEM). SAED showed the first diffraction spots only at 700 °C indicating the formation of crystalline olivine. HRTEM performed using the K2 camera revealed the early nucleation of nanocrystals (about 2 nm) within amorphous olivine already at 400 °C, indicating an onset of glass transition in olivine. Significant growth in the number and size of these nuclei was seen as the temperature was increased to 700 °C, where the film almost entirely transformed to the nanocrystalline phase (see Fig. 1). The microstructural evolution of the nanocrystalline olivine was studied by ACOM-TEM which showed progressive grain growth during heating from 700 °C to 1000 °C. Our results coupled with similar investigations on stress-induced amorphous grain boundaries could unravel the structural evolution during burial and exhumation through the Earth’s LAB.  

References
[1] P. Baral, A. Orekhov, R. Dohmen, M. Coulombier, J.-P. Raskin, P. Cordier, H. Idrissi, T. Pardoen, Acta Materialia, 2021, 219, 117257.
[2] V. Samae, P. Cordier, S. Demouchy, C. Bollinger, J. Gasc, S. Koizumi, A. Mussi, D. Schryvers, H. Idrissi, Nature, 2021, 591, 82-86.
[3] D.W. Eaton, F. Darbyshire, R.L. Evans, H. Grütter, A.G. Jones, X. Yuan, Lithos, 2009, 109, 1-22.