On the role of grain boundary sliding and amorphization on the Lithosphere-Asthenosphere Boundary (LAB)
In this paper, appearing in the Volume 591 Issue 7848, 4 March 2021 of Nature, we report the observation of amorphous layers at the interface between grains of olivine deformed under high stresses during deformation by grain boundary sliding.
These amorphized grain boundaries are found in specimens deformed at 300 MPa by Gasc et al. (2019 (https://doi.org/10.1016/j.tecto.2019.04.009) ) in the Paterson Press of S. Demouchy’s lab:
They are not present in the starting material. They have the same composition as the adjacent olivine grains and cannot result from melting since deformation temperatures are lower than those experienced during synthesis.
These amorphous layers result from stress-induced amorphization of the grain boundaries
They are also found in specimens deformed at 5 GPa by Bollinger et al. (2019 (https://doi.org/10.2138/am-2019-6629)) in a multianvil press of the Bayerisches Geoinstitut (Bayreuth):
This demonstrates that pressure does not inhibits amorphization. The micrograph of the specimen deformed at 1000 °C and 5 GPa exhibits evidence for shear across the boundary (displacement highlighted by the stars and presence of nano shear bands (arrows) in the amorphous layer).
The presence of these amorphous layers has tremendous implications on the mechanical properties of the aggregate. Both Gasc et al. and Bollinger et al.’s experiments show a rheological transition in the vicinity of 1000 °C. Indeed, a glass has very specific mechanical properties compared to crystalline matter. In particular the viscosity drops near a characteristic temperature called the glass transition temperature.
Below the glass transition temperature, the glass is rigid, brittle, and samples break along grain boundaries. From the glass transition temperature and above, glass flows and promotes grain boundary sliding:
In his 1914-1915 papers collected and gathered in one volume under the title "The Strength of the Earth's Crust.", Barrell proposed that below the lithosphere lies a thick, hot, shell of weakness: the asthenosphere. The transition between the rigid lithosphere and the convecting asthenosphere takes place without major mineralogical and compositional changes in mantle rocks.
To date a clear agreement has not been reached on the origin of the lithosphere-asthenosphere boundary (LAB). The only consensus is the satisfying match of the viscosity drop at the LAB with a 1300-1400 K isotherms.
Assuming that the origin of the LAB is linked to the rheology of olivine-rich rocks, our results suggests a mechanism controlled by the onset of ductility at amorphized grain boundaries. Once the glass transition temperature of olivine glass is reached, amorphized grain boundary flow, enhancing sliding and leading to a marked viscosity drop of the mantle rocks. The consistency between the isotherm defining the LAB and the glass transition temperature of olivine glass supports our model.
The transformation from crystal to amorphous affects many properties, including electrical conductivity. In olivine there are several evidences that shock-induced amorphization is associated with a large increase in electrical conductivity. Recently, Pommier et al. (2018) reported evidence of a large increase of grain-boundary related electrical conductivity in melt-free polycrystalline olivine (Fo90) samples deformed at 1200 °C to large shear which can well be indicative of amorphized grain boundaries in agreement with our proposition. Hence electrical conductivity might be a test for our model.
Another property that should be strongly affected by grain boundary amorphization is seismic attenuation. Debayle et al. (2020) have recently interpreted global shear attenuation and velocity models as evidence of partial melt occurring within the LVZ. We speculate that glass above the transition temperature would exhibit properties very comparable to melts and would be supported by Debayle et al.’s interpretation as well.
- Barrell, J. (1914) The Strength of the Earth's Crust. The Journal of Geology 22:7, 655-683
- Bollinger C., Marquardt, K., & Ferreira F. Intragranular plasticity vs. grain boundary sliding (GBS) in forsterite: microstructural evidence at high pressures (3.5-5.0 GPa) (2019) Am. Mineral. 104 (2), 220-231 https://doi.org/10.2138/am-2019-6629
- Debayle, E., Bodin, T., Durand, S., Ricard, Y. (2020) Seismic evidence for partial melt below tectonic plates. Nature, 586, 555-558. https://doi.org/10.1038/s41586-020-2809-4
- Eaton, D.W., Darbyshire, F., Evans, R.L., Grütter, H., Jones, A.G., Yuan, X. The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons (2009) Lithos 109, 1–22. doi:10.1016/j.lithos.2008.05.009
- Fischer, K.M., Ford, H.A., Abt, D.L. & Rychert, C.A.. The Lithosphere-Asthenosphere Boundary (2010) Annu. Rev. Earth Planet. Sci. 38, 551–575 doi:10.1146/annurev-earth-040809-152438
- Gasc, J., Demouchy, S., Barou, F., Koizumi, S. & Cordier, P. Creep mechanisms in the lithospheric mantle Inferred from deformation of iron-free forsterite aggregates at 900-1200 °C (2019) Tectonophysics, 761, 16-30, https://doi.org/10.1016/j.tecto.2019.04.009
- Pommier, A., Kohlstedt, D.L.,Hansen, L.N., Mackwell, S., Tasaka, M., Heidelbach, F., Leinenweber, K. Transport properties of olivine grain boundaries from electrical conductivity experiments (2018) Contrib. Mineral. Petrol. 173:41 https://doi.org/10.1007/s00410-018-1468-z
To learn more:
V. Samae, P. Cordier, S. Demouchy, C. Bollinger, J. Gasc, S. Koizumi, A. Mussi, D. Schryvers & H. Idrissi (2021) Stress-induced amorphization triggers deformation in the lithospheric mantle. Nature 591, 82–86. https://doi.org/10.1038/s41586-021-03238-3