Unexpected softening of a synthetic peridotite analogue (magnesium germanate) in a narrow temperature window: the Lithosphere-Asthenosphere Boundary accidentally reproduced?

Seeking to better investigate the mechanism of transformational faulting (i.e. dynamic olivine phase transition considered as the main earthquake mechanism in the deep Upper Mantle), experiments were carried out in a 1-atm rig using synthetic samples of peridotite analogues. The samples consisted of Mg₂GeO₄ with minor MgGeO₃. In brief, using Ge instead of Si allows studying deep processes without increasing the confining pressure. At 1 atm, γ-Mg₂GeO₄ (high-pressure olivine polymorph) transitions to α-Mg₂GeO₄ (olivine) at 810 °C. The hope was to generate and document strain localization features due to local phase transition and/or locally nucleate the phase transition due to strain localization. Working with this material in this apparatus between 760 and 860 °C could have appeared clever.

The experimental setup allowed runs lasting a few hours up to a full day. However, after one day at 800 °C and an axial stress > 300 MPa, the synthetic samples, characterized by a small grain size and a high homogeneity, remained perfectly elastic. Runs were too cold and dry for any nucleation of the high-pressure phase γ-Mg₂GeO₄. Due to both kinetic and rheological issues, beginning to observe strain localization would have taken months, which would most probably have killed the apparatus in only one run due to the corrosion of the external furnace. 

This failed experimental journey turned into an opportunity to study something else: the temperature window was shifted to 950-1250°C. Some experiments revieled a significant viscosity reduction in a narrow temperature window (1000-1150°C), which we propose to interpret as an analogue of the lithosphere-Asthenosphere boundary (LAB). Until recently, melting was the only “transformation” considered in the interrogations about the reduced viscosity of the LAB. In this study, based on our unexpected experimental results, we document a solid-state viscosity reduction that seems to be associated with grain-boundary instability in the context of a competition between diffusive and displacive processes (i.e. premelting). We propose to broaden the discussion including solid-state transformations and potential metastable phases that are not yet fully understood. Although the most studied mineral, olivine has not revealed all its secrets. Additional experiments are required to fully understand what happened.