Reduced Viscosity of Mg2GeO4 with Minor MgGeO3 between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere Boundary

Abstract

Tectonic plates are thought to move above the asthenosphere due to the presence of accumulated melts or volatiles that result in a low-viscosity layer, known as lithosphere–asthenosphere boundary (LAB). Here, we report experiments suggesting that the plates may slide through a solid-state mechanism. Ultrafine-grained aggregates of Mg2GeO4 and minor MgGeO3 were synthetized using spark plasma sintering (SPS) and deformed using a 1-atm deformation rig between 950 °C and 1250 °C. For 1000 < T < 1150 °C, the derivative of the stress–strain relation of the material drops down to zero once a critical stress as low as 30–100 MPa is reached. This viscosity reduction is followed by hardening. The deformation curves are consistent with what is commonly observed in steels during the shear-induced transformation from austenite to martensite, the final material being significantly harder. This is referred to as TRansformation-Induced Plasticity (TRIP), widely observed in metal alloys (TRIP alloys). It should be noted that such enhanced plasticity is not necessarily due to a phase transition, but could consist of any kind of transformation, including structural transformations. We suspect a stress-induced grain-boundary destabilization. This could be associated to the transient existence of a metastable phase forming in the vicinity of grain boundaries between 1000 and 1150 °C. However, no such phase can be observed in the recovered samples. Whatever its nature, the rheological transition seems to occur as a result of a competition between diffusional processes (i.e., thermally activated) and displacive processes (i.e., stress-induced and diffusionless). Consequently, the material would be harder at 1200 °C than at 1100 °C thanks to diffusion that would strengthen thermodynamically stable phases or grain-boundary structures. This alternative scenario for the LAB would not require volatiles. Instead, tectonic plates may slide on a layer in which the peridotite is constantly adjusting via a grain-boundary transformation.

Ultra-low-velocity anomaly inside the Pacific Slab near the 410-km discontinuity

“…Our preferred explanation of this ultra-low-velocity zone inside the slab is the existence of a layer of destabilized olivine which is associated with a significant grain size reduction: spineloid olivine (β- and/or ω-olivine and/or other sheared spineloid phase) would form as lamellae discordant with the host olivine crystal, disorganizing the structure homogeneity, and yielding a seismic wave speed reduction. Subsequent instabilities of grain boundaries are expected to maintain small grain sizes within the transformation loop 40 , 41 . In an experiment with ultrasonic interferometry and in-situ X-ray diffraction, a reduction of shear-wave velocity (i.e., −7.5% to −1.8%) has been observed at the onset of the olivine-wadsleyite transformation and is interpreted as resulting from the existence of an intermediate spineloid phase (ω-olivine or similar spineloid phase) 40 .…”

“…This intermediate phase, known either as ε*-phase or ω-olivine 11 – 16 , was at first theoretically predicted 11 , then observed in meteorites 15 , 16 . It was recently named poirierite 16 and its transient (meta)stability could impact the rheology of the mantle 40 , 41 . Our seismological observations could highlight the transient (meta)stability of poirierite, under substantial shear stress, within the cold sinking slab.…”

Editorial for Special Issue “Mantle Strain Localization—How Minerals Deform at Deep Plate Interfaces”

“…Based on acoustic emissions recorded during deformation of Germanium olivine, they show how an analogue of the olivine-ringwoodite transformation may interact with strain localization and rock embrittlement. Finally, this volume is closed by the experimental study of Ferrand and Deldique [8] and a review study by Ferrand [9], both focusing on the nature of the lithosphere-asthenosphere boundary (LAB). While experimental data highlight a solid-state process commonly observed in metals to account for rock weakening through the LAB, the second study discusses and suggests the presence of garnet-rich pyroxenite layers at the LAB to explain the anomalies of electrical conductivity within the Cocos and Nazca plates offshore Nicaragua.…”