Reduced Viscosity of Mg<sub>2</sub>GeO<sub>4</sub> with Minor MgGeO<sub>3</sub> between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere Boundary
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....
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| Format: | Article |
| Language: | English |
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MDPI AG
2021-06-01
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| Series: | Minerals |
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| Online Access: | https://www.mdpi.com/2075-163X/11/6/600 |
| _version_ | 1797531557836095488 |
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| author | Thomas P. Ferrand Damien Deldicque |
| author_facet | Thomas P. Ferrand Damien Deldicque |
| author_sort | Thomas P. Ferrand |
| collection | DOAJ |
| description | 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 Mg<sub>2</sub>GeO<sub>4</sub> and minor MgGeO<sub>3</sub> were synthetized using spark plasma sintering (SPS) and deformed using a 1-atm deformation rig between 950 °C and 1250 °C. For 1000 < <i>T</i> < 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. |
| first_indexed | 2024-03-10T10:45:28Z |
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| institution | Directory Open Access Journal |
| issn | 2075-163X |
| language | English |
| last_indexed | 2024-03-10T10:45:28Z |
| publishDate | 2021-06-01 |
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| series | Minerals |
| spelling | doaj.art-ffa761a8e2a64d1495e81e61e83fa7202023-11-21T22:38:18ZengMDPI AGMinerals2075-163X2021-06-0111660010.3390/min11060600Reduced Viscosity of Mg<sub>2</sub>GeO<sub>4</sub> with Minor MgGeO<sub>3</sub> between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere BoundaryThomas P. Ferrand0Damien Deldicque1Institut des Sciences de la Terre d’Orléans, Université d’Orléans—CNRS UMR-7327, 45100 Orléans, FranceLaboratoire de Géologie, CNRS UMR 8538, Ecole Normale Supérieure, PSL Research University, 75005 Paris, FranceTectonic 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 Mg<sub>2</sub>GeO<sub>4</sub> and minor MgGeO<sub>3</sub> were synthetized using spark plasma sintering (SPS) and deformed using a 1-atm deformation rig between 950 °C and 1250 °C. For 1000 < <i>T</i> < 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.https://www.mdpi.com/2075-163X/11/6/600plate tectonicslithosphereasthenospherelithosphere–asthenosphere boundaryplasticityperidotite |
| spellingShingle | Thomas P. Ferrand Damien Deldicque Reduced Viscosity of Mg<sub>2</sub>GeO<sub>4</sub> with Minor MgGeO<sub>3</sub> between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere Boundary Minerals plate tectonics lithosphere asthenosphere lithosphere–asthenosphere boundary plasticity peridotite |
| title | Reduced Viscosity of Mg<sub>2</sub>GeO<sub>4</sub> with Minor MgGeO<sub>3</sub> between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere Boundary |
| title_full | Reduced Viscosity of Mg<sub>2</sub>GeO<sub>4</sub> with Minor MgGeO<sub>3</sub> between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere Boundary |
| title_fullStr | Reduced Viscosity of Mg<sub>2</sub>GeO<sub>4</sub> with Minor MgGeO<sub>3</sub> between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere Boundary |
| title_full_unstemmed | Reduced Viscosity of Mg<sub>2</sub>GeO<sub>4</sub> with Minor MgGeO<sub>3</sub> between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere Boundary |
| title_short | Reduced Viscosity of Mg<sub>2</sub>GeO<sub>4</sub> with Minor MgGeO<sub>3</sub> between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere Boundary |
| title_sort | reduced viscosity of mg sub 2 sub geo sub 4 sub with minor mggeo sub 3 sub between 1000 and 1150 °c suggests solid state lubrication at the lithosphere asthenosphere boundary |
| topic | plate tectonics lithosphere asthenosphere lithosphere–asthenosphere boundary plasticity peridotite |
| url | https://www.mdpi.com/2075-163X/11/6/600 |
| work_keys_str_mv | AT thomaspferrand reducedviscosityofmgsub2subgeosub4subwithminormggeosub3subbetween1000and1150csuggestssolidstatelubricationatthelithosphereasthenosphereboundary AT damiendeldicque reducedviscosityofmgsub2subgeosub4subwithminormggeosub3subbetween1000and1150csuggestssolidstatelubricationatthelithosphereasthenosphereboundary |