Al, Si-Interdiffusion in Bridgmanit und die Viskosität des unteren Mantels
Physik des Erdkörpers
Thermodynamik und Kinetik sowie Eigenschaften der Phasen und Gefüge von Werkstoffen
Zusammenfassung der Projektergebnisse
Experimental investigation of diffusion in minerals is a fundamental tool for understanding the rheological properties of the Earth's mantle. Bdm is the major component in the Earth's lower mantle. Previous theoretical studies on deformation rates show that the crucial mechanism for the deformation of Bridgmanite (Bdm) is the diffusion-driven creep of the slowest element, Si. Bdm is a magnesium silicate which crystallizes in a perovskite structure with end-member formula MgSiO3 and can contain significant amounts of Fe and Al. Al can be incorporated by substitution of Si, with charge balancing occurring through the formation of oxygen vacancies (OV) or charge-coupled (CC) substitution of Mg and Si by 2 Al. Understanding the interdiffusion of Al and Si in Bdm is, therefore, essential for understanding the geodynamics of the mantle. This study aimed to investigate Al-Si interdiffusion in Bdm experimentally. Therefore, Bdm diffusion couples were synthesized from 0 – 5 mol % Al2O3-bearing MgSiO3 enstatite and glass samples and from Mg-excess glass with 2.7 wt. % Al2O3 at 24 GPa and 1,750 – 2,000 °C using conventional high-pressure and high-temperature multi-anvil presses. The synthesised samples could be further made into diffusion couples with a total volume of 1 mm3 by careful sample preparation and well-polished diffusion surfaces. The diffusion couples were run in a conventional multi-anvil press under the same pressure and temperature conditions as during synthesis (24 GPa and 1,750 – 2,100 °C). The diffusion couples were analysed after the experiments with a scanning transmission electron microscope (STEM) and an energy-dispersive X-ray spectrometer (EDXS). STEM-EDXS was used to evaluate elemental composition at the nanometre scale and to determine Al concentrations. During chemical analysis, the element concentration was measured along a line perpendicular to the diffusion interface determined from two-dimensional elemental distribution maps. The resulting ASCII data were converted into diffusion profiles and used to determine the diffusion rate using the semi-infinite diffusion model. The diffusion coefficients were determined from ten of the 15 diffusion experiments that were carried out at different temperatures and for different annealing times. The number of experiments may appear to be small, but they include a large investment in analytical effort to establish a high degree of confidence in the dataset. The Al-Si interdiffusion was calculated to be DAl-Si = 4.2 ± 0.9 × 10^-11 exp[-400 [kJ mol^-1]/RT] m2/s . Results differ profoundly from previous measurements, indicating a 1 to 2 orders of magnitude slower diffusion rate in Bdm than previously experimentally determined Si and Mg self-diffusion measured at similar pressures and temperatures as well as Fe-Mg interdiffusion in Bdm. Potential factors leading to differences with previous data are discussed, as well as the effect of sample surface preparation, diffusing species, and mineral structure. Following models that suggest diffusion-controlled creep can be assumed in magnesium silicate Bdm, the obtained diffusion rates provide insight into deformation rates of Bdm in the lower mantle. For this purpose, the temperature dependence of the Al-Si diffusion rate of Bdm was examined. This enabled rheology of the lower mantle to be approximated and implications for mantle rheology to be considered.
Projektbezogene Publikationen (Auswahl)
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DMG (Virtual) Poster Session 2020, online: "Aluminous bridgmanite for Al, Si interdiffusion experiments by using a multianvil press. TEM-EDS evaluations"
Czekay, L.; Miyajima, N.; Mccammon, C. & Frost, D. J.
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Aluminous bridgmanite for Al, Si interdiffusion experiments by using a multianvil pres
Czekay, L.; Miyajima, N.; Mccammon, C. & Frost, D. J.
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EMC2020, 3rd European Mineralogical Conference, Cracow, Poland (virtual): "Al, Si interdiffusion under lower mantle conditions: analytical TEM study of Al-bearing bridgmanite" EMPG-XVII (online) Poster März. 2021
Czekay, L.; Miyajima, N.; Mccammon, C. & Frost, D.
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EMPG-XVII (virtual): "Al, Si - interdiffusion in bridgmanite and the viscosity of the lower mantle" DMG Poster-Session 2020 (online) Poster Dez. 2020
Czekay, L.; Miyajima, N.; Mccammon, C. & Frost, D. J.
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Multianvil Workshop – UltraLVP, Bayerisches Geoinstitut, Bayreuth, Germany (virtual): "Aluminous bridgmanite for Al, Si interdiffusion experiments by using a multianvil press"
Czekay, L.; Miyajima, N.; Mccammon, C. & Frost, D. J.
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GeoMinKöln 2022, Köln, Germany: "Al, Si interdiffusion in Al-bearing bridgmanite under lower mantle conditions: Analytical TEM study"
CZEKAY, L.; MIYAJIMA, N.; MCCAMMON, C. & FROST, D.
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GeoMinKöln 2022, Köln, Germany: "Earth's lower mantle may be harder than expected: Al, Si diffusion in bridgmanite"
CZEKAY, L.; MIYAJIMA, N.; MCCAMMON, C. & FROST, D.
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Al, Si diffusion in bridgmanite to estimate the Earth's lower mantle rheology. Copernicus GmbH.
Czekay, Laura; Miyajima, Nobuyoshi & Frost, Daniel
