Single-crystal elasticity of Al-rich phases in the Earth’s transition zone and lower mantle

Understanding the Earth’s internal convection process is one of the major frontiers in Earth sciences. The subduction of oceanic lithosphere must produce chemical heterogeneities in the mantle, which may explain some instances of lateral heterogeneity in seismic wave velocities. Seismic observations of the Earth’s interior provide the only method to investigate the mantle at the depth and scale required to understand its chemistry and structure and for tracing the chemical anomalies potentially caused by subduction. The velocities of seismic waves through the Earth depend on the elastic properties of the minerals through which they travel. By comparing observations of seismic wave velocities with estimates for what these velocities should be given particular mineral models and mineral elastic properties, it is possible to relate these observations to the chemical and thermal state of the interior. The P-V-T equation of state and shear (Vs) and longitudinal (Vp) sound velocities of single crystals of minerals that form at conditions compatible with the Earth’s transition zone and lower mantle have been determined in this study by combining X-ray diffraction and Brillouin scattering. In particular majoritic-garnet solid solutions and the NAL (new aluminium phase) phase have been studied. The combination of these techniques makes it possible to determine the full elastic tensor of these minerals at high pressures and temperatures which allows us to accurately determine the pressures of these measurements without the use of calibrant materials and to build mineral models that describe the seismic velocity and density of different rock lithologies within the transition zone and lower mantle. Single-crystal elastic properties of two majoritic garnets (Mg3.24Al1.53Si3.23O12 and Mg3.01Fe0.17Al1.68Si3.15O12), synthesized at 1900 °C and 17 GPa, were determined as a function of density, temperature and composition under hydrostatic conditions by combining single-crystal Brillouin scattering with X-ray diffraction. Experiments were performed up to ~30 GPa and ~600 K in an externally heated diamond anvil cell. In comparison to studies performed on the pyrope garnet end member, substitution of the majorite component is found to lower both the bulk (Ks) and shear modulus (G) of garnet. The substitution of Fe for Mg affects the compression mechanism and therefore the elastic properties of majoritic garnets. The Ks and G values of both samples are similar at room pressure, however, at pressures of the transition zone, Fe-bearing majoritic garnet (Fe-Mj) becomes more compressible due to its smaller (∂K / ∂P) T value. High pressure crystal structure refinements demonstrate that this behaviour is related to the high-pressure response of the tetrahedral site of the garnet structure, which appears slightly more compressible in Fe-Mj with respect to Mj. This effect is unlikely to be linear with garnet Fe content. Single-crystals of the Na0.41[Na0.125Mg0.79Al0.085]2[Al0.79 Si0.21]6O12 (NAL) phase were synthesized at 2260 °C and 20 GPa. The single-crystal structure refinement of NAL, which is consistent with the space group P63/m, reveals dynamic disorder of Na atoms along channels within the structure, which likely influences transport properties of this phase such as electrical conductivity. The complete elastic tensor was experimentally determined for the first time at ambient conditions and at high pressures by Brillouin scattering spectroscopy. The elastic moduli obtained from the Voigt-Reuss-Hill approximation using the elastic constants determined here are KS = 206 GPa and G = 129 GPa, while the isotropic compressional and shear sound velocities are VP = 9.9 km/s and VS = 5.8 km/s. At ~ 1000 km, NAL is expected to transform to the calcium ferrite type aluminum phase (CF). This transformation has been proposed to cause a seismic discontinuity observed at this depth, however, the experimental data imply that the velocity change upon this transformation would be too small to be seismically observed. The NAL phase is elastically anisotropic, displaying 13.9 % compressional and shear wave anisotropy. The anisotropy of NAL decreases as a function of pressure showing 10.9 % compressional (AVp) and 12.71 % shear wave anisotropy (AVs) at 19.93 GPa. However, upon transformation to the CF phase a significant change in the seismic wave anisotropy would occur, which could lead to a seismically detectable discontinuity if the crystallographic preferred orientation were favourable. The elastic properties of majoritic garnets and the NAL phase were used along with literature data to invert the seismic velocities for pyrolite, harzburgite and MORB (mid-ocean ridge basalt) bulk compositions in the depth range between 480 and 780 km. In the transition zone, the mineralogy of these lithologies mostly differs in terms of the proportion and chemistry of garnet. Using the garnet experimental results, elastic properties of majorite and almandine end members were refined using a thermo-elastic model. By refining end member properties using solid solution data instead of the end member data, the compositional range of the refinement remains closer to that over which the model will be employed. This enabled velocities and densities of garnet compositions in the system Fe-CMAS (CaO-MgO-Al2O3-SiO2) to be calculated at mantle conditions. An extrapolation of this model to mantle temperatures was found to reproduce ultrasonic Vs measurements on complex garnet compositions, although not Vp. A similar model was refined to the NAL experimental data. A thermodynamic model was used to estimate mineral modes and compositions in different lithologies. Along an adiabat with a potential temperature of 1673 K, the Vs predicted for a pyrolite mantle bulk composition is 0.2 km/s slower than global seismic models between 500 and 660 km. Vp is closer but still lower than PREM although consistent with AK135. Temperatures would have to be approximately 600 K lower at this depth for the Vs model to be in agreement with seismic observations. The low Vs of majoritic garnet is chiefly responsible for this difference. Consequently models for a MORB bulk composition are also over ~0.3 km/s slower in Vs over the same depth interval. A harzburgite model with 80 % by volume of (Mg,Fe)2SiO4 phases is very close to seismic observations but is still ~0.1 km/s slower in Vs, a difference which would require temperatures to be lower by 200 K for a perfect match to be obtained. The models for all three lithologies, however, match velocities for the top of the lower mantle. The best explanation for these observations would be that average temperatures in the lower transition zone deviated globally from a normal adiabat. This may reflect the occurrence of large lateral cold anomalies at the base of the transition zone that result from the stagnation of subducting slabs. Additionally, lower temperatures in the harzburgitic layer could stabilize akimotoite at the expenses of garnet. Higher velocities expected for akimotoite would further help to explain the mismatch in calculated and observed Vs. Subducted MORB material is estimated to be intrinsically denser than pyrolite or harzburgite at the base of the transition zone even at ambient mantle temperatures. However, neither harzburgite nor MORB are denser than pyrolite or global reference model densities at the top of the lower mantle. This may act to focus these subduction related chemical heterogeneities, even at ambient mantle temperatures, at the base of the transition zone. Below 1000 km, however, the phase transformation of NAL to the CF phase likely renders MORB compositions to be as dense as the surrounding mantle.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Pamato, Martha Giovanna

Contributors dc:contributor

• Dubrovinsky, Leonid

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2082/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2082