title: Bridgmanite crystal chemistry and iron content in the Earthʹs lower mantle
Creators: name: Huang, Rong
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Bridgmanite
subject: Fe3+
subject: oxygen fugacity
subject: Fe content
subject: 550

description: Although the pyrolite model is widely accepted as providing the chemical composition of the Earth’s fertile upper mantle, it is still not clear whether it is also representative of the lower mantle. A comparison between seismic wave velocities in the lower mantle with models for what these velocities should be if the lower mantle has a pyrolitic composition is the only way to ultimately test this assumption. This requires data on mineral elastic properties and a method for determining the proportions and compositions of minerals as a function of depth and bulk composition. The Earth’s lower mantle is comprised mainly of the mineral bridgmanite (Brg), with lesser amounts of ferropericlase (Fp) and CaSiO3 perovskite. In this study multi-anvil experiments have been performed to derive a methodology for determining the compositions of Brg and Fp as a function of bulk composition in the lower mantle. As Brg can contain significant proportions of ferric iron it is also important to predict the composition of Brg as a function of oxygen fugacity. Brg-Fp assemblages have been synthesized with different proportions of Fe and Al and at different oxygen fugacities at 25 GPa and 1973 K. Water was added to some assemblages to induce the growth of large Brg single crystals with differing amounts of Fe2+, Fe3+ and Al. This produced hydrous partial melts that were also examined. In the first part of the study, single crystal X-ray diffraction measurements were made on ten Brg single crystals with different compositions. Based on these measurements and some data from the literature the partial molar volumes of the main Brg components were determined assuming a linear volume-composition relationship. These volumes, which are essential for the thermodynamic description of Brg chemistry at high pressures, decrease in the order: FeFeO3 &gt; MgFeO2.5 &gt; FeAlO3 &gt; MgAlO2.5 &gt; AlAlO3&gt; FeSiO3 &gt; MgSiO3. All lattice parameters, B-O bond distances, octahedral tilting and polyhedral distortion as well as the spontaneous strain components, e4 and etx, increase with charge coupled M3+M3+O3 (M3+=Al3++Fe3+) and oxygen vacancy MgM3+O2.5 substitution, whereas the octahedral tilting and A-site distortion decrease with Fe2+SiO3 substitution. Calculations based on the bond strengths of individual B-O and A-O lengths suggest that the octahedral tilting of Fe,Al-bearing Brg increases with pressure and point to a more compressible octahedral site and less compressible A-site with respect to the MgSiO3 end-member. In the second part of this study, the Fe3+ content of Brg was investigated experimentally as a function of composition and oxygen fugacity (fo2). Recovered samples were analyzed using the electron microprobe and Mössbauer spectroscopy. The Brg Fe3+/ΣFe ratio increases with Brg Al content and fo2 and decreases with increasing total Fe content and with temperature. The fo2 dependence was found to decrease with increasing Al content. Thermodynamic models were calibrated to describe Brg and Fp compositions in the Fe-Mg-Si-O and Fe-Al-Mg-Si-O systems as well as the inter-site partitioning of 3+ cations in Brg. These models fit the experimental data very well, particularly given the small number of adjustable terms. The models allow the mineral compositions for plausible mantle bulk compositions to be calculated as a function of fo2 and can be extrapolated to higher pressures using data on the partial molar volumes of Brg components. The results show that Fe-Mg partitioning between Brg and Fp is strongly fo2 dependent, which allows the results of previous contradictory studies to be brought into agreement. For a pyrolite bulk composition with an upper mantle bulk oxygen content, a lower mantle fo2 of IW – 0.8 is indicated with a Brg Fe3+/ΣFe ratio of 0.51 and a bulk rock ratio of 0.28. This requires the formation of 0.6 wt.% Fe-Ni alloy. With increasing pressure, the model predicts a gradual increase in the Fe3+/ΣFe ratio in Brg in contrast to several previous studies. Oxygen vacancies in Brg decrease to practically zero by 40 GPa, likely influencing transport properties in the top portion of the lower mantle. Lastly, using analyses of melts coexisting with Brg and Fp a thermodynamic model was developed to describe the Fe-Mg exchange between Brg and hydrous melt. Based on this model melt compositions were calculated at different assumed melt fractions (0.1 wt.%- 1 wt.%) for a pyrolite composition and the corresponding melt density was calculated along a mantle geotherm from 22-28 GPa. The density of a 1 wt.% hydrous partial melt is significantly lower than the surrounding lower mantle but is close to neutral buoyancy at the base of the transition zone. This raises the possibility that hydrous melts might rise out of the lower mantle but pond or freeze on entering the transition zone. If such a process occurs in regions of down-welling then melt bearing regions may be continuously dragged into the lower mantle before grain scale migration allows melts to flow upwards, potentially creating long term seismically observable low velocity layers.