Octahedral tilting transitions in perovskites are usually identified by the significant lattice distortions which accompany them. The underlying mechanism of coupling between the tilts and the macroscopic strain also gives rise to large anomalies in single crystal and bulk elastic moduli. Landau theory provides an effective framework for describing these different changes in properties and relating them, quantitatively, to the evolution of the driving order parameter for the transition. This approach has been used to analyse the overall elastic behaviour of perovskites belonging to the CaTiO3-SrTiO3 (CST) solid solution, which is expected to be closely analogous to the behaviour of silicate perovskites at higher pressures and temperatures. Pm3m ↔ I4/mcm and I4/mcm ↔ Pnma transitions in CST perovskites are marked by changes in the shear modulus of ∼ 10-30%. The evolution of the order parameter and, hence, of the octahedral tilt angles through these can be followed through the variations of spontaneous strains extracted from high resolution lattice parameter data. Contributions to the elastic softening which are due to strain/octahedral tilt coupling have been calculated using a fully parameterised Landau model of the Pm3m ↔ I4/mcm transition as a function of temperature, pressure and composition. Differences between calculated elastic constants and experimental data from Dynamical Mechanical Analysis, pulse-echo ultrasonics and Resonant Ultrasound Spectroscopy suggest that a proportion of the total softening in tetragonal samples may be due to anelastic effects. The anelastic contributions are observed at frequencies of both a few Hz and 10's of MHz, and can be understood in terms of strain contributions arising from movements of transformation twin walls in response to an externally applied shear stress. Similar transitions in other perovskites are likely to display small anomalies in the bulk modulus, due to weak coupling between octahedral tilts and volume strain, but much larger anomalies in the shear modulus. The elastic properties of tetragonal and orthorhombic structures are likely to be quite different due to different anelastic contributions from twin wall displacements.
This paper illustrates how the recent development of microanalytical techniques for major- and trace-elements in fluid and melt inclusions contributes to the quantitative understanding of lithosphere-scale chemical transport by hydrothermal fluids, including the formation of mineral deposits. After an introductory remark on aspects of instrumentation and data quality assessment, the first-order controls of crustal fluid compositions are discussed, indicating that trace-element concentrations broadly follow silicate rock buffered conditions. However, field-based studies of texturally controlled ore-forming liquid and vapour inclusions in specific upper-crustal magmatic centres show that effective ore formation is a disequilibrium process, requiring deviations of fluid composition from the crustal rock buffers. Microanalytical techniques are also changing experimental approaches in the laboratory, allowing fluid trapping experiments for investigating mineral solubilities, melt-fluid distribution constants, and the chemical properties of fluids responsible for selective element transfer from the downgoing slab to the melting region of calc-alkaline magmas in the mantle wedge.
The maximum attainable pressure of the Kawai-type multi anvil apparatus has been limited to 28 GPa when using tungsten carbide as the anvil material. Recently a remarkable innovation has been made for the apparatus by adopting sintered diamond (SD) as the anvil material. So far pressures up to 63 GPa have been confirmed in SD anvil assemblies without any reduction of sample volume. Therefore phase equilibrium and melting experiments have been extended to conditions to deep into the lower mantle.
Remnants of the materials that were present at the formation of the early solar system are preserved in cometary dust particles collected in the Earth's stratosphere. Coordinated analyses of these materials using ion and electron beam instruments have identified preserved circumstellar silicates, supernova grains, and molecular cloud organic matter within the particles. These exotic grains have isotopic compositions that lie far outside the range exhibited by solar system materials. The mineralogy, composition, and microstructure of the presolar grains provide additional insights their sources and formation. The laboratory data serve as ground truth for astrophysical models based on spectroscopic measurements.
Iron is the only major element in the Earth with multiple electronic configurations (oxidation and spin state). In the upper mantle and transition zone iron is predominantly Fe2+, but the small amount of Fe3+ that is present significantly affects properties that are sensitive to defect chemistry, including electrical conductivity, diffusivity and hydrogen solubility. Fe3+ also determines the oxygen fugacity, where the upper mantle is relatively oxidised due to the high Fe3+/ΣFe ratio in spinel, even though the overall Fe3+ concentration in the upper mantle is low due to its concentration in modally minor phases. The transition zone contains a similar amount of Fe3+, but its distribution among all the major phases results in a significantly lower oxygen fugacity, near metal saturation. In the lower mantle the transition to (Mg,Fe)(Si,Al)O3 perovskite involves the creation of significant Fe3+ (approximately 50% of available iron), even at reducing conditions, which is likely balanced in the bulk lower mantle by disproportionation of Fe2+ to form Fe3+ and metallic iron, and potentially in subducting slabs through reduction of oxidised species in the slab. The nature of Fe3+ charge balance in (Mg,Fe)(Si,Al)O3 perovskite largely determines the influence on physical and chemical properties, where electrical conductivity is enhanced, diffusivity is reduced, and elasticity and hydrogen solubility vary depending on the substitution mechanism. If Fe2+ is more stable in the post-perovskite phase relative to (Mg,Fe)(Si,Al)O3 perovskite as suggested by experiments, both Fe3+/ΣFe and the nature of its influence on physical and chemical properties may be different in the post-perovskite phase. The influence of spin crossover on mantle properties remains unclear. Recent models show that the growth of an (Mg,Fe)(Si,Al)O3 perovskite-rich lower mantle during core formation would progressively oxidise the mantle to present levels as a portion of the disproportionated iron-rich metal phase became incorporated into the core, and the increase in oxygen fugacity during the later stages of Earth accretion would alter the partitioning behaviour of elements between mantle and core, resolving puzzles such as the siderophile element anomaly and the discrepancy between U-Pb and Hf-W systematics in the early Earth. Redox reactions involved in the movement of material across the lower mantle boundary could be related to the formation of deep diamonds, and potentially the rise of atmospheric oxygen through a mantle avalanche in the late Archean that disturbed the balance of volatile element cycles.
The subduction factory processes raw materials such as oceanic sediments and basaltic crust, selectively extracts particular subduction components and manufactures magmas, their solidified materials and continental crust as products. The waste materials from the factory, such as chemically modified oceanic materials and delaminated mafic arc lower crust are transported down to the deep mantle modified their compositions and ultimately recycled as mantle plumes. Andesite composes the bulk continental crust and therefore is the major product in the subduction factory. Two types of andesites, calk-alkalic and tholeiitic series, are commonly recognized in a single arc volcano. We propose a new mechanism for production of these two magma series on the basis of data obtained by Sr isotopic micro-analyses of plagioclase in volcanic rocks from Zao Volcano, NE Japan. Tholeiitic magmas having constant and enriched isotopic signatures are produced by anatexis of the preexisting mafic lower crust, whereas calc-alkalic magmas, having compositions similar to the bulk continental crust, are products of mixing a mantle-derived, hence isotopically depleted, basaltic magma and crust-derived felsic tholeiites.