DI13B-01 INVITED
Major element chemical heterogeneity and disequilibrium in Earth's mantle
Radial and lateral variations in major element chemistry have been a prominent feature of mantle models for some time, motivated, among other factors by the apparent difference in the Mg/Si ratio between C1 carbanaceous chondrites and the MORB source, the presence of multiple mantle reservoirs with distinctive trace element chemistry, and features of seismic tomographic models that are difficult to explain in terms of lateral variations in temperature alone. However, demonstration of large scale major element chemical heterogeneity has proved elusive. Radial and lateral heterogeneity are the product of variations in temperature, and phase assemblage, as well as composition. There are at least six major oxide components (MgO, SiO2, CaO, FeO, Na2O, Al2O3) that influence seismologically observable structure through their effect on the mean atomic mass, and the phase assemblage. Our exploration of this compositional space has emphasized the origin of heterogeneity in terms of known processes. Chemical heterogeneity is continuously injected into the mantle at subduction zones in the form of the layered oceanic lithosphere. The lithologic constrast between basalt and harzburgite is likely to be long-lived given our present understanding of chemical diffusion and mixing processes. We explore the influence of variations in basalt fraction on seismic structure in the context of two end-member models: the equilibrium assemblage model (EA) in which basalt and harzburgite are assumed fully to re-equilibrate via diffusion, and the mechanical mixture model (MM) in which lithologic integrity is preserved. We have found that these two models have different phase equilibria for the same basalt fraction and that the MM model agrees better with seismological observations of the transition zone. We will discuss further insights derived from our models including the development of a transition zone thermometer based on SS-precursor intervals computed directly from the mineralogical model, anomalous thermal structure of the oceanic low velocity zone, and the possibility of radial variations in basalt fraction.
DI13B-02
Brillouin scattering in polycrystalline materials at high-pressure
Implementing a new method for measuring seismological compressional- and shear-wave velocities, VP and VS, on polycrystalline samples at high pressures (± high temperatures), we determined wave velocities of NaCl (halite), MgO (periclase) and argon to peak pressures of 26 GPa at room temperature. Our method, based on Brillouin spectroscopy, is applicable to pressures of the entire mantle and shows good agreement with previous ultrasonic and single crystal Brillouin results: we find VP = 4.86 to 6.95 (± 0.09) km/s and VS = 2.8 to 3.2 (± 0.2) km/s for NaCl at 0.8 - 18 GPa, and VP = 2.28 to 7.06 (± 0.08) km/s for Ar at 1.6 - 26 GPa. Our experiments are carried out using a five-pass Fabry-Perot interferometer and 0.25-0.5 W of 532 nm radiation from a Nd:Y-VO4 laser in backscatter and equal-angle scatter geometry through a diamond-anvil cell, allowing us to measure the optical index of refraction as well as the seismic-wave velocities as a function of pressure. NaCl and argon are of interest as calibrants and pressure-transmitting media, and our measurements on MgO represent a step toward routine wave-velocity measurements on Earth-relevant polycrystalline samples at upper- and lower-mantle pressures and variable temperatures. Such measurements are directly applicable to the interpretation of results from seismic tomography, hence clarifying the degree to which velocity variations observed throughout the mantle are due to temperature and compositional changes (including hydration).
DI13B-03
Laboratory-based Interpretation of Upper-Mantle Seismic Tomograms: Progress and Prospects
During the past decade, torsional forced-oscillation techniques have been used intensively to probe the high- temperature viscoelastic behaviour of fine-grained synthetic polycrystalline olivine. The result has been substantial progress in understanding the grain-boundary processes responsible for the attenuation and associated shear modulus dispersion in such materials. We review the experimental dataset from the ANU and University of Wisconsin laboratories for fine-grained olivine polycrystals, derived from either natural or synthetic precursors, and for a dunite mylonite. The ANU data have been re-processed to correct for the newly documented influence of interfacial compliance as part of an updated assessment – with emphasis on the grain-size sensitivity of the viscoelastic relaxation and the influence of a small melt fraction. Alternative models for description of the contrasting behaviour of genuinely melt-free and melt-bearing olivine are compared and contrasted, and their seismological implications briefly reviewed. Finally, we highlight emerging opportunities for experimental studies of dislocation damping and the role of water.