The MgSiO_3 system at high pressure: Thermodynamic properties of perovskite, postperovskite, and melt from global inversion of shock and static compression data

We present new equation-of-state (EoS) data acquired by shock loading to pressures up to 245 GPa on both low-density samples (MgSiO_3 glass) and high-density, polycrystalline aggregates (MgSiO_3 perovskite + majorite). The latter samples were synthesized using a large-volume press. Modeling indicates that these materials transform to perovskite, postperovskite, and/or melt with increasing pressure on their Hugoniots. We fit our results together with existing P-V-T data from dynamic and static compression experiments to constrain the thermal EoS for the three phases, all of which are of fundamental importance to the dynamics of the lower mantle. The EoS for perovskite and postperovskite are well described with third-order Birch-Murnaghan isentropes, offset with a Mie-Grüneisen-Debye formulation for thermal pressure. The addition of shock data helps to distinguish among discrepant static studies of perovskite, and for postperovskite, constrain a value of K' significantly larger than 4. For the melt, we define for the first time a single EoS that fits experimental data from ambient pressure to 230 GPa; the best fit requires a fourth-order isentrope. We also provide a new EoS for Mg_2SiO_4 liquid, calculated in a similar manner. The Grüneisen parameters of the solid phases decrease with pressure, whereas those of the melts increase, consistent with previous shock wave experiments as well as molecular dynamics simulations. We discuss implications of our modeling for thermal expansion in the lower mantle, stabilization of ultra-low-velocity zones associated with melting at the core-mantle boundary, and crystallization of a terrestrial magma ocean

The Effects of Disequilibrium and Deformation on the Mineralogical Evolution of Quartz Diorite During Metamorphism in the Eclogite Facies

In the Sesia Zone, Western Alps, a large volume of orthogneiss formed as a result of eclogite fades metamorphism and deformation of quartz diorite during early Alpine underthrusting and subduction. Rare lenses of undeformed metaquartz diorite, preserved within the orthogneiss, represent an early stage in the evolution of this latter rock type. The metamorphic and microstructural evolution of the orthogneiss in the eclogite fades has been reconstructed from studies of gradational contacts between undeformed and strongly deformed rocks. High pressure transformations of the original igneous plagioclase + biotite + quartz assemblage to jadeitic pyroxene (Jd0.95 -0.85 + zoisite + quartz + garnet + 2 muscovites developed prior to deformation. Slow intergranular diffusion resulted in a state of disequilibrium between small textural domains in the metaquartz diorite. The compositions of the phases of the undeformed metaquartz diorite do not reflect the bulk rock composition, but were controlled by their position relative to reactant phases. The jadeitic pyroxenes, for example, formed in localized domains which originally consisted of sodic plagioclase whereas omphacite was the equilibrium pyroxene for the bulk rock composition. Mineralogical changes which occurred during subsequent deformation of the metaquartz diorite are interpreted as resulting from a progressive enlargement of equilibrium domains and the partial equilibration of mineral compositions to the bulk rock composition rather than from changes in pressure and temperature. Initially during high-strain deformation, fine-grained aggregates of jadeitic pyroxene + quartz + zoisite (originally pseudomorphing plagioclase) are inferred to have deformed by a mechanism of grain boundary sliding accommodated by diffusive mass transfer. Muscovite and garnet compositions homogenized during the deformation but due to slow intracrystalline diffusion, pyroxene compositions (Jd0.95 -0.80) remained metastable. The coarsening of pyroxene eventually terminated deformation by grain boundary sliding and this mineral subsequently deformed by intracrystalline plastidty. This latter process was accompanied by and perhaps catalysed a change in pyroxene composition from metastable jadeite towards omphacite by a reaction involving the resorption of garnet and the nucleation and growth of paragonite. The resulting orthogneiss consists of quartz + omphadte + garnet + phengite + paragonite + zoisite. The rock is characterized by a broad range of pyroxene compositions (Jd0.8 -0.5) due to the incomplete equilibration of this mineral to the bulk rock composition and a lack of Fe-Mg exchange equilibrium between pyroxene and garnet. However, in contrast to the undeformed metaquartz diorite, there are no obvious textural indications of disequilibrium between phases in the orthogneis

The Dynamics of Silica Melts under High Pressure: Mode-Coupling Theory Results

The high-pressure dynamics of a computer-modeled silica melt is studied in the framework of the mode-coupling theory of the glass transition (MCT) using static-structure input from molecular-dynamics (MD) computer simulation. The theory reproduces the experimentally known viscosity minimum (diffusivity maximum) as a function of density or pressure and explains it in terms of a corresponding minimum in its critical temperature. This minimum arises from a gradual change in the equilibrium static structure which shifts from being dominated by tetrahedral ordering to showing the cageing known from high-density liquids. The theory is in qualitative agreement with computer simulation results.Comment: Presented at ESF EW Glassy Liquids under Pressure, to be published in Journal of Physic

A new large-volume multianvil system

Abstract A scaled-up version of the 6-8 Kwai-type multianvil apparatus has been developed at the Bayerisches Geoinstitut for operation over ranges of pressure and temperature attainable in conventional systems but with much larger sample volumes. This split-cylinder multianvil system is used with a hydraulic press that can generate loads of up to 5000 t (50 MN). The six tool-steel outer-anvils define a cubic cavity of 100 mm edge-length in which eight 54 mm tungsten carbide cubic inner-anvils are compressed. Experiments are performed using Cr 2 O 3 -doped MgO octahedra and pyrophyllite gaskets. Pressure calibrations at room temperature and high temperature have been performed with 14/8, 18/8, 18/11, 25/17 and 25/15 OEL/TEL (octahedral edge-length/anvil truncation edge-length, in millimetre) configurations. All configurations tested reach a limiting plateau where the sample-pressure no longer increases with applied load. Calibrations with different configurations show that greater sample-pressure efficiency can be achieved by increasing the OEL/TEL ratio. With the 18/8 configuration the GaP transition is reached at a load of 2500 t whereas using the 14/8 assembly this pressure cannot be reached even at substantially higher loads. With an applied load of 2000 t the 18/8 can produce MgSiO 3 perovskite at 1900 • C with a sample volume of ∼20 mm 3 , compared with <3 mm 3 in conventional multianvil systems at the same conditions. The large octahedron size and use of a stepped LaCrO 3 heater also results in significantly lower thermal gradients over the sample