Ulrich H. Faul

Simple shear deformation of olivine aggregates

Simple shear deformation of synthetic olivine aggregates to high strains shows that dynamic recrystallisation has strong eVects on the mechanical behaviour and the development of lattice preferred orientations. At 1473 K, a pronounced strain hardening is observed associated with grain elongation and limited dynamic recrystallisation along grain boundaries. The lattice preferred orientations of relict olivine grains generally follow the strain ellipsoid up to a shear strain of 1.1. Both [100] and [001] axes have peaks parallel to the maximum grain elongation direction. At 1573 K and shear strains >0.6, a moderate strain softening was associated with the development of significant dynamic recrystallisation. The lattice preferred orientation of relict olivine grains is characterised by point maxima with [100] axes parallel to the shear direction, [010] axes perpendicular to the shear plane, and [001] axes within the shear plane and perpendicular to the shear direction. The results suggest that at 1473 K both the b=[100] dislocations and b=[001] dislocations contribute to plastic deformation; at 1573 K dynamic recrystallisation relaxed constraints on deformation at grain boundaries, leading to a situation where a single slip system with b=[100] dislocations controls the rheology and the fabric. The lattice orientations of dynamically recrystallised olivine grains were measured using the electron backscatter diVraction technique. The measurements reveal a bimodal pattern of [100] axes: one parallel to the shear direction and the other perpendicular to the maximum principal compressive stress. Analysis of the results shows that the development of the stress-controlled orientations is closely associated with grain boundary migration processes during recrystallisation and growth. As a consequence, the direction of the fastest seismic velocity would not be parallel to the shear direction for olivine aggregates when grain boundary migration has a strong influence on the fabric.

Permeability of partially molten upper mantle rocks from experiments and percolation theory

Experiments with olivine and a basaltic melt were conducted to analyze the melt distribution in partially molten aggregates at low melt fractions. Grain size and melt distribution at the start of an experiment are transient; to reach steady state conditions requires run durations from 2 to 3 weeks at 1 GPa and 1300° to 1400°C. Quantitative analysis of the melt distribution from backscattered electron images of postrun samples shows that most of the melt resides in low aspect ratio, disk-shaped inclusions on two-grain boundaries at melt contents from 0.8 to 3.3 vol. %. Tubules along three-grain edges most likely interconnect the melt at all melt fractions, but their shape and size varies even at the lowest melt fractions due to the effects of anisotropy and nonuniform grain sizes. These triple junction tubules can therefore not be approximated by uniform, equally spaced cylinders with near constant cross-sectional area for the purpose of calculating the permeability of the matrix. Moreover, the tubules are much smaller than the disk-shaped inclusions; tubule-like geometries contain only ∼10% of the total melt content in each experiment. The permeability of the triple junction tubule network only is therefore low (k ∼ 10−17 m2), and segregation velocities are less than 1 mm yr−1 The permeability of the aggregate increases substantially only after the disk-shaped inclusions become interconnected. This melt fraction is calculated from percolation theory, using parameters of the melt inclusions obtained from the backscattered electron images. The threshold melt content, as calculated from the experimental melt distribution, lies between 2 and 3 vol. %.

Effects of crystalline anisotropy on fluid distribution in ultramafic partial melts

The textures of experimentally produced ultramafic partial melts show consistent and significant deviations from the morphology predicted by isotropic equilibrium theory. Flat crystalline interfaces are pervasive in these systems and they coexist with smoothly curved boundaries which are predicted by the isotropic theory. Long-duration experimental runs on olivine-basalt mixtures held at pressures between 1.0 and 2.0 GPa and temperatures from 1350°C to 1400°C were evaluated. Scanning electron microscope images of samples with 2.5 or more volume percent melt showed at least 20% of observable grain boundaries to be wetted by the melt. In addition, approximately 60% of the melt tubules occurring along triple junctions in this sample were found to have at least one flat interface, an effect which increases the ratio of permeability to porosity. Both the experimental evidence and theoretical considerations indicate that the flat faces are stable equilibrium or steady state features in these partial melts, and that they are crystallographically controlled. The crystal-melt morphology is influenced by the crystalline equilibrium habit (obtained from minimization of surface energies of individual crystals) under the constraints of polycrystalline aggregates. A dependence of the style of melt distribution on melt fraction was observed in these runs. At very low melt fractions (less than 1 vol %) the texture is dominated by melt-filled triple junctions and mostly dry grain boundaries, whereas at higher melt fractions (but below 5 vol %) more melt pockets and melt films along grain boundaries appear. An interpretation of the observed texture is made, applying established crystal growth and interface theories to steady state partially molten systems. The extensive occurrence of flat or faceted crystallographic faces in partial melts requires major changes in the modeling of their permeabilities, as well as bulk elastic, anelastic, and electrical properties, from existing models of melt distribution. In regions of the upper mantle where olivine lattice preferred orientation is expected (e.g., in the vicinity of mid-ocean ridges) the presence of faceted faces and associated changes in melt distribution will produce anisotropic permeabilities and changes in seismic attenuation.

Melt retention and segregation beneath mid-ocean ridges.

Geochemical models of melting at mid-ocean ridges—particularly those based on trace elements and uranium-decay-series isotopes—predict that melt segregates from the matrix at very low porosities, of order 0.1%. Some of these models also require that the melt ascends rapidly. But these predictions appear to conflict with seismic data obtained by the mantle electromagnetic and tomography (MELT) experiment. These data reveal, beneath the East Pacific Rise (at 17 °S), a region of low velocities several hundred kilometres wide, which is best explained by the presence of 1–2% melt, distributed on a grain scale in disk-shaped geometries. Here I show that these apparently contradictory constraints can be reconciled by taking into account the geometry and resulting permeability of the intergranular network of melt, together with the changing character of the melt as it ascends. A deep, volatile-rich melt with low viscosity and density is mobile at 0.1% porosity, but basaltic melt only becomes mobile at a porosity above 1%. While the volumetric contribution of the volatile-rich melt to the erupted basalts is small, the isotopic disequilibria (except for radium) generated by porous flow of this melt are preserved if melt transport is rapid at the onset of high-productivity melting. Also, because of incomplete extraction, some melt is retained in a broad zone, consistent with the MELT observations.

Nonvolcanic seafloor spreading and corner‐flow rotation accommodated by extensional faulting at 15°N on the Mid‐Atlantic Ridge: A structural synthesis of ODP Leg 209

Drilling during ODP Leg 209, dredging, and submersible dives have delineated an anomalous stretch of the Mid-Atlantic Ridge north and south of the 15°20′N Fracture Zone. The seafloor here consists dominantly of mantle peridotite with gabbroic intrusions that in places is covered by a thin, discontinuous extrusive volcanic layer. Thick lithosphere (10–20 km) in this region inhibits magma from reaching shallow levels beneath the ridge axis, thereby causing plate accretion to be accommodated by extensional faulting rather than magmatism. The bathymetry and complex fault relations in the drill-core suggest that mantle denudation and spreading are accommodated by a combination of high-displacement, rolling-hinge normal faults and secondary lower-displacement normal faults. These extensional faults must also accommodate corner flow rotation (up to 90°) of the upwelling mantle within the shallow lithosphere, consistent with remnant magnetic inclinations in denuded peridotite and gabbro from Leg 209 core that indicate up to 90° of sub-Curie-temperature rotation.

Dislocation damping and anisotropic seismic wave attenuation in Earth's upper mantle.

Upper Mantle Dislocations The driving forces behind plate tectonics act on a relatively weak upper mantle, such that the stress accumulated from colliding plates in the crust dissipates with depth. The physical properties of common mantle minerals, such as olivine, may be important in controlling mantle rheology, but they are difficult to measure directly. Farla et al. (p. 332) monitored the deformation of randomly oriented olivine crystals at the pressures and temperatures of the upper mantle. Linear defects, known as dislocations, dissipated energy in samples that should normally be stable in several regions of the mantle, including below oceanic crust and around actively subducting slabs. Contrary to previous models, dislocations may dampen low-frequency seismic waves traveling through Earths interior. Stress built up from plate tectonic collisions dissipates at dislocations in mantle minerals. Crystal defects form during tectonic deformation and are reactivated by the shear stress associated with passing seismic waves. Although these defects, known as dislocations, potentially contribute to the attenuation of seismic waves in Earth’s upper mantle, evidence for dislocation damping from laboratory studies has been circumstantial. We experimentally determined the shear modulus and associated strain-energy dissipation in pre-deformed synthetic olivine aggregates under high pressures and temperatures. Enhanced high-temperature background dissipation occurred in specimens pre-deformed by dislocation creep in either compression or torsion, the enhancement being greater for prior deformation in torsion. These observations suggest the possibility of anisotropic attenuation in relatively coarse-grained rocks where olivine is or was deformed at relatively high stress by dislocation creep in Earth’s upper mantle.

Constraints on the Melt Distribution in Anisotropic Polycrystalline Aggregates Undergoing Grain Growth

It has long been recognised that at elevated temperatures surface energy is the driving force for the distribution of melts and fluids among crystalline grains. While for ideal isotropic systems only two parameters, the dihedral angle (the ratio of grain boundary energy to solid-liquid surface energy) and the melt fraction are needed to completely constrain the melt distribution, anisotropic systems present a more complex problem. Surface energy minimisation includes, in addition to surface (or interface) area reduction, also interface rotation. Grain growth, driven by surface area reduction of the aggregate as a whole, means that locally interfaces constantly have to readjust their orientation, a feature not present in isotropic systems. In contrast to isotropic systems, where the geometry of the melt network is the same at all melt fractions, no unique link exists between melt fraction and melt geometry for anisotropic systems. This link is the basis for the high permeability calculated for isotropic aggregates. The degree of anisotropy and therefore the deviation from the ideal isotropic model depends not only on the solid but also on the melt or fluid involved. For the system olivine + basaltic melt the differences to the isotropic model are substantial. Since for anisotropic systems no model exists which can predict the texture of a partially molten aggregate, experimentally produced samples are evaluated in order to determine bulk physical properties of partial melts. Due to the high permeability predicted by the isotropic model, the in situ melt fraction in partially molten regions in the upper mantle would be so small that seismic velocities or the dynamic behaviour would remain essentially unaffected by the presence of melt. In contrast, the experimentally observed melt distribution indicates that a finite melt fraction is needed before efficient segregation can begin, which will affect seismic velocities and influence the dynamic behaviour of partially molten regions.

Slip-system and EBSD analysis on compressively deformed fine-grained polycrystalline olivine

Abstract A slip-system analysis was performed on two synthetic compressively deformed olivine aggregates, derived from experimental solution–gelation (sol–gel) and natural San Carlos precursors to determine how dislocation density relates to Schmid factor for slip in olivine. Individual grain orientations were measured with electron backscatter diffraction. Using decorated dislocations, grain populations were separated into subsets of high versus low dislocation density. Analysis of preferred orientations and distributions of Schmid factors suggests that there is only weak correlation between Schmid factor and dislocation density, slip on (010)[100] in San Carlos grains but (001)[100] in sol–gel material, with multiple slip or stress heterogeneity in both.

Redox-influenced seismic properties of upper-mantle olivine

Lateral variations of seismic wave speeds and attenuation (dissipation of strain energy) in the Earth’s upper mantle have the potential to map key characteristics such as temperature, major-element composition, melt fraction and water content. The inversion of these data into meaningful representations of physical properties requires a robust understanding of the micromechanical processes that affect the propagation of seismic waves. Structurally bound water (hydroxyl) is believed to affect seismic properties but this has yet to be experimentally quantified. Here we present a comprehensive low-frequency forced-oscillation assessment of the seismic properties of olivine as a function of water content within the under-saturated regime that is relevant to the Earth’s interior. Our results demonstrate that wave speeds and attenuation are in fact strikingly insensitive to water content. Rather, the redox conditions imposed by the choice of metal sleeving, and the associated defect chemistry, appear to have a substantial influence on the seismic properties. These findings suggest that elevated water contents are not responsible for low-velocity or high-attenuation structures in the upper mantle. Instead, the high attenuation observed in hydrous and oxidized regions of the upper mantle (such as above subduction zones) may reflect the prevailing oxygen fugacity. In addition, these data provide no support for the hypothesis whereby a sharp lithosphere–asthenosphere boundary is explained by enhanced grain boundary sliding in the presence of water.

The importance of grain size to mantle dynamics and seismological observations

Grain size plays a key role in controlling the mechanical properties of the Earths mantle, affecting both long-timescale flow patterns and anelasticity on the timescales of seismic wave propagation. However, dynamic models of Earths convecting mantle usually implement flow laws with constant grain size, stress-independent viscosity, and a limited treatment of changes in mineral assemblage. We study grain size evolution, its interplay with stress and strain rate in the convecting mantle, and its influence on seismic velocities and attenuation. Our geodynamic models include the simultaneous and competing effects of dynamic recrystallization resulting from dislocation creep, grain growth in multiphase assemblages, and recrystallization at phase transitions. They show that grain size evolution drastically affects the dynamics of mantle convection and the rheology of the mantle, leading to lateral viscosity variations of six orders of magnitude due to grain size alone, and controlling the shape of upwellings and downwellings. Using laboratory-derived scaling relationships, we convert model output to seismologically-observable parameters (velocity, attenuation) facilitating comparison to Earth structure. Reproducing the fundamental features of the Earths attenuation profile requires reduced activation volume and relaxed shear moduli in the lower mantle compared to the upper mantle, in agreement with geodynamic constraints. Faster lower mantle grain growth yields best fit to seismic observations, consistent with our re-examination of high pressure grain growth parameters. We also show that ignoring grain size in interpretations of seismic anomalies may underestimate the Earths true temperature variations.