Showing papers by "David L. Kohlstedt published in 2015"

Constitutive Equations, Rheological Behavior, and Viscosity of Rocks

Abstract: This chapter reviews micromechanical mechanisms of deformation and the associated constitutive equations as applied to describing the rheological behavior of rocks and compares predictions based on laboratory-derived flow laws with the viscosity structure of Earth's upper mantle determined from geophysical observations. First, the essential roles of lattice defects – point defects (e.g., vacancies), dislocations, and grain–grain interfaces (e.g., grain boundaries) – in permitting individual grains and collections of grains to deform without fracturing in response to an applied differential stress are examined. Second, the flow laws resulting from the diffusion of ions, movement of dislocations, and sliding on grain–grain interfaces are analyzed. In this context, both high-temperature dislocation creep and low-temperature dislocation plasticity are included, and the effects of water, melt, and crystallographic texture on rock viscosity are considered. Finally, extrapolations of experimentally determined flow laws are compared to global average values of mantle viscosity obtained from analyses of glacial isostatic adjustment in response to the retreat of continental ice sheets and to the regional viscosity structure of the western United States determined from analyses of filling and drying of lakes and crustal deformation following earthquakes. For olivine-rich rocks with a water concentration of ∼ 1000H/10 6 Si, laboratory-derived flow laws yield a viscosity of 10 20 –10 21 Pa s in good agreement with global average values of viscosity. Experimentally determined flow laws predict a lower value for mantle viscosity of 10 18 –10 19 Pa s, similar to that reported for the western United States, provided that rocks are water-saturated as has been proposed for this region in association with rehydration of the upper mantle due to the long history of flat-slab subduction of the Farallon Plate.

Experimental constraints on the electrical anisotropy of the lithosphere–asthenosphere system

Abstract: Electrical anisotropy measurements at high temperatures and quasi-hydrostatic pressures on previously deformed olivine plus melt samples show that electrical conductivity is much higher in the direction of deformation; this is confirmed with a layered electrical model of the asthenosphere and lithosphere that reproduces existing field data. Anne Pommier and co-authors present laboratory measurements of the electrical anisotropy of deformed olivine aggregates and sheared partially molten rocks at high temperatures and pressures. At temperatures of less than 900°C, they find that sheared samples containing both olivine and mid-ocean ridge basalt have up to 100 times greater conductivity in the shear direction than normal to shear. To obtain such high electrical anisotropy, the authors propose a model whereby layers of sheared olivine are alternated with layers of a combined sheared olivine and mid-ocean ridge basalt or pure melt. They find that their experimental results and model can reproduce observed mantle conductivity-depth profiles. The relative motion of lithospheric plates and underlying mantle produces localized deformation near the lithosphere–asthenosphere boundary1. The transition from rheologically stronger lithosphere to weaker asthenosphere may result from a small amount of melt or water in the asthenosphere, reducing viscosity1,2,3. Either possibility may explain the seismic and electrical anomalies that extend to a depth of about 200 kilometres4,5. However, the effect of melt on the physical properties of deformed materials at upper-mantle conditions remains poorly constrained6. Here we present electrical anisotropy measurements at high temperatures and quasi-hydrostatic pressures of about three gigapascals on previously deformed olivine aggregates and sheared partially molten rocks. For all samples, electrical conductivity is highest when parallel to the direction of prior deformation. The conductivity of highly sheared olivine samples is ten times greater in the shear direction than for undeformed samples. At temperatures above 900 degrees Celsius, a deformed solid matrix with nearly isotropic melt distribution has an electrical anisotropy factor less than five. To obtain higher electrical anisotropy (up to a factor of 100), we propose an experimentally based model in which layers of sheared olivine are alternated with layers of sheared olivine plus MORB or of pure melt. Conductivities are up to 100 times greater in the shear direction than when perpendicular to the shear direction and reproduce stress-driven alignment of the melt. Our experimental results and the model reproduce mantle conductivity–depth profiles for melt-bearing geological contexts. The field data are best fitted by an electrically anisotropic asthenosphere overlain by an isotropic, high-conductivity lowermost lithosphere. The high conductivity could arise from partial melting associated with localized deformation resulting from differential plate velocities relative to the mantle, with subsequent upward melt percolation from the asthenosphere.

Reaction infiltration instabilities in experiments on partially molten mantle rocks

Abstract: To investigate channelization of a reactive melt in mantle rocks, we imposed a gradient in fluid pressure across a partially molten rock composed of olivine and clinopyroxene, sandwiched between a source of alkali basalt melt and a sink of porous alumina. We performed experiments at a confining pressure of 300 MPa and pore pressures of 0.1–300 MPa, resulting in fluid pressure gradients of 0–88 MPa/mm at temperatures of 1200–1250 °C. When the gradient in fluid pressure is zero, only a planar reaction layer composed of olivine + melt develops, in agreement with previous experiments. However, if the gradient in fluid pressure is greater than zero, in addition to the planar reaction layer, finger-like melt-rich channels that contain olivine + melt develop and propagate into the rock, significantly past the interface between the melt reservoir and the partially molten rock. Channelization of the melt results in a significant increase in permeability and hence in the flux of melt through the partially molten rock.

Creep behavior of Fe‐bearing olivine under hydrous conditions

Abstract: To understand the effect of iron content on the creep behavior of olivine, (MgxFe(1 − x))2SiO4, under hydrous conditions, we have conducted tri-axial compressive creep experiments on samples of polycrystalline olivine with Mg contents of x = 0.53, 0.77, 0.90, and 1. Samples were deformed at stresses of 25 to 320 MPa, temperatures of 1050° to 1200°C, a confining pressure of 300 MPa, and a water fugacity of 300 MPa using a gas-medium high-pressure apparatus. Under hydrous conditions, our results yield the following expression for strain rate as a function of iron content for 0.53 ≤ x ≤ 0.90 in the dislocation creep regime: e˙=e˙0.901−x0.11/2exp226×1030.9−xRT. In this equation, the strain rate of San Carlos olivine, e˙0.90, is a function of T, σ, and fH2O. As previously shown for anhydrous conditions, an increase in iron content directly increases creep rate. In addition, an increase in iron content increases hydrogen solubility and therefore indirectly increases creep rate. This flow law allows us to extrapolate our results to a wide range of mantle conditions, not only for Earth's mantle but also for the mantle of Mars.

Experimental test of the viscous anisotropy hypothesis for partially molten rocks

Abstract: Chemical differentiation of rocky planets occurs by melt segregation away from the region of melting. The mechanics of this process, however, are complex and incompletely understood. In partially molten rocks undergoing shear deformation, melt pockets between grains align coherently in the stress field; it has been hypothesized that this anisotropy in microstructure creates an anisotropy in the viscosity of the aggregate. With the inclusion of anisotropic viscosity, continuum, two-phase-flow models reproduce the emergence and angle of melt-enriched bands that form in laboratory experiments. In the same theoretical context, these models also predict sample-scale melt migration due to a gradient in shear stress. Under torsional deformation, melt is expected to segregate radially inward. Here we present torsional deformation experiments on partially molten rocks that test this prediction. Microstructural analyses of the distribution of melt and solid reveal a radial gradient in melt fraction, with more melt toward the center of the cylinder. The extent of this radial melt segregation grows with progressive strain, consistent with theory. The agreement between theoretical prediction and experimental observation provides a validation of this theory.