Mark E. Zimmerman

High‐temperature deformation of dry diabase with application to tectonics on Venus

We have performed an experimental study to quantify the high-temperature creep behavior of natural diabase rocks under dry deformation conditions. Samples of both Maryland diabase and Columbia diabase were investigated to measure the effects of temperature, oxygen fugacity, and plagioclase-to-pyroxene ratio on creep strength. Flow laws determined for creep of these diabases were characterized by an activation energy of Q = 485±30 kJ/mol and a stress exponent of n = 4.7±0.6, indicative of deformation dominated by dislocation creep processes. Although n and Q are the same for the two rocks within experimental error, the Maryland diabase, which has the lower plagioclase content, is significantly stronger than the Columbia diabase. Thus the modal abundance of the various minerals plays an important role in defining rock strength. Within the sample-to-sample variation, no clear influence of oxygen fugacity on creep strength could be discerned for either rock. The dry creep strengths of both rocks are significantly greater than values previously measured on diabase under “as-received” or wet conditions [Shelton and Tullis, 1981; Caristan, 1982]. Application of these results to the present conditions in the lithosphere on Venus predicts a high viscosity crust with strong dynamic coupling between mantle convection and crustal deformation, consistent with measurements of topography and gravity for that planet.

Herpes Simplex Virus Resistant to Acyclovir: A Study in a Tertiary Care Center

STUDY OBJECTIVE To determine the sensitivity of herpes simplex virus isolates to acyclovir and the importance of resistant isolates in hospitalized patients. DESIGN Retrospective incidence cohort study. SETTING All herpes simplex virus isolates cultured over 1 year from patients followed at a tertiary care center. PATIENTS Consecutive herpes simplex virus isolates were collected from 207 patients, including immunocompetent patients, patients with malignancy, neonates, bone marrow and organ transplant recipients, and patients seropositive for human immunodeficiency virus. MEASUREMENTS AND MAIN RESULTS A rapid nucleic acid hybridization method was used to assess susceptibility to acyclovir. Acyclovir-resistant herpes simplex viruses were recovered from 7 of 148 immunocompromised patients (4.7%) but from none of 59 immunocompetent hosts. Clinical disease was found in all 7 patients with resistant herpes simplex virus and was more severe in pediatric patients. All resistant isolates were from acyclovir-treated patients and had absent or altered thymidine kinase activity by plaque autoradiography. CONCLUSION Herpes simplex virus resistant to acyclovir arises relatively frequently in immunocompromised patients and may cause serious disease. Rapid detection of resistance permits antiviral therapy to be individualized. Antiviral susceptibility testing to monitor viral resistance should be encouraged, especially in tertiary care settings.

Melt distribution in mantle rocks deformed in shear

Shear experiments on olivine-basalt aggregates provide compelling evidence that the dynamic distribution of melt is controlled by the magnitude and orientation of the differential stress. Our results suggest that deformed, partially molten upper mantle rocks will have highly anisotropic physical properties including seismic wave velocities and melt permeability. In addition, our results provide a basis for interpreting geophysical observations, such as shear-wave splitting and for modeling melt migration processes beneath mid-ocean ridges, specifically focused flow of melt toward the ridge axis.

Laboratory measurements of the viscous anisotropy of olivine aggregates

A marked anisotropy in viscosity develops in Earth’s mantle as deformation strongly aligns the crystallographic axes of the individual grains that comprise the rocks. On the basis of geodynamic simulations, processes significantly affected by viscous anisotropy include post-glacial rebound, foundering of lithosphere and melt production above subduction zones. However, an estimate of the magnitude of viscous anisotropy based on the results of deformation experiments on single crystals differs by three orders of magnitude from that obtained by grain-scale numerical models of deforming aggregates with strong crystallographic alignment. Complicating matters, recent experiments indicate that deformation of the uppermost mantle is dominated by dislocation-accommodated grain-boundary sliding, a mechanism not activated in experiments on single crystals and not included in numerical models. Here, using direct measurements of the viscous anisotropy of highly deformed polycrystalline olivine, we demonstrate a significant directional dependence of viscosity. Specifically, shear viscosities measured in high-strain torsion experiments are 15 times smaller than normal viscosities measured in subsequent tension tests performed parallel to the torsion axis. This anisotropy is approximately an order of magnitude larger than that predicted by grain-scale simulations. These results indicate that dislocation-accommodated grain-boundary sliding produces an appreciable anisotropy in rock viscosity. We propose that crystallographic alignment imparts viscous anisotropy because the rate of deformation is limited by the movement of dislocations through the interiors of the crystallographically aligned grains. The maximum degree of anisotropy is reached at geologically low shear strain (of about ten) such that deforming regions of the upper mantle will exhibit significant viscous anisotropy.

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.

Reaction infiltration instabilities in experiments on partially molten mantle rocks

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.

Comparison of microstructures in superplastically deformed synthetic materials and natural mylonites: Mineral aggregation via grain boundary sliding

We conducted compressional, tensile, and torsional creep experiments on fine-grained forsterite plus Ca-bearing pyroxene aggregates. A distinct microstructure with aggregation of the same phase in the direction of compression was formed in our samples after all the experiments. The stress–strain rate relationship, grain-size dependent flow strength, and the achievement of large tensile strain all indicate that samples underwent creep due to grain boundary sliding (GBS). As a result of GBS, grain-switching events allow dispersed phases to contact grains of the same phase and orient in the direction of compression. We identify similar aggregated microstructures in previously reported micrographs of polymineralic granite-origin ultramylonites. Mineral phase mixing through GBS, which helps to retain fine grain size in rocks due to grain boundary pinning, has been speculated to occur during formation of mylonites. However, our results contradict this hypothesis because mineral aggregation through GBS promotes demixing rather than mixing of the mineral phases. GBS processes alone will not promote a transformation of well-developed monomineralic bands to polymineralic bands during mylonitization.

Creep behavior of Fe-bearing olivine under hydrous conditions

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 Earths mantle but also for the mantle of Mars.

Evolution of the rheological and microstructural properties of olivine aggregates during dislocation creep under hydrous conditions

Since hydrogen plays an important role in dynamic processes in Earths mantle, we conducted torsion experiments to shear strains of 0.6 to 5.0 on Fe-bearing olivine aggregates [(Mg0.5Fe0.5)2SiO4: Fo50] under hydrous conditions at T = 1200 °C and P = 300 MPa. We deformed samples to high enough strains that a steady-state microstructure was achieved, which allowed us to investigate the evolution of both the rheological and microstructural properties. The stress exponent of n ≈ 5.0 and the grain size exponent of p ≈ 0 determined by fitting the strain rate, stress, and grain size data indicate that our samples deformed by dislocation creep. Fourier transform infrared (FTIR) spectroscopy measurements on embedded olivine single crystals demonstrated that our samples were saturated with hydrogen during the deformation experiments. The lattice preferred orientation (LPO) of olivine changes as a function of strain due to competition among three slip systems: (010)[100], (100)[001], and (001)[100]. Observed strain weakening can be attributed to geometrical softening associated with development of LPO, which reduces the stress by ~1/3 from its peak value in constant strain rate experiments. The geometrical softening coefficient determined in this study is an important constraint for modeling and understanding dynamical processes in upper mantle under hydrous conditions.

Rheological weakening of olivine + orthopyroxene aggregates due to phase mixing, Part2: Microstructural development

To understand the processes involved in phase mixing during deformation and the resulting changes in rheological behavior, we conducted torsion experiments on samples of iron-rich olivine plus orthopyroxene. The experiments were conducted at a temperature, T, of 1200°C and a confining pressure, P, of 300 MPa using a gas-medium, deformation apparatus. Samples composed of olivine plus 26% orthopyroxene were deformed to outer radius shear strains up to γ ≈ 26. In samples deformed to lower strains of γ ≲ 4, elongated olivine and pyroxene grains form a compositional layering. Already by this strain, mixtures of small equant grains of olivine and pyroxene begin to develop and continue to evolve with increasing strain. The ratios of olivine to pyroxene grain size in deformed samples follow the Zener relationship, indicating that pyroxene grains effectively pin the grain boundaries of olivine and inhibit grain growth. Due to the reduction in grain size, the dominant deformation mechanism changes as a function of strain. The microstructural development forming more thoroughly mixed, fine-grained olivine-pyroxene aggregates can be explained by the difference in diffusivity among Me (Fe or Mg), O, and Si, with transport of MeO significantly faster than that of SiO2. These mechanical and associated microstructural properties provide important constraints for understanding rheological weakening and strain localization in upper mantle rocks.