Stephen J. Mackwell

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.

High‐temperature creep of olivine single crystals 1. Mechanical results for buffered samples

To investigate the rheological behavior of the Earths upper mantle, over 100 high-temperature deformation experiments have been performed on single crystals of San Carlos olivine in controlled chemical environments at a total pressure of 0.1 MPa. Constitutive equations have been determined which describe the dependence of creep rate on applied stress, temperature and oxygen fugacity in terms of power law relations. In addition, the effects of orthopyroxene activity and loading orientation on the creep behavior have been investigated. For samples of each of three compression directions, [101]c, [011]c and [110]c, buffered by orthopyroxene or magnesiowustite, either two or three power law equations are required to describe the dependence of strain rate at fixed stress on temperature and oxygen fugacity over the full range of experimental conditions. It is proposed that in each power law regime a different creep mechanism controls the creep rate. For all of the experimental conditions, the activation energy for creep is independent of the stress level and the stress exponent is constant at 3.5±0.1. Activation energies for the various creep mechanisms varied from 230 to 1000 kJ/mol; and oxygen fugacity exponents lie in the range −0.03 to 0.4. From the constitutive equations determined based on the power law equations for all of the creep mechanisms, e˙-T-fo2 deformation maps were constructed at a stress of 1 MPa for olivine.

Shear deformation experiments of forsterite at 11 Gpa - 1400°C in the multianvil apparatus

Synthetic forsterite samples were shear-deformed at 11 GPa, 1400°C in the multianvil apparatus. The deformation microstructures have been characterised by SEM, EBSD, X-ray diffraction peak broadening and strain anisotropy analysis, and TEM. Different time durations have been characterised with a view to follow the evolution of strain and stress in high-pressure deformation experiments. A high density of [001] dislocations is introduced during pressurization at room temperature although no significant macroscopic shear or crystal preferred orientations are induced at this stage. The deviatoric stress is probably on the order of 1.5 GPa. Heating at 1400°C leads to a rapid decrease of the density of these dislocations. The shear deformation at high-temperature leads to measurable strain and development of crystal preferred orientations after one hour. Stress and strain-rate continue to decrease with time, such that eight hour experiments exhibit microstructures where recovery is apparent. At this stage, the stress level is estimated at ca. 100 MPa from dislocation density measurements. Crystal preferred orientations and TEM characterisation show that glide of [001] dislocations on (100) or (010) is the dominant deformation mechanism. Further investigation is needed to determine whether inhibition of [100] glide in these experiments is due to the role of water or whether a physical effect of pressure is also contributing.

Structure and elasticity of single‐crystal (Mg,Fe)O and a new method of generating shear waves for gigahertz ultrasonic interferometry

investigated Fe 3+ -bearing (Mg,Fe)O single crystals prepared by interdiffusion having Fe/(Fe + Mg) = 0.06, 015, 0.24, 0.27, 0.37, 0.53, 0.56, 0.75, and 0.79, with ferric iron contents ranging from � 1 to 12% of the total Fe. The elastic constants (c11, c12, c44) are determined from compressional and shear wave velocities in the (100) and (111) propagation directions in the range of 0.5-1.2 GHz. The c11 and c44 elastic constants soften from periclase to wustite, whereas the c12 elastic constant increases. The rate of change in the elastic constants with composition (@cij/@x) is greatest between MgO and (Mg,Fe)O with � 25 mol % FeO implying that substitution of Fe into periclase has a greater effect on the elastic properties than adding Mg to wustite. The elastic anisotropy of (Mg,Fe)O has rather unusual behavior, being essentially constant for the range 0-25 mol % FeO but then decreases linearly with Fe content such that wustite is elastically isotropic. The elastic properties of (Mg,Fe)O having similar total Fe but varying Fe 3+ contents are identical within uncertainty. The isothermal compressibility of samples with Fe/(Fe + Mg) = 0.27, 0.56, and 0.75 is determined by single-crystal X-ray diffraction in a diamond anvil cell to � 9 GPa. For these samples, K0T = 158.4(4), 155.8(9), and 151.3(6) GPa with @KT/@P = 5.5(1), 5.5(1), and 5.6(2), respectively (where values in parentheses indicate standard deviations). The deviation of @KT/@P from 4.0 corresponds to a difference in calculated density of about one percent for ferropericlase (Mg0.8Fe0.2)O at 30 GPa from the value predicted by second-order truncation of the Birch- Murnaghan equation of state. INDEX TERMS: 3620 Mineralogy and Petrology: Crystal chemistry; 3909 Mineral Physics: Elasticity and anelasticity; 3919 Mineral Physics: Equations of state; 3924 Mineral Physics: High-pressure behavior; KEYWORDS: magnesiowustite, elastic constants, ultrasonics, crystal chemistry, bulk moduli

Intercalibration of FTIR and SIMS for hydrogen measurements in glasses and nominally anhydrous minerals

Abstract We present new Fourier Transform Infrared Spectroscopy (FTIR) and ion microprobe/secondary ion mass spectrometry (SIMS) analyses of 1H in 61 natural and experimental geological samples. These samples include 8 basaltic glasses (0.17 to 7.65 wt% H2O), 11 rhyolitic glasses (0.143 to 6.20 wt% H2O), 17 olivines (~0 to 910 wt. ppm H2O), 9 orthopyroxenes (~0 to 263 wt. ppm H2O), 8 clinopyroxenes (~0 to 490 wt. ppm H2O), and 8 garnets (~0 to 189 wt. ppm H2O). By careful attention to vacuum quality, the use a Cs+ primary beam, and a resin-free mounting technique, we routinely achieve hydrogen backgrounds equivalent to less than 5 ppm by weight H2O in olivine. Compared to previous efforts, the new calibration extends to a wider range of H2O contents for the minerals and is more reliable owing to a larger number of standards and to characterization of anisotropic minerals by polarized FTIR on oriented crystals. When observed, discrepancies between FTIR and SIMS measurements are attributable to inclusions of hydrous minerals or fluid inclusions in the crystals. Inclusions more commonly interfere with FTIR analyses than with SIMS, owing to the much larger volume sampled by the former. Plots of H2O determined by FTIR vs. (1H/30Si) × (SiO2), determined by SIMS and electron microprobe (EMP) yield linear arrays and for each phase appear to be insensitive to bulk composition. For example, basalt and rhyolite calibration slopes cannot be distinguished. On the other hand, calibration slopes of different phases vary by up to a factor of 4. This reflects either phase-specific behavior of 1H/30Si secondary ion ratios excited by Cs+ ion beams or discrepancies between phase-specific FTIR absorption coefficient schemes.

Creep of dry clinopyroxene aggregates

We have determined diffusional and dislocation creep rheologies for clinopyroxenite Ca1.0Mg0.8Fe0.2Si2O6 under dry conditions by deforming natural and hot-pressed samples at confining pressures of 300–430 MPa and temperatures of 1100°–1250°C with the oxygen fugacity buffered by either nickel-nickel oxide or iron-wustite powders. The coarse-grained natural Sleaford Bay clinopyroxenite yielded a stress exponent of n = 4.7 ± 0.2 and an activation energy for creep of Q = 760 ± 40 kJ mol−1, consistent with deformation in the dislocation creep regime. The strength of the natural clinopyroxenite is consistent with previous high-temperature measurements of dislocation creep behavior of Sleaford Bay clinopyroxenite by Kirby and Kronenberg [1984] and Boland and Tullis [1986]. Fine-grained clinopyroxenite was prepared from ground powders of the natural clinopyroxenite. Hot-pressed samples were deformed under similar conditions to the natural samples. Mixed-mode deformation behavior was observed, with diffusional creep (n = 1) at lower differential stresses and dislocation creep (with n and Q similar to those of the natural samples) at higher differential stresses. Within the dislocation creep field the predried hot-pressed samples generally yielded creep rates that were about an order of magnitude faster than the natural samples. Thus, even at the highest differential stresses, a component of strain accommodation by grain boundary diffusion was present in the hot-pressed samples. Optical and electron microscope investigations of the deformation microstructures of the natural and hot-pressed samples show evidence for mechanical twinning and activation of dislocation slip systems. When extrapolated to geological conditions expected in the deep crust and upper mantle on Earth and other terrestrial planets, the strength of dry single-phase clinopyroxene aggregates is very high, exceeding that of dry olivine-rich rocks.

Hydrogen in diopside: Diffusion profiles

Abstract The kinetics of diffusion for hydrogen in diopside single crystals from Jaipur, India, were determined by performing room pressure dehydration experiments at temperatures from 700-850 °C and an oxygen fugacity of 10-14 bar. The hydrogen diffusivities were determined for the [100], [010], and [001]* directions either from concentration profiles for hydroxyl in samples after annealing or from bulk hydroxyl concentrations as a function of anneal time for sequential dehydration experiments. The rate of diffusion is anisotropic, with fastest transport along the [100] and [001]* axes and slowest along the [010] axis. Fits of the data to an Arrhenius law yield activation energies and preexponential terms of 181 ± 38 kJ/mol and 10-2.1±1.9 m2/s for diffusion parallel to [100], and 153 ± 32 kJ/mol and 10-3.4±1.6 m2/s for diffusion parallel to [001]*. For diffusion parallel to [010], the data were measured over an insufficient temperature range to calculate the activation energy for diffusion. However, these diffusivities were approximately an order of magnitude slower than those for diffusion parallel to [100] or [001]*. The measured rates and anisotropy for self-diffusion of hydrogen in diopside are consistent with those determined from hydrogen-deuterium exchange in Russian diopside (Hercule and Ingrin 1999). The hydrogen diffusivities are also similar in magnitude to those for olivine (Mackwell and Kohlstedt 1990) and are large enough that the hydrogen content of millimeter- size diopside grains with compositions near Jaipur diopside will adjust to changing environmental conditions in time scales of hours at temperatures as low as 800 °C. As xenoliths ascending from the mantle remain at high temperatures (i.e., >1000 °C) but experience a rapid decrease in pressure, diopside grains may dehydrate during ascent. Thus, low water contents for diopside crystals from xenoliths cannot be taken as indicative of low water contents in the mantle.

High-temperature rheology of enstatite: Implications for creep in the mantle

High-temperature deformation experiments have been performed on oriented single crystals of enstatite at 1 atm under controlled conditions of oxygen fugacity and silica activity. The results indicate that creep on the weakest high temperature slip system in enstatite within the protoenstatite stability field is independent of oxygen fugacity and obeys a power law relation with a stress exponent of n = 3.8±0.5 and activation energy of Q = 820±80 kJ/mol. A decrease in creep rate with increasing aluminum and iron content suggests that aluminum or iron ions play an active role in maintaining charge neutrality within the crystals. The single crystal strengths are greater than those reported for polycrystalline enstatite deforming predominantly on the same slip system, suggesting that the polycrystalline samples may have been water weakened. A comparison of the single crystal results with those for olivine deformed at 1 atm indicates that enstatite is probably stronger than olivine under upper mantle conditions, unless the presence of water results in a greater weakening of enstatite than olivine.

Pressure dependence of H solubility in magnesiowüstite up to 25 GPa: Implications for the storage of water in the Earth's lower mantle

The solubility of hydrogen in (Mg 0.93 Fe 0.07 )O was studied from 5 to 25 GPa at 1200°C under oxidizing conditions, using FTIR spectroscopy on quenched single crystals. OH solubility increases with increasing pressure, concomitant with a decrease in ferric iron content. This result suggests that under lower mantle conditions, the charge neutrality condition could change from [Fe . Me ] = 2[V Me ] to [(OH) . O ] = 2[V Me ], which would have important implications for the rheology of that region. The OH solubility data are well explained by incorporation of H as isolated hydroxyl groups via reduction of ferric iron. The amount of H incorporated at 25 GPa and 1200°C is ∼100 H/10 6 Me (Me = Mg, Fe), i.e. ∼20 ppm wt H 2 O, under water-saturated conditions. When integrated over the entire mass of the lower mantle, these 20 ppm wt H 2 O amount to about 7.3 x 10 18 kg H 2 O, a storage capacity of ∼0.5% of the ocean mass.

Dislocation creep of magnesiowüstite (Mg0.8Fe0.2O)

Abstract Magnesiowustite is likely the second most abundant and probably weakest lower-mantle phase. Its deformation properties will therefore influence the rheological and dynamic behaviour of this region. In this study magnesiowustite aggregates with a composition of Mg0.8Fe0.2O have been deformed under dry conditions in axial compression at a confining pressure of 300 MPa and temperatures from 1200 to 1400 K (0.41