John D. Fitz Gerald

Relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks

New microstructural data on experimentally deformed “wet” and “dry” natural olivine rocks (Anita Bay and Aheim dunite), together with the other reliable experimental data, indicate that the experimental stress-recrystallized grain size relationship in olivine-rocks is largely independent of water content and temperature, and is only slightly dependent on the flow properties of the material. The experimental data cover a stress range of 30–300 MPa, water contents from <30 ppm to 300 ppm, and temperatures in the range 1100–1650°C. Local melt contents of up to 10 volume% cannot be demonstrated to have a significant effect on the stress—grain size relationship.

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.

A solid-state process for formation of boron nitride nanotubes

The formation of boron nitride (BN) nanotubes via a solid-state process is demonstrated. The nanotubes are produced by first ball-milling hexagonal BN powder to generate highly disordered or amorphous nanostructures, followed by annealing at temperatures up to 1300 °C. The annealing leads to the nucleation and growth of hexagonal BN nanotubes of both cylindrical and bamboo-like morphology. Unlike previous mechanisms for nanotube formation, the reordering and solid-state growth process of our nanotubes does not involve deposition from the vapor phase nor chemical reactions.

Quantitative absorbance spectroscopy with unpolarized light: Part II. Experimental evaluation and development of a protocol for quantitative analysis of mineral IR spectra

Abstract The predictions of the theory of light propagation in weakly absorbing anisotropic minerals are tested against systematic measurements of the infrared absorbance spectra of calcite, olivine, and topaz oriented in both principal and random sections, using both polarized and unpolarized light. We show that if the linear polarized maximum absorbance is smaller than ~0.3, or if the ratio of maximum and minimum absorbance is close to unity, then (1) the polarized maximum and minimum absorbances as well as the unpolarized absorbance are, to a good approximation, linearly proportional to thickness, regardless of the direction of the incident light; (2) the angular variation of polarized light absorption is indistinguishable from the theoretical predictions within the uncertainty of the measurements; (3) for any section the unpolarized absorbance is the mean of the polarized maximum and minimum absorbance; and (4) the average unpolarized absorbance of randomly oriented grains is one third of the Total Absorbance (defined as the sum of the three principal absorbances). Therefore, calibrations relating Total Absorbance to absorber concentration in minerals that have been developed from measurements with polarized light parallel to the principal axes may be applied to measurements with unpolarized light on a population of randomly oriented sections. We show that 10 such measurements are sufficient to achieve a petrologically useful accuracy. The method enables water concentrations in nominally anhydrous minerals to be determined from samples where the preparation of oriented specimens is not feasible, such as high-pressure experimental runs and fine-grained mantle xenoliths. The method may also be used for obtaining quantitative measurements on low-symmetry minerals.

Nanoporous carbon produced by ball milling

A nanoporous structure was produced in the samples of graphite after ball milling at ambient temperature. The specific internal surface area of the micropores, as determined using the t-plot method, is higher than the external surface area of particles and mesopores. Phase transformations from hexagonal to turbostratic, and to amorphous structures were investigated using x-ray diffraction analysis and transmission electron microscopy. Formation of the nanoporous structure is associated with that of the disordered carbon. The disordered and nanoporous structure is probably fullerene-like in nature.

Diffusion of 40Ar in metamorphic hornblende

Isothermal, hydrothermal experiments were performed on two compositionally contrasting hornblendes from amphibolites in order to examine Ar diffusion behavior in metamorphic hornblendes. Ten experiments on sample RF were performed at temperatures of 750°C, 800°C, and 850°C and pressures of 1 kbar using measured grain radii of 158, 101, and 34 μm. Eight experiments on sample 118576 were performed under the same conditions using measured grain radii of 145, 77, and 25 μm. Minor (<5%) alteration was observed in high temperature runs. Diffusion coefficients were calculated from measured radiogenic 40Ar loss following treatment assuming a spherical geometry for the mineral aggregate. Diffusivities calculated for different grain sizes vary by up to an order of magnitude for a given temperature indicating that the effective diffusion radius was less than the measured grain radius. Diffusivities for RF and 118576 calculated for grain radii of 101 and 145 μm, respectively, form a linear array on an Arrhenius diagram with slopes indicating activation energies of ∼ 60 kcal/mol. No correlation between Mg number (100 Mg/(Mg+Fe)) and activation energy was observed. Diffusivities calculated for these experiments are higher than previously reported results from similar experiments performed on hornblendes. A comparison of results for 34 μm splits from these two studies indicates higher apparent diffusivities (by a factor of 5), which probably result from observed phyllosilicate inter-growths (chlorite) and/or exsolution lamellae that partition the metamorphic hornblendes into smaller subdomains. Diffusivities calculated for experiments performed on 65 μm and 34 μm splits of 40Ar/39Ar standard MMhb-1 at 800°C and 1 kbar are consistent with a previously reported activation energy of 65 kcal/mol. Arrhenius parameters which emerge from the empirical model of Fortier and Giletti (1989) agree with experimental results to within analytical uncertainty. Although results of these experiments support previously reported estimates of the activation energy of 40Ar in hornblende (∼60 kcal/mol), phyllosilicate intergrowths and/or microstructures such as exsolution lamellae within the two metamorphic hornblendes result in extremely small diffusion domains, which may lead to lower Ar retentivities and lower closure temperatures. The effective diffusion dimension for 40Ar in hornblende is not likely to be defined by dislocations but rather by some larger structure within the crystal. TEM and SEM studies may provide some insight into the effective diffusion dimension for 40Ar in amphiboles, thereby enabling better estimates of closure temperatures and more precise temperature-time reconstructions.

Saddle-shaped 40Ar39Ar age spectra from young, microstructurally complex potassium feldspars

Abstract A suite of young potassium feldspars show markedly saddle-shaped 40 Ar 39 Ar age spectra as a result of incorporating 10−10 to 10−9 mol/g of excess 40Ar. The minima of these age spectra record reasonable cooling ages, based on the known thermal history and geology of the samples. Acid etching of one sample indicates that excess 40Ar is concentrated near grain margins. The release of a substantial portion of this excess Ar at high temperatures in the laboratory requires that this component be situated in a more retentive site than radiogenic 40Ar. Anion vacancies have been proposed to act in this role in plagioclase, and we speculate that this is so in K-feldspar as well. Such a mechanism would explain the observation that relative to radiogenic 40Ar, excess 40Ar is incorporated at low temperatures in nature but is released at high temperatures in the laboratory. Oxygen diffusion provides an appropriate analogy for this phenomenon, being relatively fast under natural, hydrothermal conditions, but extremely slow in anhydrous environments such as an Ar-extraction system. TEM observations made on two of the samples confirm that their effective grain sizes for diffusion are likely to be on the order of ten microns, due to the presence of such microstructures as incoherent exsolution lamellae, dislocations, and stepped twins. TEM observations also reveal the presence in one sample of orthoclase enclaves in a microcline host.

Grain boundary melt films in an experimentally deformed olivine-orthopyroxene rock: Implications for melt distribution in upper mantle rocks

An unexpected microstructure, with important implications for melt distribution in the upper mantle, has been found in a 70% olivine +30% orthopyroxene rock experimentally deformed at 1500K, 300 MPa, where incipient partial melting occurred. As found in previous studies of comparable systems, most melt in the olivine-orthopyroxene sample resided in a network of grain-edge tubes and occasional thick (50–500 nm) layers. We infer, using electron microscopy at its highest resolution, that melt also existed in another form as glass films 1.0–1.5 nm thick, along all grain boundaries, with total film fraction (F) of 0.0002. All of these melts are unusually SiO2-rich. Further work is needed to confirm that the thin films are not transient, although their coexistence with smoothly curved solid-melt interfaces and flat crystal faces suggests they are stable. If thin high-silica melt films are stable they might influence physical properties and melt extraction processes in regions of incipient melting and metasomatism in the upper mantle.

Quantitative absorbance spectroscopy with unpolarized light: Part I. Physical and mathematical development

Abstract A new approach to the use of spectroscopic absorbance measurements for anisotropic crystals allows results to be extracted using unpolarized light incident on random crystal orientations. The theory of light propagation in anisotropic absorbing crystals is developed from Maxwell’s equations to devise an expression for the transmittance of linearly polarized light traveling in an arbitrary direction in weakly absorbing media. This theory predicts the distribution of transmittance and absorbance as a function of direction and polarization angle of incident light. It is shown how a previously deduced empirical expression, commonly used in infrared spectroscopy, is a good approximation to the full theory under a wide range of conditions. The new theory shows that principal polarized absorbances correspond to the eigenvalues of an absorbance ellipsoid. An expression is derived for the unpolarized absorbance as a function of the angles describing incident light direction, Aunpol(ϕ,ψ), and the principal polarized absorbances, Aa, Ab, Ac in an anisotropic crystal Aunpol(ϕ,ψ) = ½[Aa(cos2ϕcos2ψ + sin2ψ) + Ab(cos2ϕsin2ψ + cos2ψ) + Acsin2ϕ]. Integration of this expression over all incident angles leads to a simple relationship between total measured unpolarized absorbance and the three principal polarized absorbances. Using this theory, a procedure is proposed for estimating both total (Aa + Ab + Ac) and principal absorbances from spectroscopic measurements of absorbance using unpolarized light on a set of randomly oriented crystals.

Microstructures in water‐weakened single crystals of quartz

Models of hydrolytic weakening based on the influence of chemical environment on the concentrations of point defects stimulated a series of experiments by Ord and Hobbs [1986] in which single crystals of natural quartz were deformed in a solid medium apparatus under a range of buffered oxygen, water, and hydrogen fugacities. Microstructures in these specimens have been examined using visible light and transmission electron microscopy in an attempt to identify processes responsible for the observed relationships between chemical environment and strength. Preheating treatments, conducted for ∼20 hours at the pressure, temperature, and chemical conditions of each ensuing deformation experiment, modified each specimen microstructurally such that the quartz was not in the form of a low-dislocation-density single crystal when deformation testing began. Specimens fractured axially during pretreatment, particularly those in chemical environments for which water fugacity remained high. Fractures later healed giving rise to arrays of fluid inclusions in regions with moderate densities of dislocations. Some deformation and recrystallization, concentrated at healed fractures and specimen end zones, also accompanied pretreatment. “Mn-buffered” specimens, for which f O2 and f H2O.were high, were densely fractured; in less strained regions, subsequent deformation was restricted to the vicinity of healed fractures, while regions of higher plastic strain and recrystallization show evidence from inclusions for abundant early fractures, now obliterated. “Ta-buffered” specimens, of low f O2 and f H2O, are also deformed close to axial fractures, but here fractures are comparatively rare. The marked variation in the degree of fracturing seems to be an example of stress-corrosion cracking in response to chemical environment. Dislocations are developed in material away from healed fractures in one deformed Ta-buffered specimen. If point defects introduced by diffusion are responsible for water weakening of quartz crystals, the complex microstructures seen in the deformed specimens of Ord and Hobbs [1986] require that point defect distributions were neither uniform nor controlled by bulk diffusion from the sample edges. The clear association between plastic strain and healed fracture does point to important local effects either of “water” upon dislocation activity or of healed-fracture structure having been involved in the nucleation of dislocations. Regardless of the details, we infer from the dependence of the degree of fracturing upon chemical environment, together with the high correlation between locations of plastic strain and regions of earlier fracture, that the relationship between chemical environment and macroscopic strength of natural quartz crystal is not simply a matter of water weakening as a direct result of high point defect concentrations. Instead, fracture, inferred to be important in natural fluid-rock interactions, also seems to be an experimentally critical process which might nucleate the dislocations necessary to initiate plastic deformation in the laboratory.