Pamela C. Burnley

Thermodynamic properties and hydrogen speciation from vibrational spectra of dense hydrous magnesium silicates

Infrared absorption measurements were taken from 100 to 5000 cm−1 of a natural chondrodite and three dense hydrous magnesium silicates: phase A, phase B, and superhydrous phase B (shy-B). Raman spectra were also acquired from phase B and the chondrodite. Roughly half of the lattice modes are represented and our data are the first report of the low frequency modes. Comparison of our new spectra to symmetry analyses suggests that multiple sites for hydrogen exist for all the phases. The shy-B we examined crystallizes in P21nm with two OH sites. Models for the density of states are constructed based on band assignments for the lattice modes and for the OH stretching vibrations. Heat capacity CP and entropy S calculated using Kieffers formulation should be accurate within 3% from 200 to 800 K. Model values for CP at 298 K are 299.6 J/mol-K for chondrodite, 421.5 J/mol-K for phase A, 529.4 J/mol-K for shy-B, and 618.9 J/mol-K for phase B. Model values for S2980 are 234.2 J/mol-K for chondrodite, 303.5 J/ mol-K for phase A, 377.9 J/mol-K for shy-B, and 473.3 J/mol-K for phase B. Debye temperatures are near 1000 K.

Synthesis of high-pressure hydrous magnesium silicates : Observations and Analysis

Abstract Specimens of dense high-pressure hydrous magnesium silicates, notably phase A, phase B, and superhydrous phase B, have been synthesized for use in calorimetric and spectroscopic experiments. X-ray diffraction and electron microprobe analysis were used to study both major and minor phases occurring in the experimental products. We observed the formation of phase A at 16 GPa, 1172 °C; phase B at 15 GPa, 1158 °C; chondrodite at 12 GPa, 1158 °C; and clinohumite at 12 GPa, 1170 °C. These pressures and temperatures are higher than those reported in previous studies. We also observed the formation of phases E and superhydrous B at conditions at which they have been previously observed. However, we propose that there is a high-low transition in superhydrous B. We have not identified phase C in the pressure-temperature-bulk compositional space where it has been observed in previous studies. We conclude that this phase may be identical with superhydrous B, as previously suggested, or that in some studies a humite may have been misidentified as phase C.

The mobility of Nb in rutile-saturated NaCl- and NaF-bearing aqueous fluids from 1–6.5 GPa and 300–800 °C

Abstract Rutile (TiO2) is an important host phase for high field strength elements (HFSE) such as Nb in metamorphic and subduction zone environments. The observed depletion of Nb in arc rocks is often explained by the hypothesis that rutile sequesters HFSE in the subducted slab and overlying sediment, and is chemically inert with respect to aqueous fluids evolved during prograde metamorphism in the forearc to subarc environment. However, field observations of exhumed terranes, and experimental studies, indicate that HFSE may be soluble in complex aqueous fluids at high pressure (i.e., >0.5 GPa) and moderate to high temperature (i.e., >300 °C). In this study, we investigated experimentally the mobility of Nb in NaCl- and NaF-bearing aqueous fluids in equilibrium with Nb-bearing rutile at pressure-temperature conditions applicable to fluid evolution in arc environments. Niobium concentrations in aqueous fluid at rutile saturation were measured directly by using a hydrothermal diamondanvil cell (HDAC) and synchrotron X‑ray fluorescence (SXRF) at 2.1 to 6.5 GPa and 300-500 °C, and indirectly by performing mass loss experiments in a piston-cylinder (PC) apparatus at ~1 GPa and 700-800 °C. The concentration of Nb in a 10 wt% NaCl aqueous fluid increases from 6 to 11 μg/g as temperature increases from 300 to 500 °C, over a pressure range from 2.1 to 2.8 GPa, consistent with a positive temperature dependence. The concentration of Nb in a 20 wt% NaCl aqueous fluid varies from 55 to 150 μg/g at 300 to 500 °C, over a pressure range from 1.8 to 6.4 GPa; however, there is no discernible temperature or pressure dependence. The Nb concentration in a 4 wt% NaF-bearing aqueous fluid increases from 180 to 910 μg/g as temperature increases from 300 to 500 °C over the pressure range 2.1 to 6.5 GPa. The data for the F-bearing fluid indicate that the Nb content of the fluid exhibits a dependence on temperature between 300 and 500 °C at ≥2 GPa, but there is no observed dependence on pressure. Together, the data demonstrate that the hydrothermal mobility of Nb is strongly controlled by the composition of the fluid, consistent with published data for Ti. At all experimental conditions, however, the concentration of Nb in the fluid is always lower than coexisting rutile, consistent with a role for rutile in moderating the Nb budget of arc rocks.

Lessons on the Role of Fun/Playfulness from a Geology Undergraduate Summer Research Program

This paper examines past and current experiences with fun and playfulness of participants in two summers of an NSF funded summer research experiences for undergraduates (REU) geosciences program. Thirty students responded to questionnaires on the role of play in their previous learning and their playful, inspirational, or “ah-ha” feelings while doing their summer research. They reported a sense of playfulness during science classes, promoted by engagement with interesting phenomena, ability to work independently, and a relaxed atmosphere. Their descriptions of playfulness in the program were similar to those of scientists describing playfulness while doing research. They described the fun of the work itself, the opportunities for playful social interactions with peers, and excitement at finding results. Implications for science education involve the inclusion of playfulness and fun in the modeling of scientific inquiry and the structuring of science classes and labs to allow more students? input into their own learning, the provision of field experiences, and the allowance of some socialization.

The failure mechanism for deep-focus earthquakes

Abstract Experimental deformation of Mg2GeO4 olivine at pressures between 1 and 2 GPa in the spinel stability field has led to discovery of a faulting instability that develops at the kinetically-controlled threshold of transformation. Very fine-grained olivine and spinel are found in fault zones. Deformation at lower temperatures is ductile; transformation is inhibited and specimens are very strong. Deformation at higher temperatures also is ductile but transformation is rapid and specimens are much weaker. Detailed examination of the microstructures of specimens deformed in the faulting regime lead to an anticrack theory of faulting that explains the experimental data and provides a fundamentally new mechanism for deep-focus earthquakes. The new mechanism is analogous to the Griffith theory of fracture; nucleation and growth of spinel under stress produces spinel-filled microanticracks normal to the maximum compressive stress that link up to produce faulting. The friction paradox for deep earthquakes is resolved because this faulting process provides a fine-grained, superplastic, ‘lubricant’ for faults. The temperature distribution within subducting slabs of lithosphere requires that the conditions of instability are reached as a natural consequence of subduction; metastable olivine in the interior of deep slabs warms to a critical temperature where faulting ensues in the presence of a shear stress.

Elastic geothermobarometry: Corrections for the geometry of the host-inclusion system

Elastic geothermobarometry on inclusions is a method to determine pressure-temperature conditions of mineral growth independent of chemical equilibrium. Because of the difference in their elastic properties, an inclusion completely entrapped inside a host mineral will develop a residual stress upon exhumation, from which one can back-calculate the entrapment pressure. Current elastic geobarometric models assume that both host and inclusion are elastically isotropic and have an ideal geometry (the inclusion is spherical and isolated at the center of an infinite host). These conditions do not commonly occur in natural rocks, and the consequences for inclusion pressures can only be quantified with numerical approaches. In this paper, we report the results of numerical simulations of inclusions with the finite element method on elastically isotropic systems. We define and determine a geometrical factor (Γ) that allows measured residual pressures to be corrected for the effects of non-ideal geometry. We provide simple guidelines as to which geometries can safely be used for elastic geobarometry without correcting for the geometry. We also show that the discrepancies between elastic and conventional geobarometry reported in literature are not due to geometrical effects, and therefore result from other factors not yet included in current models.

ECCI, EBSD and EPSC characterization of rhombohedral twinning in polycrystalline α-alumina deformed in a D-DIA apparatus

Rhombohedral twinning in alumina (aluminium oxide, α-Al2O3) is an important mechanism for plastic deformation under high-temperature–pressure conditions. Rhombohedral twins in a polycrystalline alumina sample deformed in a D-DIA apparatus at 965 K and 4.48 GPa have been characterized. Three classes of grains were imaged, containing single, double and mosaic twins, using electron channeling contrast imaging (ECCI) in a field emission scanning electron microscope. These twinned grains were analyzed using electron backscatter diffraction (EBSD). The methodology for twin identification presented here is based on comparison of theoretical pole figures for a rhombohedral twin with experimental pole figures obtained with EBSD crystal orientation mapping. An 85°〈02{\overline 2}1〉 angle–axis pair of misorientation was identified for rhombohedral twin boundaries in alumina, which can be readily used in EBSD post-processing software to identify the twin boundaries in EBSD maps and distinguish the rhombohedral twins from basal twins. Elastic plastic self-consistent (EPSC) modeling was then used to model the synchrotron X-ray diffraction data from the D-DIA experiments utilizing the rhombohedral twinning law. From these EPSC models, a critical resolved shear stress of 0.25 GPa was obtained for rhombohedral twinning under the above experimental conditions, which is internally consistent with the value estimated from the applied load and Schmid factors determined by EBSD analysis.

Deformation Analysis of Forsterite Olivine Using Electron Channeling Contrast Imaging and Electron Backscatter Diffraction

The dynamics of the Earth system is expressed in a number of ways such as earthquakes, mountain building and plate tectonics. The rheology of Earth materials at the conditions of Earth’s interior dictates the nature of these phenomena and therefore, is an important field of research in Earth sciences. The flow strength in rocks is a function of minerology, crystal defects substructures, grain size, shape and crystallographic orientations. Furthermore, flow strength in rocks strongly depends on temperature and pressure levels. Experimental deformation studies focus on understanding these various dependencies in rocks during deformation processes in Earth’s interior. In particular, the in-situ synchrotron x-ray diffraction technique integrates the multi-anvil high-pressure system with the synchrotron in order to quantify plastic properties of Earth’s materials at pressures relevant to the Earth’s upper and lower mantle [1].

Modeling background radiation using geochemical data: A case study in and around Cameron, Arizona

This study compares high resolution forward models of natural gamma-ray background with that measured by high resolution aerial gamma-ray surveys. The ability to predict variations in natural background radiation levels should prove useful for those engaged in measuring anthropogenic contributions to background radiation for the purpose of emergency response and homeland security operations. The forward models are based on geologic maps and remote sensing multi-spectral imagery combined with two different sources of data: 1) bedrock geochemical data (uranium, potassium and thorium concentrations) collected from national databases, the scientific literature and private companies, and 2) the low spatial resolution NURE (National Uranium Resource Evaluation) aerial gamma-ray survey. The study area near Cameron, Arizona, is located in an arid region with minimal vegetation and, due to the presence of abandoned uranium mines, was the subject of a previous high resolution gamma-ray survey. We found that, in general, geologic map units form a good basis for predicting the geographic distribution of the gamma-ray background. Predictions of background gamma-radiation levels based on bedrock geochemical analyses were not as successful as those based on the NURE aerial survey data sorted by geologic unit. The less successful result of the bedrock geochemical model is most likely due to a number of factors including the need to take into account the evolution of soil geochemistry during chemical weathering and the influence of aeolian addition. Refinements to the forward models were made using ASTER visualizations to create subunits of similar exposure rate within the Chinle Formation, which contains multiple lithologies and by grouping alluvial units by drainage basin rather than age.

Applications of Electron Channeling Contrast Imaging (ECCI) in Failure Analysis of In-Situ Synchrotron X-Ray Diffraction Deformation Experiments

Significant advances in high pressure deformation of Earth materials have been made in the last decade using in-situ synchrotron X-ray diffraction deformation experiments. The high pressure D-DIA apparatus [1], used for many of these experiments allows monitoring the internal stress within the sample via synchrotron X-ray diffraction. Polycrystalline α-alumina is commonly used as material for deformation pistons in sample assemblies for the D-DIA apparatus. Despite the higher strength of alumina relative to geological materials under study, the alumina pistons occasionally shorten during compression, resulting in an underestimate of the strength of the samples. In order to investigate the failure mechanism of alumina pistons microstructural evaluation is necessary. The primary goal of this study is to showcase the value of using electron channeling contrast imaging (ECCI) in a field emission scanning electron microscope (FE-SEM) for analysis of the deformation mechanisms operating in plastically deformed polycrystalline α-alumina. Improving our understanding of the plastic deformation of the alumina piston material will lead to improved reproducibility in D-DIA data sets and will also contribute to the general understanding of deformation mechanisms in alumina ceramics.