Browsing by Subject "Crystallization"

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Basaltic volcanism : deep mantle recycling, Plinian eruptions, and cooling-induced crystallization
(2010-08) Szramek, Lindsay Ann; Lassiter, John C.; Gardner, James Edward, 1963-; Carlson, William; Houghton, Bruce; Rowe, Michael C.
Mafic magma is the most common magma erupted at the surface of the earth. It is generated from partial melting of the mantle, which has been subdivided into end-members based on unique geochemical signatures. One reason these end members, or heterogeneities, exist is subduction of lithospheric plates back into the mantle. The amount of elements, such as Cl and K, removed during subduction and recycled into the deep mantle, is poorly constrained. Additionally, the amount of volatiles, such as Cl, that are recycled into the deep mantle will strongly affect the behavior of the system. I have looked at Cl and K in HIMU source melts to see how it varies. Cl/Nb and K/Nb suggest that elevated Cl/K ratios are the result of depletion of K rather than increased Cl recycled into the deep mantle. After the mantle has partially melted and mafic melt has migrated to the surface, it usually erupts effusively or with low explosivity because of its low viscosity, but it is possible for larger eruptions to occur. These larger, Plinian eruptions, are not well understood in mafic systems. It is generally thought that basalt has a viscosity that is too low to allow for such an eruption to occur. Plinian eruptions require fragmentation to occur, which means the melt must undergo brittle failure. This may occur if the melt ascends rapidly enough to allow pressure to build in bubbles without the bubbles expanding. To test this, I have done decompression experiments to try to bracket the ascent rate for two Plinian eruptions. One eruption has a fast ascent, faster than those seen in more silicic melts, whereas the other eruption is unable to be reproduced in the lab, however it began with a increased viscosity in the partly crystallized magma. After fragmentation and eruption, it is generally thought that tephra do not continue to crystallize. We have found that crystallinity increases from rim to core in two basaltic pumice. Textural data along with a cooling model has allowed us to estimate growth rates in a natural system, which are similar to experimental data.
Coupled compositional and three-dimensional textural analysis of garnet porphyroblasts
(1995) Chernoff, Carlotta Bridget; Carlson, William, 1952-
Integration of garnet compositional data with quantitative textural data for garnet positions acquired from high-resolution computed X-ray tomography (X-ray CT) allows the growth zoning in a crystal to be assessed with a full knowledge of the locations and sizes of all other garnets in the specimen. This study of a specimen of garnetiferous quartzite from the Picuris Range of north-central New Mexico is the first to evaluate garnet compositional profiles for hundreds of crystals with a clear knowledge of the degree of isolation of each garnet. Using the X-ray CT imagery as a guide, a 47.9 cm³ volume of rock containing 1248 garnet crystals was serially sectioned by grinding to expose the central section of each garnet. Electron microprobe analyses through the centers of 470 garnets include profiles of Mn, Mg, Fe, Ca, Al, and Si at 10-20 μm spacing across 372 of the crystals and analyses of the cores of the remaining 98. Part I of this thesis presents results from several textural and compositional analysis techniques applied to the data set in order to assess the mechanisms that governed crystallization of garnet porphyroblasts. Textural data enable determination of statistical measures of the extent of ordering, clustering, and impingement of adjacent porphyroblasts and characterization of the crystal size distribution and degree of isolation of each porphyroblast. These data may be compared with predictions of quantitative textural features for porphyroblastic rocks resulting from different crystallization mechanisms. Likewise, normalized radius-rate relations computed from compositional zoning patterns in the garnets are compared with predicted relations for several growth-rate laws. Interpretation of these results indicates the dominance of diffusionally-influenced nucleation and diffusion-controlled growth mechanisms. In Part II, the effects of competition among adjacent porphyroblasts for garnet-forming nutrients during a diffusion-controlled nucleation-and-growth process are investigated. Measures of crystal isolation when compared with crystal sizes fail to provide a measure of diffusional competition, although detailed analysis of sizes of closely adjacent crystals reveals clear evidence for the effects of competition during growth. These data lead to the conclusion that the evidence for competitive growth is obscured by the effects of inhomogeneities in the precursor. Part III presents evidence for Ca disequilibrium during garnet growth. Compositional data for Mn, Mg and Fe indicate that their compositions evolved systematically during crystallization and achieved equilibrium in the intergranular medium at hand-sample scale, whereas Ca did not. Instead, Ca concentrations appear to reflect the local extent of reaction around each individual garnet crystal, yielding equilibrium for Ca at scales no larger than that of an individual porphyroblast. These observations are consistent with the speculation that the relative rate of diffusion of Mn, Mg and Fe through the intergranular medium is more rapid than that of Ca, permitting rock-wide equilibration for these species. Slower relative diffusion of Ca prevents equilibration at scales much larger than that of a single garnet crystal. These unexpected results challenge the current thinking about garnet crystallization and call into question the accuracy of thermobarometers based on cation-exchange and phase equilibria involving garnet
Crystallization of amorphous solid films
(2003-05) Safarik, Douglas Joseph; Mullins, C. B.
Below ~130 K, H2O can exist for prolonged periods in a thermodynamically unstable, non-crystalline solid form known as amorphous solid water (ASW). When warmed to above 135 K, ASW crystallizes to the thermodynamically favored state, cubic ice I, on a laboratory time scale. Despite the relevance of ASW crystallization to a variety of scientific problems ranging from astrophysical phenomena to cryopreservation, the kinetics of this transformation are largely uncharacterized, and its mechanism is not fully understood. In the present work, the crystallization kinetics of vapor-deposited, nonporous ASW films less than one micron thick are investigated experimentally near 140 K. The amorphous to crystalline transition is characterized using a probe molecule, chlorodifluoromethane (CHF2Cl), whose adsorbed states and hence desorption kinetics are sensitive to the crystallinity of solid water surfaces. The transformation kinetics of very thick ASW films are found to be both independent of specimen size and consistent with simultaneous homogeneous nucleation and isotropic growth of crystalline ice grains. As the ASW film thickness is reduced from 385 nm to 55 nm, however, the rate of surface crystallization decelerates, in apparent conflict with a homogeneous nucleation and growth mechanism. In an attempt to explain this behavior, a geometrical model of phase transition kinetics at the surface of solids, with special consideration of finite specimen size in one dimension, is constructed. For materials in which nucleation occurs spatially randomly, phase change is predicted to decelerate when film thickness is reduced below the mean crystal grain size. This phenomenon originates from a reduction in the number of crystallites available to transform the surface as the sample becomes thinner. Good quantitative agreement between this simple model and the experimental data is attained using a minimum of kinetic parameters, suggesting it captures the essential physics of ASW crystallization. These model fits also yield preliminary estimates of crystalline ice growth and nucleation rates. Finally, an experimental protocol and corresponding model of phase change that together permit accurate quantification of nucleation and growth kinetics (when both processes occur simultaneously) is developed. Using the outlined methodology, crystalline ice growth and nucleation rates near 140 K are found to be ~1 Å/s and ~1010 cm-3s -1, respectively, and to exhibit Arrhenius temperature dependencies with activation energies of ~50 and ~170 kJ/mol.
Effects of various additives on crystal habit and other properties of petroleum wax solutions
(1964) Birdwell, Bertie Franklin, 1931-; Not available
Gas transport properties of poly(n-alkyl acrylate) blends and modeling of modified atmosphere storage using selective and non-selective membranes
(2007-12) Kirkland, Bertha Shontae, 1976-; Paul, Donald R.
The gas transport properties of side-chain crystalline poly(n-alkyl acrylate) and poly(m-alkyl acrylate) blends are determined as a function of temperature for varying side-chain lengths, n and m, and blend compositions. The side chains of poly(n-alkyl acrylate)s crystallize independently of the main chain for n [is greater than or equal to] 10 which leads to an extraordinary increase in the permeability at the melting temperature of the crystallites. The compatibility of these polymers are examined and macroscopic homogeneity is observed for a small range of n and m when the difference /n - m/ is between 2 - 4 methylene units. Thermal analysis shows that the blend components crystallize independently of one another; at the same time, the crystallization of each component is hindered by the presence the other component. The permeation responses of these blends show two distinct permeation jumps as the crystallites from each component melt at their respective melting temperatures. Blends with continuous permeation responses are found to have higher effective activation energies than observed for common polymers. Thermal analysis proved to be a useful tool to help predict the permeation response for poly(alkyl acrylates); thus the thermal behavior of poly(n-alkyl acrylate) blended with n-aliphatic materials and random copolymers of poly(n-alkyl acrylates) are briefly examined. A bulk modified atmospheric storage design is proposed where produce is stored in a rigid chamber that is equipped with both selective and non-selective membrane modules that help regulate the oxygen entering and the carbon dioxide leaving the produce compartment. The design enables control of the atmosphere inside the chamber by modulating gas flow, i.e. the gas flow rate and composition, through the non-selective membrane by delivering fresh air upstream of the non-selective membrane. The model shows that the choice of materials for the selective and non-selective membranes dictate the range of concentrations achievable; however, the air flow rate allows the control between these ranges. The method to design a practical chamber from this model is also described.
Hydroaromatic bases in the kerosene distillate of California petroleum
(1932) Armendt, Bradshaw Frederick; Bailey, James Robinson