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Browsing by Subject "Exsolution"

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    Magnetic and physical characteristics of magnetite associated with deformation and exsolution.
    (2011-10) Till, Jessica Lynn
    This thesis contains a collection of laboratory-based studies designed to characterize the magnetic properties and physical aspects of magnetite that result from deformation or high temperature growth. In Chapter 2, a detailed rock magnetic characterization of rocks containing nanoscale magnetite exsolved from volcanic glass identifies the location of domain-state thresholds through distinct transitions in remanence and susceptibility properties. This unique material is an excellent candidate for standard material to be used in studies of magnetite granulometry. In Chapter 3, theoretical timescales for the growth of sub-microscopic magnetite needles during exsolution from plagioclase are calculated using results of diffusion experiments. Measured diffusivities are modeled to calculate the amount of diffusion-limited growth possible under different conditions of nucleation temperature and cooling rate. In Chapters 4 and 5, the development and evolution of magnetic fabrics are investigated through deformation experiments on synthetic rock-analogues at high temperatures and ductile conditions. Stress-induced changes in rock magnetic properties after deformation are significant. Examination of deformation-induced remagnetization demonstrates that a primary remanence can survive conditions equivalent to moderate metamorphism in certain cases and that petrofabric can play an important role in determining the remanence stability. High-temperature deformation experiments result in a pattern of anisotropy development that indicates plastic deformation of magnetic grains, which is distinct from anisotropy development resulting from different magnetite strain responses. Experimental data are combined with theoretical magnetic anisotropy models and used to estimate effective magnetite strains and strain partitioning from magnetic fabric data in deformed samples. Finally, observations of strong shape-preferred orientation and deformation-induced microstructures in magnetite grains from high-temperature shear experiments indicate plastic deformation of magnetite. Microstructural observations place constraints on the rheological behavior of magnetite and the conditions in which dislocation creep is dominant. These observations prompt a re-examination of the previously established magnetite flow laws which are modified and used to construct new deformation mechanism maps.
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    Warming and stratification changes in Lake Kivu, East Africa
    (2013-08) Aaberg, Arthur Allen
    To investigate changes in the temperature and stratification structure in Lake Kivu, we have installed a string of temperature recorders and performed CTD casts. The obtained data have been compared to historical profiles and the heat budget for the lake was analyzed. Lake Kivu is a meromictic lake characterized by an anomalous temperature distribution with a temperature minimum close to the base of the seasonally mixed layer. Warming rate at the depth of the temperature inversion is consistent with the historical warming rate of the surface layer of ∼0.14 ±0.02 °C per decade. Atmospheric warming rates since the 1970's in East Africa are between 0.20 and 0.25 °C per decade. Reported warming in surface waters of other East-African rift lakes is ∼0.13 °C per decade. Deep waters (greater than 350 m) in Lake Kivu exhibit variability in temperature and are currently warming at a rate of &sim0.06±0.02 °C per decade based on the increase in heat content since the 1970's and the increase in temperature seen in the deepest measurements between our 2011 and 2012 profiles. The monimolimnion of Lake Kivu cannot be considered to be in a steady state. The depth of wind-induced surface mixing during the dry season varies significantly between years. Mixing to 80 m (the present depth of the temperature inversion) requires continuous winds blowing from the south at 9–10 m s-1, whereas typical wind speed maxima are around 5–6 m s-1 and capable of mixing to around 65 m depth. Occasional stronger winds cause episodic mixing closer to the inversion which removes heat, but this does not happen on a regular basis. As the temperature inversion in recent historical profiles has been as shallow as 65 m, mixing to the temperature inversion depth is possible during years with stronger than average winds. With heat diffusing towards the temperature inversion from both above and below, the temperature at the inversion depth will continue to rise, resulting in a reduced transport of heat out of the deep waters that may increase the rate at which the water column is warming.