Browsing by Subject "Olivine"

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    The Development Of Olivine Textures In Complex Deformation Geometries
    (2023) Wagner, Nicole
    The analysis of crystallographic preferred orientations (CPOs) in olivine is a crucial tool in our understanding of the Earth’s upper mantle. The development of CPOs is controlled by the activation of slip systems, which are sensitive to various thermochemomechanical conditions such as stress, temperature, and water content. Laboratory experiments have offered insight on the conditions in which different olivine CPOs develop. However, the relationship between these thermochemomechanical conditions and CPOs is complicated. Recent studies have challenged some of our current understanding on olivine CPO development and have brought attention to the importance of kinematics. Here, we aim to experimentally investigate the role of kinematics in the development of olivine CPOs, particularly pertaining to complex deformation geometries involving simultaneous pure and simple shear. To explore this topic, cylindrical samples of dry, polycrystalline San Carlos olivine were deformed in either simultaneous extension and torsion or simultaneous shortening and torsion at a temperature of 1523 K and confining pressure of 300 MPa in a Paterson gas-medium apparatus at the University of Minnesota. Following deformation, the CPO was measured at different sections along the sample radius using electron backscatter diffraction analysis. For each section, the kinematic vorticity number, the equivalent strain, the J-index, and the M-index were determined. The fabric index angle was also calculated to classify the resulting CPO for each section. In total, two samples were deformed in extension and torsion and three samples in shortening and torsion. Of the two extension and torsion experiments, one sample localized thus allowing for multiple sections along the length to be analyzed. Moreover, the samples subjected to extension and torsion went out to axial strains of 0.15, 0.26, and 0.36 and to shear strains between 1.5 and 2.2. These samples generally produced stronger textures and displayed an evolution from C-type to E-type to D-type CPO with increasing kinematic vorticity. The presence of an E-type CPO aligns with numerical simulations of CPO development in simultaneous extension and simple shear geometries and suggests that olivine does not necessarily develop as a function of water content. Conversely, the samples deformed in shortening and torsion went to axial strains of -0.12, -0.28, and -0.35 and shear strains between 1.0 and 1.7. These samples had weaker textures and typically evolved from an AG-type to an A-type CPO with increasing kinematic vorticity, which also reasonably aligns with numerical simulations of this given geometry. Although different textural evolutions were observed between these two deformation geometries, the fabric index angles began to converge as kinematic vorticity increased for both sets of experiments, ultimately resulting in textures expected in simple shear. The geometries achieved in this study were then simulated using a modified director textural model. The simulations produced from this model reasonably reproduced the CPO characteristics observed in the experimental samples, but the CPO classification, as determined by the fabric index angle, did not align as well. All in all, the findings presented in this study highlight the role of kinematics in CPO development, emphasize the caution needed when interpreting the thermochemomechanical conditions of the mantle from seismic anisotropy and exhumed peridotites, and contribute valuable information on how we can use olivine CPOs as a field tool.
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    Emplacement and Crystallization Histories of Cu-Ni-PGE Sulfide-mineralized Peridotites in the Eagle and Eagle East Intrusions
    (2018-06) Mulcahy, Connor
    The Eagle and Eagle East intrusions, located about 40 kilometers northwest of Marquette, MI, are two small, partially exposed, sub-vertical, funnel-shaped mafic/ultramafic intrusions emplaced in Paleoproterozoic black slates. Both intrusions host economic Ni-Cu-(PGE) sulfide deposits, the Eagle intrusion in its main body and the Eagle East intrusion in its feeder at depth. The Eagle deposit has been being mined by the Lundin Mining Corporation since 2014, which is now also expanding its operation to mine the Eagle East deposit. Transmitted light petrography, scanning electron microscopy, and electron microprobe analyses were performed on samples from six drill cores in the Eagle system, three from Eagle and three from Eagle East. Lundin additionally provided whole-rock geochemistry for five of these cores at ~1.5m intervals. The concentration of Ni in olivines in the Eagle and Eagle East intrusions were measured by electron microprobe. A bimodal distribution of Ni concentration in olivine - i.e., both enriched and unenriched populations being present - may have been evidence for multiple magma pulses in the Eagle system. However, olivine in both intrusions were determined to be universally Ni-enriched, which means that this line of inquiry was not useful for determining the number of magma pulses. The cumulate nature of samples were determined by whole-rock geochemistry, wherein incompatible trace elements including Zr, and La were used as proxies for the amount of intercumulus material present in a sample, as well as by visual estimation using transmitted-light petrography. Counter to the conclusions of Ding et el. (2010), variations in incompatible trace element ratios in various rock types in the Eagle system were satisfactorily explained by the cumulate nature and high sulfide content of the samples, with no need to invoke multiple parental magmas in the explanation. The intrusive breccia (or “IBRX”) lithology present in both the Eagle and Eagle East intrusions was studied with transmitted light petrography. It was determined to occur in at least two variations. Both variations have a feldspathic lherzolite matrix with subangular clasts, but in one type heavy sulfide mineralization (up to 30% by volume) occurs in the clasts and in the other type the clasts are nearly devoid of sulfides. In both cases the clasts have high pyroxene abundances and are devoid of olivine, but clasts with high sulfide content tend to have more plagioclase and are more heavily altered. In the Eagle East intrusion, sampled clasts were only of the low-sulfide variety. The IBRX clasts may be a slower-cooling version of the PRX lithology also present in the Eagle system. Alternatively, they may be part of an older intrusion that the Eagle system parental magma cannibalized at depth during emplacement. The main body of the Eagle East intrusion was studied by petrographic examination of a core that profiled its depth. While there was no significant change in cumulate rock type, the core did show modest cryptic variation with depth. Notably, a horizon of increased olivine abundance indicated the potential recharge of the intrusion with the same, homogeneous parental magma. The lithological similarity of the Eagle and Eagle East intrusions indicates that they likely formed from the same parental magma. The main petrographic differences between the two intrusions were the poikilitic nature of clinopyroxene and the lower abundance of plagioclase in the Eagle East intrusion. These differences may be explained by the larger size and thus presumed longer cooling time of the Eagle East intrusion.
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    Hydrogen incorporation mechanisms and diffusion of hydrous defects and silicon ions in olivine: the major rock-forming mineral of the Earth's upper mantle
    (2015-12) Li, Yang
    Water solubility of olivine was investigated with controlled pyroxene activity at 0.3 to 5 GPa and 1473 K. Its dependence on pyroxene activity and water fugacity with the activation volume was studies to place constraints on the hydrogen incorporation mechanisms in olivine. Results of water solubility in olivine indicate that hydrogen incorporated with metal vacancies is the dominant mechanism for samples buffered with pyroxene at all pressures. Hydration and dehydration experiments were carried out to study the kinetic properties of hydrogen ions in olivine to determine the diffusivity of hydrous defects associated with individual peaks in IR spectra. Results of diffusivity indicate that hydrogen diffuses in olivine as interstitial hydrogen ions regardless of the incorporation mechanisms. Silicon diffusivity was also study to better understand the point defect chemistry of olivine.
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    Stress-driven melt redistribution in partially molten rocks deformed in torsion: from pressure shadows to base-state segregation
    (2014-10) Qi, Chao
    The redistribution of melt in partially molten rocks during deformation plays an important role in the evolution and dynamics of Earth's mantle. Previous studies discovered different scales of melt redistribution: melt alignment and melt segregation to form melt-enriched bands , both of which have demonstrated their importance to the deformation of the mantle. In this dissertation, two new forms of stress-driven melt redistribution in deformed partially molten rocks are produced: a formation of pressure shadows around rigid particles and a large-scale, base-state melt segregation. For pressure shadows, observations on the microstructure around the rigid particles revealed the melt distribution and solid flow field, which will provide a constraint on the bulk viscosity of the partially molten rock, if associated with theoretical studies. The presence of base-state melt segregation validated a hypothesis of viscous anisotropy, which provides explanations for melt segregation processes and will cause a significant impact to the dynamic of the mantle. Therefore, the studies of stress-driven melt redistribution in this dissertation are of great significance that will influence the future studies of Earth's mantle.
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    Stress-driven melt segregation and reactive melt in ltration in partially molten rocks deformed in torsion with applications to melt extraction from Earth's mantle.
    (2010-10) King, Daniel S. H.
    Melt extraction from Earth's upper mantle requires transport of magma from regions of partial melting at depth to the Earth's surface. During its ascent, melt interacts chemically and mechanically with the rock matrix. Melt reduces the viscosity of the partially molten rock compared to that of a melt-free rock. This weakening is a potential mechanism of strain localization that could have significant geodynamical implications. Magma interacts chemically with mineral phases during its ascent, dissolving phases in which it is undersaturated and precipitating phases in which it is oversaturated. Such melt-rock reaction can be a driving force for melt migration. Water and other volatiles also partition into the melt from minerals and are then expelled to Earth's oceans or atmosphere. This process leaves behind stronger dehydrated rocks, and it could be the mechanism by which the oceanic lithosphere (mechanical boundary layer) is formed. The work presented here is an experimental investigation of several mechanisms that influence the distribution of melt within a viscously deformable partially molten rock. Three mechanisms are considered, either alone or in various combinations. (1) An applied shear stress causes melt to align and segregate into melt-rich bands with a consistent geometrical relationship to the shear geometry. In Chapter 2, we investigate possible means of scaling the bands that form in experimental samples to Earth's mantle and explore the evolution of melt-rich bands at high shear strain. (2) Interfacial tension driven flow acts to homogenize the distribution of melt within a partially molten sample. In Chapter 3, we investigate the evolution of melt distribution during static annealing of a sample containing melt-rich bands. We compare the experimental results with models of interfacial tension driven flow to determine which mechanisms control the rate of melt redistribution. (3) A melt source that is undersaturated in some component, when coupled with a sink that is rich in that component, will infiltrate into the sink through reactive flow. This reactive flow can develop into an instability in which fingers of high melt fraction propagate into the sink. In Chapter 4 we investigate this process both under static conditions and in combination with stress-driven melt segregation.
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    Suitability of chondrules for studying the magnetic field of the early solar system: an examination of synthetically produced dusty olivine
    (2009-12) Hu, Yifan
    Chondritic meteorites are rare, yet incredibly valuable windows into the geophysical and geochemical environment of the early solar system. Dusty olivine grains containing exsolved nanometer scale iron nickel alloy inclusions are present in many chondritic meteorites and their remanent magnetization may give insight into the strength of the solar dynamo at the time of chondrule formation. Laboratory methods for determining the paleointensity of these rare materials must be optimized prior to conducting experiments on actual meteorite samples. To this end, we have used high temperature recrystallization techniques to produce synthetic dusty olivine samples with textures remarkably similar to those observed in chondritic meteorites. The olivine grains used in these annealing experiments are from the 13 kya Haleyjabunga picritic basalt flow in Iceland and have compositions of Fo90, which closely resembles the olivine composition observed in chondritic meteorites. Samples were annealed at 1350 ˚C, 1315 ˚C and 1425 ˚C either under vacuum in the presence of graphite or under controlled oxygen fugacity using pure CO gas. The laboratory produced magnetic mineral assemblages in 4 different types of samples as well as the starting material have been characterized using low and high-temperature magnetic measurements, hysteresis loops, FORC diagrams, and scanning electron microscopy. The room temperature remanence properties of these materials have been explored using stepwise IRM and ARM acquisition and alternating field demagnetization. These synthesis techniques allow us to produce a wide range of iron nickel grain sizes with correspondingly large variations in coercivity (between 0 and 500 mT). High-temperature measurements of saturation magnetization show that all the samples reach their Curie temperatures at ~760 ˚C, consistent with kamacite, a low Ni high Fe metal alloy. Multiple experiments have shown that care must be taken to rigorously control the atmosphere in which the samples are heated and cooled in order to avoid forming trace amounts of magnetite on the surface of the samples. Future research will explore the feasibility of using modified Thellier protocols or the Shaw method to determine the paleointensity of laboratory induced thermoremanent magnetizations.