Browsing by Subject "Magnetite"

Now showing 1 - 2 of 2

Low temperature scanned probe microscopy studies of magnetic oxides
(2011-05) Lee, Alfred K.; de Lozanne, Alejandro L.; Shih, Chih-Kang; Markert, John; Demkov, Alexander; Ruoff, Rodney
This dissertation is divided into two parts. In the first, the general paradigm of scanned probe microscopy is outlined with a focus on atomic force microscopy and a few of its variations. Magnetic force microscopy is covered in detail as it forms the basis of the second part of this dissertation. The core elements and extra features of the instrument are described with attention paid to the upgrades made by the author. In the second part of this dissertation, background information on perovskite oxides and the inverse spinel system, magnetite, is given. Magnetic force microscopy studies were done on three thin film systems and are detailed. In the first study, ferromagnetic manganite films were subjected to discontinuous changes in strain due to structural transitions in their barium titanate substrates. The resulting effect on the magnetic domains was observed. In the second study, the ferromagnetism of a tensile-strained LaCoO₃ film was studied across temperatures from 4.3 K to 90 K and applied fields up to [mu]₀H=1.1 T. Finally, the properties of antiphase domains in magnetite films of varying film strain due to transition metal buffer layers was probed by imaging the magnetic domains which are pinned to the antiphase boundaries.
Novel (U-Th)/He thermochronometric constraints on serpentinized ultramafic rocks
(2017-08) Cooperdock, Emily Jane; Stockli, Daniel F.; Ketcham, Richard; Barnes, Jaime; Lavier, Luc; Müntener, Othmar; Klein, Frieder
Serpentinization, the hydration of peridotite, is a widespread process that impacts rheological properties along plate boundaries, fluid-mobile element cycling and biogeochemical processes. While numerous studies exist on the petrology, structure and geochemistry of serpentinites, geochronological analysis has been elusive due to the lack minerals with well-established dating techniques. With technological advances in radiogenic isotope analysis in the past decade, it is now possible to address the timing of serpentinization by dating the growth of magnetite that forms as a direct result of the breakdown of primary peridotite phases in the presence of water. This study develops and applies a (U-Th)/He-based method that is tailored to analyze magnetite that grow during the alteration of ultramafic rocks. The technique includes procedures to screen for grain quality in opaque phases with X-Ray Computed Tomography, physical air abrasion to remove the alpha implantation/ejection zone in the outer grain boundary, and analytical procedures to measure low parent nuclide concentrations typical of these lithologies. The method is then modified to test the applicability to date magmatic spinel, a primary mineral phase in peridotite, to date peridotite exhumation by tectonic and volcanic processes. The magnetite (U-Th)/He technique is applied to serpentinites within an exhumed subduction complex in Syros, Greece, uplifted serpentinites within the Malenco Ultramafic Unit, Italian Alps, and to carbonated serpentinite in Wadi Fins, Oman. The results of these studies provide evidence for episodic or continuous magnetite growth over millions of years, which implies that magnetite formation can continue to occur in serpentinites under changing metamorphic conditions. These studies also show that multiple generations of magnetite growth can be distinguished based on magnetite grain size and trace element chemistry, and that hydrothermal magnetite can record changing chemical conditions and act as an important host phase of U and Th in serpentinites. Overall, the magnetite (U-Th)/He chronometer can be used to distinguish between multiple episodes of fluid-rock alteration, which has implications for the cooling history and geochemical exchanges in serpentinized peridotite. This technique, coupled with spinel (U-Th)/He, has broad applicability to investigate the timescales of mantle exhumation and alteration of ultramafic rocks in a variety of tectonic settings.