Browsing by Subject "paleomagnetism"
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Item Characterization and modeling of materials responsible for planetary crustal magnetism(2016-08) Strauss, BeckyEarth and Mercury are the only terrestrial planets in our solar system with present-day magnetic dipole fields generated by internal dynamo systems. In contrast, Mars and the Moon show evidence of past dipole fields in the form of crustal magnetic anomalies; to hold measurable magnetizations, crustal materials must have been exposed to an applied field. While the physical principles of magnetic recording are consistent between terrestrial planets, the particular conditions at each planet control the mechanisms by which crustal materials may be magnetized and limit the types of minerals that can retain magnetic remanence. As the suite of magnetic materials used for studies of remanence expands, the need for new methods follows. The integration of rock magnetic techniques with microscopy and chemical analyses enables the reconstruction of increasingly comprehensive narratives of remanence acquisition and alteration, even in materials that are challenging to study using traditional methods. This thesis demonstrates the utility of a materials approach to rock magnetism by applying techniques designed for terrestrial use in a planetary context. The first of two case studies focuses on calcite cave deposits as a means to demonstrate how novel techniques can be used to unlock previously inaccessible archives of magnetic information. Tandem magnetic and microscopic analyses improve our understanding of the rock magnetic properties of weakly magnetic stalagmites and their potential for paleomagnetic research, as well as illuminating the pathways of remanence acquisition in cave systems. The second case study addresses the magnetic anomalies recently detected by the MESSENGER orbiter at Mercury. These anomalies are consistent with remanence acquired in a dipole field. However, in the absence of physical samples, the types of magnetic minerals that could be holding remanence in Mercury’s hot, highly reducing surface environment have not yet been determined. Orbital data is combined with fundamental rock magnetic principles to constrain the magnetic mineralogy of Mercury and to propose mechanisms of magnetization and remagnetization in the lithosphere.Item Paleomagnetism and age of the Leucite Hills Volcanic Complex, Wyoming: Implications for eruptive history, landscape evolution, and the geomagnetic instability timescale (GITS)(2023-02-22) Welsh, Josie T; Feinberg, Joshua M; Schneider, Emma L; Pares, Josep M; Jicha, Brian R; Singer, Bradley S; Carroll, Alan R; feinberg@umn.edu; Feinberg, Joshua M; Institute for Rock MagnetismThe Leucite Hills Volcanic Field, southwest Wyoming comprises two dozen volcanic features including necks, flows, dikes, and plugs. It has been the focus of many petrologic studies as its volcanic and shallow intrusive rocks are one of the only surficial manifestations of ultrapotassic lamproite. We build on paleomagnetic findings of Sheriff and Shive (1980) by providing further paleomagnetic data from the Boars Tusk dike and Black Rock flows. We also characterize the magnetic mineral assemblage of these lamproites. Principal component analysis of alternating field (AF) and thermal demagnetization data indicate that the dike and breccias of Boars Tusk record a reversed magnetic polarity and the Black Rock lava records a normal polarity, both consistent with previous findings. This recording is typically carried by minerals with coercivities >15 mT and susceptibility measurements indicate magnetite, maghemite, and titanomagnetite as likely magnetic carriers. AF and thermal demagnetization experiments evince secondary magnetizations held by lower coercivity grains, likely caused by lightning strikes. 40Ar/39Ar incremental heating experiments from Boars Tusk and Black Rock give plateau ages of ~2500 ka and ~900 ka, respectively. Recent advances in the chronology of geomagnetic field reversals and excursions during the Quaternary permit integration of the Boars Tusk dike into the lower Matuyama chron, whereas the Black Rock lavas most probably record the Kamikatsura excursion. Notably, Black Rock records high inclinations that suggest the short-lived excursion achieved a full geomagnetic reversal, something not observed at other localities recording the Kamikatsura excursion. The Leucite Hills offer further opportunities to refine the Quaternary geomagnetic instability time scale (GITS), and to improve understanding of the eruptive and geomorphic evolution of this unusual volcanism.Item Pseudotachylyte remanence confirms generation along low-angle normal fault planes(2019-06) Longchamp, Benjamin MaxfieldLow-angle normal faults (LANFs) have been mapped in metamorphic core complexes (MCCs) throughout western North America, but dip too shallowly for seismic slip according to Andersonian fault mechanics. Debate over the origin of these structures is split between support for models where normal faults initiate at favorable dips and subsequently rotate to low-angle orientations (e.g. rolling hinge model) and belief that field relationships show that LANFs were active at or near their present orientation. Using paleomagnetic data from pseudotachylyte veins I show conclusively for the first time that LANFs in the South Mountains MCC in Arizona failed seismically at low-angles. Additionally, I detail many of the challenges that I faced using pseudotachylyte in a paleomagnetic study, providing a starting point for future workers seeking to recover remanence directions from similar materials.Item Updated Magnetostratigraphy for The Eocene Green River Formation, Wyoming(2022-05) Schneider, EmmaThe Green River Formation (GRF) is one of the best-preserved continuous Eocene terrestrial records in the world, allowing researchers to track phenomena in high resolution related to climate, vegetation, tectonics, and geomorphology. The preservation of the early Eocene in the GRF is particularly important as it records the Early Eocene Climatic Optimum (EECO), an analog for current greenhouse gas-driven global warming. Here we provide an updated magnetostratigraphy for the Wilkins Peak Member (WPM) of the GRF integrated with recent 238U-206Pb and 40Ar/39Ar results that more confidently identifies the geomagnetic reversals preserved in the sediments (C22n, C22r, C23n.1n, C23n1.1r, C23n.2n, and C23r) and refines their radioisotopic ages. Earlier GRF magnetostratigraphic studies were challenged by the presence of pervasive authigenic pyrrhotite in sediments of the Wilkins Peak member (Sheriff and Shive, 1982), which are confirmed in non-tuff lithologies in this study. Here, we build on the work of Tsukui and Clyde (2012) by focusing paleomagnetic sampling on ash-fall tuffs, which are more resistant to the formation of authigenic sulfides and can be dated directly using 238U-206Pb and 40Ar/39Ar techniques. The tuffs were deposited in a closed-lake basin setting and are sufficient in number to refine the stratigraphic position of geomagnetic reversals. Most tuffs show minimal post-depositional alteration and act as reliable paleomagnetic recorders. Tuffs of both normal and reversed polarity were identified using alternating field and thermal demagnetization protocols. The magnetic mineral carriers are magnetite, hematite, and their Ti-substituted equivalents and were characterized using hysteresis loops, backfield curves, and magnetic susceptibility. Fe- sulfides are present in some samples and produce secondary magnetic minerals during thermal demagnetization at temperatures >450°C. This updated terrestrial magnetostratigraphy provides an important bridge for correlating to marine records deposited across the EECO, and ongoing cyclostratigraphic work promises an even higher resolution view of the natural history preserved within the GRF.