Browsing by Subject "rock magnetism"
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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 Data for "Late Paleozoic depositional environments and sediment transport directions of the Itararé Group rocks from the state of São Paulo, Brazil, determined from rock magnetism and magnetic anisotropy" collected at the University of Sao Paulo and the University of Minnesota between 2012 and 2020.(2020-12-22) Bilardello, Dario; dario@umn.edu; Bilardello, Dario; University of Minnesota Institute for Rock MagnetismItem The effect of low‐temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites: A case for acquisition of chemical remanent magnetization(Journal of Geophysical Research (American Geophysical Union), 1991) Kelso, Paul; Banerjee, Subir; Worm, HorstItem 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 The rock magnetic fingerprint of chemical remagnetization in midcontinental Paleozoic carbonates(Geophysical Research Letters (American Geophysical Union), 1992) Jackson, Mike; Sun, Wei-wei; Craddock, JohnResults of a paleomagnetic and rock magnetic survey of Paleozoic carbonates from 39 sites in the midcontinental U.S. show that many of these sites carry a stable remanence of apparent Permian age. Many of these remagnetized sites also have relatively high anhysteretic susceptibilities, and higher saturation remanence than most of the sites where the late Paleozoic remanence is absent. However the correlation between late Paleozoic remanence and high anhysteretic susceptibility or high saturation remanence is imperfect. The most diagnostic rock magnetic parameter for recognizing remagnetized sites is a ratio of anhysteretic remanence/saturation remanence exceeding 10%. We have found high ratios in almost all remagnetized sites, but in very few sites where the Late Paleozoic remanence is absent. The high ratios reflect the presence of a significant fraction of extremely fine-grained magnetite (a few tens of nanometers), spanning the superparamagnetic-single domain threshhold.Item Rock magnetic properties of the Arunta Block, Central Australia, and their implication for the interpretation of long-wavelength magnetic anomalies(Journal of Geophysical Research (American Geophysical Union), 1993) Kelso, Paul; Banerjee, Subir; Teyssier, ChristianRock magnetic and petrologic studies of a suite of deep crustal rocks from the Arunta Block of Central Australia reveal that the granulite grade rocks are in general much more magnetic than the amphibolite grade samples irrespective of bulk rock composition. The dominant magnetic mineral in all samples is relatively pure magnetite as determined from thermomagnetic and electron microprobe analysis. The bulk magnetic properties are typical of pseudo-single-domain to multidomain size material. The samples from our study have very large remanences compared to previous crustal magnetic studies, with the granulites having a median natural remanent magnetization of 4.1 A/m and Koenigsberger ratio of 7.2. These remanences are relatively resistant to thermal demagnetization, with nearly 50% of the magnetization remaining after 400°C demagnetization. Thus remanence may contribute significantly to the observed magnetic anomalies, including long-wavelength magnetic anomalies, the source of which resides at depth and therefore at elevated temperature, where a thermoviscous remanent magnetization along the present-day field is likely to dominate. The magnetic susceptibilities of the samples are only capable of producing a magnetization of less than 1 A/m in the 0.05 mT present-day field of Central Australia. Susceptibility is nearly constant with temperature to within 30°C of the Curie temperature where it decreases rapidly, i.e., there is no significant Hopkinson peak. The granulite samples from this study have magnetizations, both remanent and induced components, that are large enough to account for most long-wavelength magnetic anomalies if they are juxtaposed with relatively nonmagnetic rocks, similar to the high-grade rocks in the Canadian Shield.Item Rock magnetism of remagnetized Paleozoic carbonates: low-temperature behavior and susceptibility characteristics(Journal of Geophysical Research (American Geophysical Union), 1993) Jackson, Mike; Rochette, Pierre; Fillion, Gerard; Banerjee, Subir; Marvin, James