Paleointensity determinations on basalts from the Emperor Seamounts and testing the properties of FORC diagrams using micromagnetic modelling and measurements.
Published: 2004
Language: eng
Subjects: Physics Theses
Description: The intensity of the geomagnetic field recorded in rocks from the Emperor Seamounts was measured. Paleolatitudes determined from thermal demagnetization in the Thellier experiments support the hypothesis of a moving Hawaiian hotspot. Because of thermal alteration and the presence of titanomaghemite, the Thellier success rate was only 5%. The virtual axial dipole moment at 56 Ma was (7.2 +/- 0.2) x 1022 Am2, about 60% higher than the average value for the 300--0.3 Ma time period, although still within the error bounds of this average.In nature, magnetic minerals usually occur as mixtures. The linear additivity of FORC diagrams was investigated, in order to study the possibility of separating magnetic mixtures into their components. Micromagnetic models of mixtures of magnetite and hematite and mixtures of magnetite with bimodal grain size distributions, as well as experimental measurements, yield linearly additive FORC diagrams.First-order reversal curve (FORC) diagrams give the coercivity and interaction field distributions of magnetic minerals and could be used to pre-select samples for paleointensity experiments. Micromagnetic modelling shows that non-interacting particles are manifested by a symmetrical single peak. When particles interact, splitting of the central peak on the interaction field (Hu) axis might explain the vertical spread observed on FORC diagrams.I compared FORC and Preisach diagrams measured on a range of natural and synthetic samples. I find a good agreement between both diagrams in the main features. Contours that are very close to the Hu axis are not imaged on a Preisach diagram. As a first approximation, FORC and Preisach diagrams are equivalent. However, the level of detail offered by FORC diagrams makes them more attractive to use than Preisach diagrams.I measured FORC diagrams on elongated SD particles at temperatures ranging from 20 K to 853 K. The median field of the profile through the FORC peak along the coercivity field (Hc) axis varies in proportion to the bulk coercive force. The half-width of a profile through the FORC peak parallel to the Hu axis is similar to independent measures of local interaction fields Hi. These properties support the interpretation of the FORC variables  Hc and Hu as being equivalent to microcoercivity Hc and interaction field  Hi, respectively, and validate the FORC method.
Notes: Adviser:  David J. Dunlop.

Thesis (Ph.D.)--University of Toronto, 2004.

Electronic version licensed for access by U. of T. users.

Source: Dissertation Abstracts International, Volume: 65-10, Section: B, page: 5041.