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A 3-D Magnetotelluric Study of the Slave Craton Lithosphere, NW Canada

  • Author / Creator
    Bettac, Sean
  • Archean cratons are comprised of thick, stable continental lithosphere that has persisted for billions of years owing to their isopycnicity (i.e. near neutral buoyancy) and higher viscosity than the surrounding mantle. These regions host diamonds at depths greater than 150 km due to the relatively cold temperatures found in the depth range where pressure is high enough for diamonds to be stable. Because of this, understanding the deep structure of cratonic lithosphere is important for regional diamond exploration. In this thesis previously collected magnetotelluric (MT) data were used to produce a 3-D resistivity model of the Slave craton's lithosphere. The resolution of this resistivity model was rigorously tested. Important features of the model and their implications include:
    (1) A region of low resistivity (< 10 Ωm) at a depth of 100 km beneath the Lac de Gras kimberlite field in the central Slave craton, which was interpreted as a layer of high density saline fluids that were emplaced from a Mesozoic subduction event. Phlogopite and grain boundary sulphides also remain as possible interpretations for the cause of the low resistivity (< 10 Ωm). However, none of these proposed interpretations are without their flaws. Fluids in the mantle would be positively buoyant and reactive suggesting that it would be difficult to retain fluids from the Mesozoic over long time periods. Large quantities of phlogopite would be required (> 20% volume fraction) to produce the low resistivity observed (< 10 Ωm), which is not strongly supported by observations in xenoliths and there is little experimental work on grain boundary sulphides in the mantle. This region of low resistivity was previously observed at depths of 80 - 120 km by Jones et al. (2003). Previous interpretations of this conductor invoked graphite films as an explanation, but it is unlikely that graphite films are stable at these depths. The spatial correlation between the low resistivity layer and the Lac de Gras kimberlite field could be the result of kimberlite magmas following zones of weakness in the lithosphere as a result of the metasomatism caused by the slab-derived fluids.
    (2) The resistivity model also showed that beneath the entire Slave craton the lithosphere-asthenosphere boundary (LAB) is at a depth of at least 200 km. Determining the depth to the LAB is important as this can determine the size of the diamond stability field within the lithosphere, which is used in regional diamond targeting.
    (3) The upper mantle resistivity can be interpreted as requiring a low water content on the order of 10 - 150 ppm by weight in the depth range 100 -170 km. Below these depths the lithosphere appears to be dry (< 5 ppm). The dry base of the lithosphere may act as a resistant boundary preventing the cratonic root from being eroded by the underlying asthenosphere.

  • Subjects / Keywords
  • Graduation date
    Spring 2020
  • Type of Item
    Thesis
  • Degree
    Master of Science
  • DOI
    https://doi.org/10.7939/r3-19h7-yd08
  • License
    Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.