Low-temperature demagnetization of natural remanent magnetization in dolerites of a Proterozoic dyke swarm near Nain, Labrador

Mackay, Robert I. (1995) Low-temperature demagnetization of natural remanent magnetization in dolerites of a Proterozoic dyke swarm near Nain, Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

Dolerite dykes are presently considered to be the most important recorders of Precambrian paleomagnetism. This is because they can often be accurately dated (using U-Pb in baddeleyite) and can often be shown to carry primary remanence (using the baked contact test). However the mechanisms by which this stable natural remanence is retained over geological time is not well understood. In this thesis, observation of changes in remanence on cooling to 100 K in zero magnetic field and warming back to room temperature is used to help understand these mechanisms. -- Twelve specimens were studied from different dykes in a Proterozoic dolerite dyke swarm (U-Pb date of 1,277±3 Ma) located near Nain, Labrador. Each specimen carried a stable westerly-directed remanence that was likely acquired soon after crystallization. Alternating-field demagnetization curves of the natural remanent magnetization (NRM) of each specimen had a quadratic shape. The median destructive field (MDF), which is the alternating field required to reduce intensity by half, ranged from 18mT to 47.5mT for NRM. These properties suggest that remanence is carried by single-domain (SD) or pseudo-single-domain (PSD) grains of magnetite rather than by large multidomain (MD) grains. Median destructive fields were similar for anhysteretic remanent magnetization (ARM) and NRM but were smaller for saturation isothermal remanent magnetization (SIRM). -- Apparatus was built to measure remanence intensity as a function of temperature in cooling cycles to near liquid nitrogen temperature. Low-temperature demagnetization experiments were done for NRM, ARM and SIRM for all specimens. For the three specimens with highest median destructive fields (~40mT for NRM) low temperature cycling had relatively little effect on remanence as expected if the remanence was controlled by shape anisotropy. Similar low-temperature behaviour was reported by others for synthetic magnetites of less than .31μm grain size. -- The rest of the specimens showed a pronounced decrease in intensity of remanence on cooling in zero field. The rate of remanence decrease was greatest on approaching 120 K, the temperature of magnetite's Verwey transition from cubic to monoclinic crystal structure. Cooling below the Verwey transition resulted in little further decrease in remanence. On warming back to room temperature remanence increased once the Verwey transition was passed. The final remanence intensity after a cycle of cooling and warming as a function of the initial intensity is termed "recovery" and averaged about ~0.75 of initial remanence for NRM and ARM and ~0.60 for SIRM. The decrease of ARM on cooling from room temperature to near 120 K is shown to roughly parallel the decrease of saturation magnetostriction, suggesting that remanence decrease is due to unpinning of magnetoelastically controlled domain walls. -- Finally, combining our measurements of recovery in magnetite after low temperature cycling with measurements of others revealed that the higher the median destructive field of the specimen, the higher the recovery. This may be due to both median destructive field and recovery increasing with dislocation density.

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