The magnetic fabric of ocean basalts : character and variability
The magnetic fabric of ocean basalts : character and variability
Magnetic susceptibility anisotropy (Magnetic fabric) measurements have been performed on a suite of 187 DSDP/IPOD Ocean Basalt cores from the Indian and Atlantic Oceans. Terrestrial pillow analogues have been measured for comparison. The Low Field Torque Magnetometer is found to be the more reliable for anisotropy measurements on extrusive igneous rocks, compared to the Digico Spinner System. Experiments show that there is often a close equivalence between the measured magnetic grain distribution and the magnetic fabric. For medium and coarse-grained basalts the opaque (magnetic) fabric mimics the feldspar fabric in both shape and orientation, whilst this does not hold true for all finer-grained variolitic basalts. The magnitude of the magnetic anisotropy, H%, is related not only to the degree of preferred orientation, but is sensitive to variations in susceptibility, oxidation and grain-size. The ocean basalts examined fall into three morpho-genetic categories: Pillows/Thin Flows; Thick Massive Flows and intrusives. Each category has a distinctive magnetic fabric style. Overall, the anisotropy values are low (mean 4.0 + 2.39%), predominantly triaxial, and comparable to terrestrial extrusives. Locally much higher values of anisotropy occur: these correlate with samples having the greatest preferred crystal alignment. The Ellwood Parameter Fb (Ellwood, 1976) is critically reassessed. It is shown to be inadequate for distinguishing thick flow basalts from intrusives; it is much better at differentiating 'fine grained' basalts from 'coarse grained'. There is a marked tendency for the maximum susceptibility axis to be inclined close to the horizontal, irrespective of texture, position within the cooling unit or local topography. A vertically-directed hydrostatic stress mechanism during the plastic, pre-solidification phase is postulated.
University of Southampton
Manighetti, Adrian Alexander
1982
Manighetti, Adrian Alexander
Manighetti, Adrian Alexander
(1982)
The magnetic fabric of ocean basalts : character and variability.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
Magnetic susceptibility anisotropy (Magnetic fabric) measurements have been performed on a suite of 187 DSDP/IPOD Ocean Basalt cores from the Indian and Atlantic Oceans. Terrestrial pillow analogues have been measured for comparison. The Low Field Torque Magnetometer is found to be the more reliable for anisotropy measurements on extrusive igneous rocks, compared to the Digico Spinner System. Experiments show that there is often a close equivalence between the measured magnetic grain distribution and the magnetic fabric. For medium and coarse-grained basalts the opaque (magnetic) fabric mimics the feldspar fabric in both shape and orientation, whilst this does not hold true for all finer-grained variolitic basalts. The magnitude of the magnetic anisotropy, H%, is related not only to the degree of preferred orientation, but is sensitive to variations in susceptibility, oxidation and grain-size. The ocean basalts examined fall into three morpho-genetic categories: Pillows/Thin Flows; Thick Massive Flows and intrusives. Each category has a distinctive magnetic fabric style. Overall, the anisotropy values are low (mean 4.0 + 2.39%), predominantly triaxial, and comparable to terrestrial extrusives. Locally much higher values of anisotropy occur: these correlate with samples having the greatest preferred crystal alignment. The Ellwood Parameter Fb (Ellwood, 1976) is critically reassessed. It is shown to be inadequate for distinguishing thick flow basalts from intrusives; it is much better at differentiating 'fine grained' basalts from 'coarse grained'. There is a marked tendency for the maximum susceptibility axis to be inclined close to the horizontal, irrespective of texture, position within the cooling unit or local topography. A vertically-directed hydrostatic stress mechanism during the plastic, pre-solidification phase is postulated.
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Published date: 1982
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Local EPrints ID: 460429
URI: http://eprints.soton.ac.uk/id/eprint/460429
PURE UUID: e33ebe7a-0aaf-47e1-a9c2-22b9e0ee772e
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Date deposited: 04 Jul 2022 18:22
Last modified: 04 Jul 2022 18:22
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Author:
Adrian Alexander Manighetti
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