Anisotropy of magnetic susceptibility and magnetic properties of obsidians: volcanic implications |
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Authors: | E Cañón-Tapia K Cárdenas |
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Institution: | (1) Department of Geology, CICESE, PO Box 434843, San Diego, CA, USA |
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Abstract: | The anisotropy of magnetic susceptibility (AMS), hysteresis and thermomagnetic curves of two sets of obsidians with contrasting
bulk compositions are reported in this work. The cooling and deformation history of one of those obsidians is perfectly known,
as these specimens were produced in the laboratory using material from a basaltic lava flow. The other samples are occurrences
of a more silicic composition and for which the AMS has been documented to have a close relationship with the distribution
of microlites. The results of our measurements indicate that although the deformation and cooling histories of the lava might
influence the exact composition of the ferromagnetic fraction, the relationship between the AMS and the deformation history
does not seem to be altered. Furthermore, the results of this work indicate that the AMS can be associated to a population
of ferromagnetic minerals of a submicroscopic size, despite of which it can be very well defined and yield large degrees of
anisotropy. It is suggested that the AMS associated to such population of small grains might indeed be the origin of the AMS
of other igneous rocks that have an optically observable fraction of mineral grains, although until present it had been overlooked
in most instances. Use of tests designed to identify the contribution of a superparamagnetic fraction (SP) in the magnetic
properties of a rock can help us to identify the presence of such a SP-related AMS in other cases. |
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