Palaeointensity results for historic lavas from Mt Etna using microwave demagnetization/remagnetization in a modifiedThellier-type experiment |
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| Authors: | Mimi J Hill & John Shaw |
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| Institution: | The Geomagnetism Laboratory, Department of Earth Sciences, The University of Liverpool, Oxford Street, Liverpool L69 7ZE, UK. E-mail: |
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| Abstract: | The bane of conventional Thellier-type palaeointensity experiments is the thermal alteration of the samples during experimentation. However, high-frequency microwaves can be used to stimulate the magnetic minerals directly, thus eliminating the need for the bulk sample to be heated ( Walton et al . 1993 ). The microwave technique has been successfully applied to ceramics ( Shaw et al . 1996 , 1999 ), and here we present results for historic lavas from Mt Etna, Sicily. 20 samples were randomly selected from 10 different historic flows dating from 1843 to 1983. Hysteresis parameters were monitored as a check for alteration during experimentation and minimal alteration was found. Rock-magnetic analysis and AF plus microwave demagnetizations were carried out on all samples, and microwave intensity analysis was carried out on those samples that were stable to microwave demagnetization (16 in total). With one exception, all samples gave high-quality intensity results. Two or three subsamples from each sample were investigated and a weighted average calculated to give a single estimate of the field. When the intensity values were compared with the SV model based on direct observatory measurements ( Bloxham & Gubbins 1985 ), it was found that those samples that contained a high multidomain (MD) component gave intensity values around 20 per cent lower than expected. This can be explained by the cooling rate effect ( Dodson & McClelland-Brown 1980 ) and/or the presence of an MD component ( McClelland et al. 1996 ). Those samples with the lowest numbers of MD grains gave field values which, within error, were the same as those for the model, or slightly too high: this is compatible with the cooling rate effect. |
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| Keywords: | Etna lava microwaves palaeointensity |
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