An experimental evaluation of Shaw’s paleointensity method and its modifications using Late Quaternary basalts |
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Authors: | Juan MoralesAvto Goguitchaichvili Jaime Urrutia-Fucugauchi |
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Institution: | Laboratorio de Paleomagnetismo y Geofisica Nuclear, Instituto de Geofisica, UNAM, Ciudad Universitaria, 04510 México DF, Mexico |
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Abstract: | Absolute paleointensity experiments were carried out using Shaw’s method Geophys. J. Res. Astr. Soc. 39 (1974) 133] and its modifications Geophys. J. Res. Astr. Soc. 54 (1978) 241; Geophys. J. Int. 80 (1985) 773; Geophys. J. Int. 118 (1994) 781] on 49 samples belonging to six Late Quaternary basaltic flows from central Mexico. Samples were selected from a large collection because of their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. Moreover, they previously yielded high quality Thellier paleointensity results, which makes them good candidates to assess the reliability of Shaw’s paleointensity method. Only 13 samples yielded acceptable results using Shaw’s original method (ARM2/ARM1 ratio varies from 0.95 to 1.05 for accepted determinations) although 6 samples do not pass the validity test proposed by Tsunakawa and Shaw Geophys. J. Int. 118 (1994) 781] and thus should be rejected for paleointensity analyses. Rolph and Shaw’s Geophys. J. Int. 80 (1985) 773] method gives reliable determination only in one case and no single determination was obtained by Kono’s Geophys. J. Res. Astr. Soc. 54 (1978) 241] modification. Our results indicate an extremely low success rate of Shaw’s paleointensity method, which may be due to magneto-chemical changes that occurred during heating of samples above their Curie temperatures. |
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Keywords: | Paleointensity Rock-magnetism Paleomagnetism Quaternary Central Mexico |
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