Comprehensive paleomagnetic study of a succession of Holocene olivine-basalt flow: Xitle Volcano (Mexico) revisited

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  • Luis M. Alva-Valdivia
    Laboratorio de Paleomagnetismo y Geofisica Nuclear, Instituto de Geofisica, Universidad Nacional Autónoma de México

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A detailed paleomagnetic study of a young Late Holocene olivine-basalt flow from the Xitle volcano in the southern Basin of Mexico was completed to evaluate the consistency and reliability of the record derived from fresh well-preserved and exposed lava flows. One-hundred oriented standard paleomagnetic cores corresponding to 10 different lava effusion episodes were collected from unit-flow V. Thermomagnetic analyses show that bulk magnetic properties and remanence is carried in most cases by Ti-poor titanomagnetite, presumably resulting from oxy-exsolution processes during the initial flow cooling. Unblocking temperature and coercivity suggests pseudo-single domain magnetic grains for these (titano) magnetites. Thermal and alternating field demagnetizations show well-defined univectorial magnetizations. Most sites present a mean direction with small angular dispersion around the dipolar direction for central Mexico. The overall mean direction (N = 10, Dec = 1.1°, Inc = 34.1°, k = 531 and α95 = 2.1°) is characterized by small angular dispersion and inclination close to the dipolar value for the locality. Anisotropy ofmagnetic susceptibility lineation agrees with the geologically-inferred flow direction. Various samples from the 10 lava flows were selected for Thellier paleointensity experiments because of their stable remanent magnetization and relatively low within-site dispersion. According to reliability parameters, the obtained paleointensities are of reasonably good quality. Nine mean paleointensities range between 48.6 and 73.9 μT. The overall mean paleointensity of 59.9-7.7 μT is higher than the present-day field of 43 μT, consistent with the global data for this time-period. Most samples presented alteration during the cooling rate test, and no correction was made to these samples. Those samples on which cooling-rate correction was applied give a flow mean lower than the raw paleointensity data, as was expected.

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